Document text
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WAR DEPARTMENT
ENGINEER
FIELD MANUAL
*
FIELD FORTIFICATIONS
FM 5-15
ENGINEER FIELD MANUAL
FIELD FORTIFICATIONS
Prepared under direction of the
Chief of Engineers
UNITED STATES
GOVERNMENT PRINTING OFFICE
WASHINGTON : 1940
For sale by the Superintendent of Documents, Washington, D. C. - Price 45 cents
WAR DEPARTMENT,
Washington, October 1, 1940.
FM 5-15, Engineer Field Manual, Field Fortifications, is
published for the information and guidance of all concerned.
[A. G. 062.11 (7-30-40).]
By order of the Secretary of War:
G. C. MARSHALL,
Chief of Staff.
Official :
E. S. ADAMS,
Mafor General,
The Adjutant General.
n
TABLE OF CONTENTS
SECTION I. General. Paragraph. Page
Definitions_ 1 1
Employment_ 2 1
Execution_,_ 3 2
Nature of work_ 4 2
Effectiveness against projectiles and
bombs- 5 3
Natural conditions affecting design
and location- 6 4
Use of existing terrain features_ 7 5
IT. Terrain appreciation.
General_ 8 6
Factors_ 9 6
Compartments- 10 8
Influence of corridors_ 11 12
Influence of cross compartments_ 12 20
Aids to study of terrain_ 13 20
HI. Organization op the Ground.
General.- 14 21
Use of hasty fortifications_ 15 23
Use of deliberate fortifications_ 16 25
Battle position- 17 25
Platoon defense area_ 18 29
Company defense area_ 19 30
Battalion defense area_ 20 31
Regimental sectors_ 21 35
Additional organization_ 22 36
Artillery support_ 23 36
Outpost area_ 24 37
Reserve battle position_ 25 41
Switch positions_ 26 42
IV. Effect of Projectiles.
General--_ 27 42
Infantry weapons_ 28 43
Artillery and aircraft_ 29 45
V. trenches.
General,-- 30 60
Trace, tracing, and profile_.1_ 31 60
Necessity for standard types_ 32 61
Classification_ 33 61
Hasty_ 34 63
Deliberate- 35 68
Drainage.—---— 36 87
Revetments_ 37 92
Breastworks- 38 103
Accessories- 39 105
in
TABLE OF CONTENTS
Section VI. Obstacles. Paragraph Page
Classification_ 40 117
Basis of location, design, and con¬
struction_ 41 117
Barbed wire_ 42 118
Abatis- 43 149
Inundations- 44 152
Underwater_ 45 153
Tank- 46 157
Summary- 47 157
VII. Emplacements.
Infantry weapons_ 48 158
Artillery_ 49 18ft
Antiaircraft materiel_ 50 201
VIIl. Protected Shelters.
Classification_ 51 206
Choice of type_ 52 209
Overhead cover- 53 213
Standard construction materials_ 54 219
Surface_ 55 220
Artillery ammunition_ 56 225
CUt-and-cover_ 57 226
Cave_ 58 240
Drainage_ 59 276
Ventilation_ 60 278
Circulation of fresh air_ 61 278
Gasproofing- 62 279
Air supply during gas attack_ 63 282
Floor space- 64 283
Time for construction_ 65 284
IX. Summary.
General- 66 286
Terrain appreciation_ 67 286
Organization of the ground_ 68 287
Effects of projectiles_ 70 287
Trenches_ 71 287
Obstacles_ 72 289
Emplacements_ 73 290
Protected shelters_ 69 291.
General rules for execution of field
Works_ 74 292
IV
FM 5-15
ENGINEER FIELD MANUAL
FIELD FORTIFICATIONS
(The matter contained herein supersedes chapter 2, part two.
Engineer Field Manual, volume XI (tentative), June 25, 1932; TR
196-5, October 1, 1926; TR 195-50, February 20, 1929; and TR
1195-65, January 28, 1926.)
Section I
GENERAL
■ 1. Definitions. — Field fortifications are works constructed
by military forces in combat operations to increase natural
defensive strength of a locality. They are of two general
classes—
a. Hasty, constructed when in contact or about to make
contact with the enemy and consisting generally of such fox
holes, open weapon emplacements, and simple obstacles as
the situation permits.
b. Deliberate, constructed out of presence of the enemy or
developed gradually from hasty fortifications as a result of
long occupation, and consisting generally of such standard
trenches, covered emplacements, obstacles, etc., as the situa¬
tion requires.
■ 2. Employment.— -a. Defense, —Field fortifications are used
in defensive operations to—
(1) Fortify any position not protected by permanent forti¬
fications.
(2) Supplement permanent fortifications.
(3) provide local security for isolated installations such as
coast defense guns.
b. Offense. —In offensive operations they are used by—
(1) Any portions of the force assigned defensive missions,
thus increasing their defensive strength and thereby releas¬
ing other troops for the offensive.
(2) Attacking troops making a temporary halt, thereby
increasing their resistance to counter attacks.
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ENGINEER FIELD MANUAL
c. In general, any occupied position should be protected by
field fortifications whether in preparation for defensive or in
connection with offensive combat. However, the primary use
Is in defense.
■ 3. Execution.— a. In general, field fortifications are laid
out and constructed by the troops occupying them.
b. In divisions, corps, and armies the unit engineer assists
in preparation of plans and in technical inspections to insure
carrying out such plans. It is seldom that small units have
engineer assistance in the planning or laying out.
c. Primary duties of engineer troops in execution are—
(1) Supply of tools and materials.
(2) Execution of works of general use such as command
and observation posts, aid stations, etc., for higher units, main
communications for supply and evacuation, and water supply
facilities. Engineer troops may also assist other units in
execution of works for which engineers are better trained
and equipped than are the other troops, such as large shelters,
obstacles of special nature, special works for drainage, and
demolitions.
d. Rear positions usually are planned and laid out by the
unit engineer under general instructions from the com*
mander. Engineer troops assisted by reserve unit and labor
troops, and civilian labor if available, usually execute these
works.
■ 4. Nature of Work.— a. Work of fortifying a locality con¬
sists generally of the following tasks, listed in the order of
importance most frequently applicable:
(1) Hasty ,— The general rule is first to concentrate the bulk
of the force on those things which most increase defensive
strength of the position.
(a) Providing camouflage, limited generally to selection of
positions affording natural cover or ease of concealment.
<b) Clearing reasonable fields of fire for flat trajectory
weapons.
(c) Digging open emplacements for machine guns.
(d> Digging fox holes (small pits for individual soldier).
(e) Erecting obstacles, principally wire entanglements.
(/) Digging shallow trenches connecting fox holes when
time permits.
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4-5
(2) Deliberate .—The general rule Is first to concentrate as
much of the force as is necessary on those things which re*
quire the longest time.
(a) Adequate communications.
(b) Providing shelter for those working on the position.
(c) Providing camouflage of important installations.
(d) Constructing splinterproof or shellproof observation
posts.
(e) Constructing protected shelters for troops, command
posts, aid stations, etc.
(/) Constructing splinterproof or shellproof emplacements
for infantry supporting weapons.
(flr) Constructing standard fire and communication
trenches.
Ui) Constructing artillery emplacements.
. (t) Constructing obstacles of all types, including antitank
obstacles.
b. In both hasty and deliberate fortifications, the above
tasks are undertaken concurrently so that the position is
fairly well coordinated for defense at all times. Provision
for camouflage must be made before work is commenced.
Camouflage is then carried on continuously throughout the
work so that as much of the position as possible is concealed
from the enemy no matter when he observes it.
e. Hasty fortifications are developed into deliberate forti¬
fications by deepening and extending trenches, strengthening
obstacles, adding protected shelters, etc. This development
normally is a continuing process, its extent depending prin¬
cipally upon length of time the position is occupied. As engi¬
neers may have to assist in development work, they should be
familiar with both hasty and deliberate fortifications.
d. Breastworks and surface shelters are used in place of
trenches, dugouts, etc., where natural conditions such as high
ground water level or rock prohibit normal excavation.
■ 5. Effectiveness Against Projectiles and bombs. — a.
Trenches and open emplacements properly constructed pro¬
vide effective protection against all small-arms fire, artillery
projectiles up to 4 inches in caliber, and fragmentation bombs
(usually 20 pounds), except direct hits. Such works can be
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5-6 ENGINEER FIELD MANUAL
destroyed by fire from larger caliber guns or bombing with
demolition bombs of 50 pounds or heavier.
b. Protected shelters and shellproof covers can be made
effective against the heaviest shells or bombs, but such com¬
plete protection is limited to the most important installations
only, because of excessive labor and materials involved. Most
installations are protected only against light or medium shells
and bombs. Degree of protection to be provided for any
given installation depends on type, amount, and accuracy of
fire or bombing which the enemy is likely to bring to bear on
it. The enemy normally will conserve ammunition and save
wear and tear on guns and airplanes by directing his heaviest
fire and bombing against those installations the destruction
of which is most important to him, and which he can shell
or bomb with reasonable accuracy and effectiveness. Hence
concealment which makes accurate fire against a single in¬
stallation more difficult, dispersion which makes effective fire
against a group of installations more difficult, and defilade
which masks flat-trajectory fire should all be used to the
greatest extent practicable In order to lessen need for exten¬
sive protective works.
■ 6- Natural Conditions Affecting Design and Location.—
a. Surface water resulting from the run-off of rainfall should
be kept out of excavated works by—
(1) Siting them so that natural drainage lines drain away
from rather than into them.
(2) Using drainage ditches to divert surface water which
would otherwise run Into them.
(3) Providing internal drainage to dispose of surface water
which enters them.
b. Ground water is free water contained in pervious soil.
Its upper level is roughly parallel to the ground surface, and
is nearest the surface at drainage lines and deepest under hill
crests. Its level at any point is the same as the level of water
in a shallow well or test hole at the point. It is generally
impracticable to excavate for field fortifications below ground
water level because of the difficulty of providing adequate
drainage (see par. 36).
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6-7
c. Rock or hardpan prohibits excavating for hasty fortifi¬
cations, and ordinarily requires too much labor to permit ex¬
cavation for deliberate fortifications.
d. Sand or similar material lacking cohesion is very good
for hasty fortifications because it is easy to dig, but is poor
for deliberate because it requires excessive revetment to make
the works reasonably permanent.
e. Terrain is the most important feature to be considered
in locating field fortifications. A detailed discussion of ter¬
rain is given in section H.
■ 7. Use of Existing Terrain Features. — a. Field fortifica¬
tion work may be . reduced by taking full advantage of all
existing terrain features such as woods, cities or towns, brick
or stone walls, railroad or highway embankments and ditches,
small gullies or folds in the ground, and many others. Such
features often may be used in their natural state to provide
either concealment or protection from fire or both. In many
cases a little labor can convert them into strong defensive
works. Such features as houses and walls of brick or stone
sometimes assist indirectly by providing a source of materials
for field works, thus reducing need for supplies from the rear.
b. Large woods have great defensive value because they
afford concealment from enemy observation and are developed
readily for protection from hostile fire. On the other hand,
small patches of woods draw artillery fire, are particularly
subject to gassing, and should be avoided. In defense of
woods, concealment offered should be used to take the enemy
under fire by surprise. The strongest resistance should be
located 100 yards or more in rear of the forward edge of the
woods. Major irregularities in the edge of the woods are
used to develop flanking fire sweeping it. When clearing
woods to develop fields of fire, care should be taken not to
destroy concealment, especially from aerial observation.
Such clearing should generally be limited to underbrush and
lower branches of trees.
c. A large town or city containing masonry buildings may
be very strongly organized for defense; however, provision
must be made for protection against heavy shell fire and
aerial bombs, and against gas because of the excellent target
provided. Buildings provide concealment and protection for
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ENGINEER FIELD MANUAL
riflemen and machine guns, cellars and heavy walls provide
shelter which can be reinforced against artillery, and higher
buildings afford means of observation. Thus an excellent
combination of fire and obstacles may be obtained. Because
they are easily set on fire wooden buildings are of little de¬
fensive value. Small villages or isolated buildings draw con¬
centrated artillery fire and should therefore be avoided.
Section II
TERRAIN APPRECIATION
■ 8. General. — a. Terrain is an area of ground considered as
to its extent and topography in relation to its use for a par¬
ticular military operation. From either direct observation or
study of maps and aerial photographs the eye gets a picture
of the ground with its drainage systems, commanding eleva¬
tions, wooded and open areas, and works of man. Terrain
appreciation is the evaluation of this picture to determine
effect of the terrain on lines of action open to opposing forces
in the area.
b. By reason of his specialized training the unit engineer
is the terrain specialist on the unit commander’s staff. As
such he must understand effect of terrain upon military
operations and must be prepared to make recommendations
thereon at all times, particularly in defensive operations.
The purpose of this section is to describe means of evaluating
and to show effects of terrain on tactical dispositions.
■ 9. Factors. — a. Terrain is always evaluated in terms of five
factors: observation, fields of fire, cover and concealment,
obstacles, and communications.
(1) Observation of the battle area is necessary for effective
combat. Points of commanding elevation form the frame¬
work for fire-control systems, and observation is essential to
have effective fire. It is of great importance to artillery in
fire adjustment, and is important not only because it makes
effective fire possible, but also because it enters largely into
influence of the four other terrain factors. Fields of fire
obviously need local observation for effective use; value of
cover is based on denial of observation to the enemy; obstacles
should be observed so that fire may be brought to bear on
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FIELD FORTIFICATIONS
9
any enemy stopped by them; and routes of communication
are the more useful the more the enemy can be denied obser¬
vation thereof. Hence, observation and denial of observation
are used as the basis for the method of studying terrain de¬
scribed in paragraphs 1&-12 below.
(2) Fields of fire are essential to defense. An ideal situa¬
tion exists where terrain offers an open stretch of ground
over which the advancing enemy may be brought under
effective fire of infantry weapons. Fields of Are may be im¬
proved by cutting or burning weeds, grass, and crops; by
clearing brush and trees; by demolishing buildings; and by
cutting lanes through woods, but concealment must be con¬
sidered in each instance. Time and labor available for sucii
improvement should be considered in evaluating terrain.
(3) Cover and concealment includes protection from fire
provided by accidents of terrain, and that provided by other
natural or artificial means. Concealment from view, from
both air and ground, affords cover only so long as the enemy
does not know that the natural or artificial features are
occupied. Cover is important to both attack and defense.
(4) Obstacles aid defense by hindering the attacker’s
movements, preferably by halting him under effective fire.
Rivers, streams, ponds, gullies and steep banks, and lakes are
some of the common natural obstacles which may be utilized
to advantage. A stream with a depth over five feet is par¬
ticularly effective against mechanized vehicles. No oppor¬
tunity for providing obstacles to mechanized attack should
be overlooked. Where natural obstacles are lacking artificial
ones usually can and should be prepared.
(5) Communications (that is, roads, railroads, waterways
and airway facilities) are important to both offense and
defense. Small bodies of troops may move off roads, but in
some situations, especially in operations of large bodies of
troops, the means of communication may be of vital im¬
portance.
b. Every line of tactical action in a given situation is an¬
alyzed with respect to pertinent terrain features. Advantages
and disadvantages of each such feature upon any particular
line of action constitute as a whole the relative effect of ter¬
rain in comparison with its effect on other possible lines of
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engineer field manual
action. Modification of the situation may completely change
influence of terrain features and so affect plan of action which
should be adopted. Therefore, effect of terrain must always
be studied in connection with the mission and the situation
of the enemy and of our own forces.
■ 10. Compartments. — a. Definition. —Based on observation
as the decisive factor, a basic unit called “terrain compart¬
ment” is considered in studying terrain. A terrain compart¬
ment is an area inclosed on at least two sides (opposite sides )
bp critical terrain features which prevent ground observation
into the area. The limiting features are usually ridges or
high ground, but may be woods, cities, towns, or wide bodies
of water.
b. Form. —Compartments may be of any size or shape.
They may be simple or complex with the interior subdivided
into smaller compartments. Limiting features at the edges
may be high or low, continuous or discontinuous. A compart¬
ment may be screened on only two sides or on all sides. The
illustrative examples in this section are based on the simplest
forms but the fundamentals discussed apply to all types.
(1) A terrain compartment in its simplest form is shown in
figure 1©. The figure shows three ridges, ABC, DEF, GHI,
separated by two valleys, JK and LM. Based on the defini¬
tion, the area ACFD is a terrain compartment. The bound¬
aries are selected so that from outside them no portion of
the compartment can be observed. Therefore the boundary
runs along the topographic crest of the adjacent ridges.
(2) In figure 1 ® boundaries of the compartments are
ridges. In figure 1 © there are no ridges but there are strips
of woods along the general lines AC and DF, and a town along
GI. They form two terrain compartments since the woods
and town limit observation just as effectively as the ridges.
Actual boundaries of the compartments, that is, the limiting
lines of observation, lie not at the edge of the woods or town
but far enough back from the edge to shut off observation
over the open ground beyond.
c. Significance. —<T) While division of terrain into com¬
partments is based upon observation, the real significance is
that from points outside their boundaries it is difficult to
bring effective fire to bear upon forces within the compart-
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10
©Formed by woods and town.
Figure 1. —Terrain compartments.
.9
DIRECTION
OF ATTACK
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ENGINEER field manual
10
FIELD FORTIFICATIONS
10
ments. In figure 1 © and © machine guns at P, Q, R, or S
can flre upon any forces within compartment ACFD, but
they cannot fire effectively with direct laying upon any forces
in the adjoining compartment DFIG, because machine gun
personnel cannot see such forces. For the same reason, ar¬
tillery observers at T or U can bring observed fire to bear
upon any forces within compartment ACFD, but not upon
forces in adjoining compartment DFIG. Similarly, machine
gunners and artillery observers within DFIG are obliged to
confine their observed fire to forces within that compartment
since they cannot see beyond it. Hence the fundamental
characteristic of terrain compartments is that they limit
observation and thereby decrease possibility of observed fires
from outside compartments.
(2) Figure 2 represents the same sized area as figure 1 ©
but instead of the two stream valleys and three ridges of
figure 1 © there is a fairly uniform slope from the line ADG
downward to the river. The contours are nearly straight,
parallel, and at about equal intervals. No longer Is the fire
power within the area ACIG divided into two groups by
terrain compartments. With terrain conditions as shown in
figure 2 there are no compartments and all of the fire power
within the area ACIG can be brought to bear upon any part
of the area. Machine gunners at P, Q, R, and S and artillery
observers at T and U can observe and hence place their fire
anywhere within the area ACIG. Therefore a better organ¬
ized, more flexible system of fire is possible where there are
no terrain compartments than is possible where they break
fire power up into groups to support each other. This is the
fundamental idea underlying influence of terrain compart¬
ments upon tactical operations.
(3) A common terrain condition is that shown in figure 3.
A main ridge, the top of which is along the line AB, is cut
by the gullies IJ, KL, and MN, leaving ridges CD, EF, and
GH. These ridges form the boundaries of two compartments,
CEFD and EGHF, ending at about the line CM. Above that
line a defender can organize his fires for mutual support
across the entire area ABMC; below the line CM he is lim¬
ited in the lateral organization of his fire by the compart¬
ments CEFD and EGHF. These compartments are subdivi-
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ENGINEER FIELD MANUAL
sions of a larger compartment, ABND, bounded by the ridge
AB and the woods DN.
d. Classification. —(1) Terrain compartments are classified
with respect to direction of movement of forces operating
therein as—
(a) Corridors, when the longer dimension lies generally in
the direction of movement or leads toward an objective.
(b) Cross compartments, when the long axis of the com¬
partment lies across the direction in which a force is moving,
or is parallel to the front.
(c) Oblique compartments, when the compartment is
oblique to such direction or front.
(2) To illustrate:
(a) In figure 1 © a force advancing westward from the
vicinity of the point K calls compartment ACFD a corridor.
A force advancing north from the valley LM would call the
same compartment ACFD a cross compartment. These two
terms, corridor and cross compartment, do not define com¬
partments that differ in a physical sense; the compartment
is the same in both cases, namely, the area ACFD. The
difference lies in the direction of the main axis of attack and
defense with respect to the main axis of the compartment.
If attack comes from east or west, both attacker and de¬
fender would call ACFD a terrain corridor; if attack comes
from north or south, both attacker and defender would call
ACFD a cross compartment.
(b) In figure 3, with the direction of attack as shown,
CEFD and EGHF are corridors within the cross compartment
ABND.
■ 11. Influence of Corridors. — a. In attack. —(1) In general,
a corridor favors attack because it limits lateral organiza¬
tion of the defender's fire. This does not mean that it
gives the attacker an advantage over the defender; it merely
means that it gives the attacker an advantage that he would
not have If there were no terrain corridors since he will be
subjected to a lesser density of fire than otherwise would be
the case. Therefore, the attacker seeks to utilize corridors
wherever they exist.
(2) Where terrain is the decisive factor, boundaries be¬
tween tactical units in attack should coincide with boundaries
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FIELD FORTIFICATIONS
11
of corridors. The reasons for this fundamental are illustrated
by the following example:
(a) In figure 4 both ® and © show two corridors, HILK
and IJML, each assumed to be a suitable width for a regi¬
ment in attack. In © the boundaries between attacking
regiments are located along centers of the corridors; in ©
the boundaries between attacking regiments coincide with
the boundaries of the corridors.
(b) In figure 4 ® the 1st Infantry has attacked and reached
the dotted line near the center of the figure. As the attack
spearhead near D appears to be the major threat, enemy ma¬
chine guns on the slope BC and along the edge of the woods
between F and J begin firing on the flanks of the 1st In¬
fantry; an artillery observer on the slope to the right of H
places fire on the left flank of the 1st Infantry. These fires,
in addition to those from within the zone of action of the
1st Infantry, may prove sufficient to stop the attack. The
regiments attacking on the right and left of the 1st Infantry
will cooperate, but if one of them succeeds in making a
threatening advance, the defender can shift some of the fire
from the zone of the 1st Infantry to aid in stopping it.
(c) In figure 4 © the 1st Infantry has again advanced to the
dotted line. However, the situation now differs materially
from that shown in figure 4 ©. Because the boundaries of
the 1st Infantry coincide with the boundaries of the corridor
HILK, the defense cannot bring to bear against the 1st In¬
fantry any direct or observed Are from outside the corridor.
As long as the 1st Infantry can continue to overcome the de¬
fensive fires within its own zone of action, that is, within its
own corridor, it can continue to advance, and by flanking
attacks to right and left it can materially aid advance of its
neighbors.
(3) Boundaries in attack usually extend at least from the
line of departure to the terrain features designated as the
objective or objectives.
b. In defense. —(1) The backbone of defense is organized
fire of automatic weapons supplemented by observed artillery
fire. Corridors entering a defensive portion break it up so
that fire power used to defend one corridor cannot be used
to assist in defending any other part of the position, There-
262375'
13
11
engineer field manual
fore the defender seeks a position' without corridors because
they weaken his defense by decreasing flexibility of his fire.
(2) In general, from the standpoint of the most advan¬
tageous use of terrain, boundaries between units in defense
should neither follow the edges of corridors nor the lines of
probable enemy approach, but should be situated somewhere
between the two in such a manner as to secure unity of com¬
mand along the most dangerous avenue of approach. The
reasons for this are illustrated by the following example:
(a) Figure 5 ®, ©, and ©, shows three different methods
of assigning boundaries, the terrain being the same in all
figures, Figure 5 © shows sector boundaries coinciding with
® Boundaries in centers of cor- (?) Boundaries at edges of cor¬
ridors (wrong). rldors (right).
Figure 4.—Corridors in attack,
i
boundaries of corridors as is proper in attack. Figure 5 ©
shows sector boundaries following lines of probable enemy
approach within the corridors. Figure 5 © shows sector
boundaries placed between those shown in © and ©. In
each case it is assumed that the boundaries mark off frontages
which can be defended by an infantry battalion. Proper
defense of the area requires that the ridges through A, B,
and C he organized for strong all around defense; the valleys
between the ridges being defended by flanking fire from
the ridges and by frontal fire from the rear portions of the
position. Location of sector boundaries is important because
14
FIELD FORTIFICATIONS
11
these boundaries allocate responsibility among the unit com¬
manders concerned.
(b) In figure 5 ® the sector boundaries pass through the
ridges at A, B, and C, which should be well organized. Two
commanders, each defending half of the ridge BO, cannot
® Boundaries along ridges ® Boundaries along valleys
(wrong). (wrong).
® Boundaries between ridges ® Boundaries excluding routes
and valleys (right). of approach (wrong).
Figtjke 5. —Corridors in defense.
do it as effectively as one commander who directs the defense
of the whole ridge. The same thing applies to the ridges at
A and C. It is true that the valley between A and B which
is a probable avenue of approach for the attacker is entirely
under control of the commander of the 1st Battalion, but his
15
u
ENGINEER FIELD MANUAL
main reliance for defense is not upon the head-on opposi¬
tion that he can offer to the attacker’s approach up the
valley but on the flanking fire made possible by continued
possession of the organized localities at A and B.
(c) In figure 5 © the ridges at A, B, and C are each placed
under control of a single commander by establishing bound¬
aries in the stream beds, that is, along the lines that the
attacker will probably seek to penetrate. Thus one source of
weakness, namely, division of responsibility for maintaining
possession of high ground, has been eliminated but another
source of weakness has been introduced. The valley is the
probable route of approach of the attacker; he will seek to
penetrate the valley and then reduce the high ground by
flanking attacks. Putting the boundaries in the valleys di¬
vides the responsibility for stopping the enemy advance up
the valleys. One commander can accomplish it more effec¬
tively than two. Unity of effort requires giving this respon¬
sibility to one commander.
(d) In figure 5 © the boundaries are drawn neither along
the ridges nor along the stream beds, but somewhere between
the two. The commander of the battalion defending ridge Bo
in this case has full control both of forces holding the ridge
at B and of those opposing advance of the enemy up the
valley between A and B. Cooperation will still be necessary;
he will expect to receive assistance from the battalions on
his right and left. But the boundaries along which this co¬
operation must be arranged do not in this case pass through
either of the two important terrain features, the ridge and
the valley adjoining it. A boundary line is always a line of
weakness, but by drawing the boundaries as they are shown in
© this weakness is much less serious to the defense than it
is if the boundaries are drawn as shown in © or ©.
(e) Figure 5 © shows the same terrain as ©, ©, and ©
but with the sector boundaries located to the right of the
streams instead of the left. This alternate arrangement of
boundaries appears at first glance to be as satisfactory as that
shown in © since the boundaries divide neither the ridges
nor the valleys between two commanders. The wooded val¬
ley between A and B, however, is more of a threat to the
battalion defending the ridge through B. It offers opportunity
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PIELP FORTIFICATIONS
11
for an attacker to advance up the valley by using the woods
for cover, capture the woods on the top of the ridge near O,
and then capture the entire ridge from O to B by attacking
downhill from the rear. The commander of the battalion
defending BO therefore has the greater interest in defense of
this approach, and its defense should consequently be as¬
signed to him by drawing the boundaries as they are shown
in ® rather than as shown in @.
(3) The preceding considerations apply particularly to
those portions of the boundaries within the position and at the
shorter ranges to the front. It may be found that their con¬
tinuation to the front at the longer infantry ranges involves
use of terrain differing from that in the close-in defense. For
example, in figure 6 the boundary AB is properly located for
close-in defense. In extending this boundary to the front in
order to place responsibility for longer range fire it would,
following the same considerations that led to establishment
of boundary AB, extend forward through F, G, and H. Then
enemy machine guns at X and Y are in the sector of the 2d
Infantry and should be covered by the Are of that unit. How¬
ever, the defilade provided by the ridge CDE prevents the 2d
Infantry from firing on X and Y, and also prevents guns at X
and Y from firing into the sector of the 2d Infantry. The
guns at X and Y can fire into the sector of the 1st Infantry,
and can be covered by fire from the 1st Infantry. If the
boundary is shifted to CDE instead of FGH, responsibility for
taking care of these machine guns is placed where it belongs,
namely, on the 1st Infantry. The ridge CDE is utilized prop¬
erly from the standpoint of terrain when the boundary line
between the 1st Infantry and 2d Infantry is drawn along the
top of the ridge, thus giving to the 1st Infantry the east slope
in which it has a paramount interest, and to the 2d Infantry
the west slope in which it has a paramount interest.
(4) The same considerations apply when boundaries of
terrain compartments are formed by villages or woods instead
of by ridges. In figure 7 the village and woods are important
features of the terrain. Responsibility for their defense
should therefore not be divided as it is in © but should be
placed under a single commander as shown in ©. (An at-
17
IX
engineer field manual
tacker would locate his boundaries as shown in ®, that is,
along edges of the corridor formed by the village and woods.)
c. Extension of boundaries .—At longer ranges to the front,
boundaries are intended primarily to coordinate artillery fires
and will be influenced by location of possible hostile assembly
areas and routes of approach. Such boundaries extending
sectors to the limits of artillery fire in front of defensive posi¬
tions frequently are placed along the crests of ridges. The
extension of boundaries to the rear in both attack and de¬
fense is influenced largely by the location of routes of com¬
munication essential to supply and movement within the
sector.
18
DIRECTION OF ATTACK
FIELD FORTIFICATIONS
11
® Wrong. ® Right.
Figure 7.—Corridors between Woods and villages in defense.
Figure 8-—Cross compartment.
19
12-13
ENGINEER field MANUAL
■ 12. Influence of Cross Compartments.— a. On defense .—
In figure 8 the ridge AB is one boundary of cross compart¬
ment ABDC. The observation available to the defender from
this boundary is very valuable; consequently he should dispose
his forces well to the front of it in order that local successes
by the attacker may not result in loss of so valuable an
asset. The cross compartment as a whole is a valuable asset
to defense because mutually supporting fires can be organ¬
ized across its entire length from AC to BD. Until the de¬
fender is actually driven from the line AB he retains his
observation over the entire area. In general therefore a
cross compartment favors the defense and handicaps the
attack.
b. On attack. —Boundaries' of cross compartments form
natural objectives for attack or limited objectives along the
line of attack. Until the entire area ABDC has been cap¬
tured and the defender driven beyond the ridge AB, the
attack or at any rate this particular phase is not complete.
When the ridge has been taken the attacker may have, a
breathing spell in which to reorganize his forces, move his
observers forward to points from which they can see into
the adjoining cross compartment, and make his plans for
the next phase of the attack which will be capture of the
next cross compartment in its entirety.
c. Unit boundaries .—Prom a terrain standpoint, the bound¬
aries between attacking units may be located with equal
effect anywhere between AC and BD. There will of course
be other factors such as known: disposition of defending
forces that will influence location of boundaries between
attacking units, but the terrain Is so uniform over this par¬
ticular area that its influence on boundaries is negligible.
The boundaries between sectors in the defense can also be
placed anywhere, as unit commanders can cooperate along
one line just as well as along another. In other words, there
are no terrain features that limit lateral visibility so location
of boundaries between units would be determined by factors
other than terrain.
■ 13. Aids to Study of Terrain.— a. Drainage lines and
ridge lines form the natural basis for the study of terrain
with respect to shape of the ground, When such study of
20
FIELD FORTIFICATIONS
13-14
ground forms is made on a map or aerial photograph it can
be aided materially by—
(1) Emphasizing drainage lines by marking heavily (fig.
9©).
(2) Drawing in heavy lines along crests of ridges, called
“ridge-lining” (fig. 9 © ).
(3) (On contoured maps.) Emphasizing certain contours
With heavy lines (fig. 9 ©> or coloring map areas between
contours with different colors to make ground forms and com¬
manding elevations more apparent. (See TM 5-220.)
b. Drainage lines always form a connected system or sys¬
tems of branching lines. Ridge lines form similar systems
of branching lines since spurs and the smaller ridges branch
off from large main ridges just as small streams and gullies
branch off from the main streams. Drainage lines and ridge
lines thus form two interlocking, branching systems which
either singly or together indicate clearly the general shape of
the ground. When both systems are emphasized on a map or
photo, different colors should be used, preferably blue for
drainage lines and brown for ridge lines to conform to usual
map colors. The more important ridge or drainage lines may
be given special emphasis by drawing them in heavier lines.
c. It will occur frequently that ridge lines and drainage
systems are not the only terrain features of outstanding im¬
portance in a tactical situation. There may be forests, towns,
railroads, etc., to which particular attention must be paid.
In such cases these features may be emphasized in much
the same ways as described above for ridge lines and drainage
systems.
d. Further details regarding tactical study of terrain, to¬
gether with a form therefor, are given in FM 101-5.
Section m
ORGANIZATION OP THE GROUND
■ 14. General. — a. Organization of the ground is the devel¬
opment of a defensive position to its full strength by—
(1) Providing fields of fire for all flat trajectory infantry
weapons.
21
® Ridge-lining.
FIELD FORTIFICATIONS
14-15
(2) Constructing field fortifications, including such camou¬
flage thereof as is practicable.
b- A defensive position, properly organized, consists of
a system of mutually supporting defensive areas organized
in depth. Depending on the time, materials, and labor avail¬
able, either hasty or deliberate fortifications may be used.
However, the fundamentals of location and layout of the
defensive areas, and the limitations as to their frontages and
the intervals between them apply whether hasty or deliberate
fortifications are used.
c. Drawings in this section are diagrammatic only. Actual
dispositions of units must be adapted to terrain.
d. The types of trenches, wire entanglements, etc., referred
to in this section are described in detail in sections V, VI
and VII.
* 15. Use of Hasty Fortifications. — a. In organizing the
ground, infantry troops normally will work either under fire
from the enemy of under threat of such fire being delivered
Within a few hours of starting work. HenCe work done under
these conditions should be such as to provide the greatest
possible defensive strength for the position in the shortest
possible time. To accomplish this, hasty fortification works
of the following types are used:
(1) Skirmisher trench. —See paragraph 34 b and figure 22.
(2) Foxhole. —See paragraph 34 c- and figure 23.
(3) Shell hole position.—See paragraph 34 d and figure 24.
(4) Slit trenches. —See paragraph 34 e and figure 25.
(5) Shallow connecting trench. —See paragraph 34 / and
figure 27.
(6) Machine-gun emplacements. —See paragraph 48 A and
figures 89, 90, and 91.
(7) 37-mm gun emplacements. —See paragraph 48 d and
figures 92 and 93.
(8) 60-mm and 81-mm mortar emplacements. —See para¬
graph 48 d and figures 94 and 95.
(9) Clearing fields of fire. —This consists usually of clear¬
ing underbrush and small trees and the lower limbs of large
trees to a depth of at least 100 yards in front of the position,
with lanes for machine-gun fire extending considerably far¬
ther (see par. 35d).
23
15
ENGINEER FIELD MANUAL
(10) Obstacles .—Barbed wire entanglements form the
major obstacles against foot troops. The double-apron fence
(see par. 42 c (4) and figs. 75 and 76) is the most efficient
type, but where time is short or materials lacking, the four-
strand fence (see par. 42e (6) and fig. 80) may be used.
Provision of antitank obstacles will ordinarily be limited to
improvement of natural obstacles as streams, ditches, etc.,'
and laying antitank mine fields.
(11) Camouflage .—Camouflage to be effective must be
executed simultaneously with the defensive works. Spoil
should be properly disposed of as soon as dug; parapets
should be sodded as fast as they are finished; emplacements
i
V
Figure 10.—Rifle platoon in lox holes with connecting trenches
(12-man squads).
should be concealed under fishnets or natural materials from
the start, and paths pointing to defensive works should be
avoided (see PM 5-20).
b. Clearing fields of fire is of first priority; however, in
most instances a small detail can complete the clearing
while some of the men dig fox holes, others prepare emplace¬
ments, etc. It usually will be advisable to start placing obsta¬
cles early in the work. Thus an enemy attack will encounter
a fairly well-organized position. As more time becomes avail¬
able, the initial hasty fortifications are gradually developed
24
FIELD FORTIFICATIONS
15-17
into deliberate fortifications. Fox holes and shallow connect¬
ing trenches are converted into standard fire and communi¬
cation trenches; open emplacements are changed to covered
types; existing obstacles are strengthened and new obstacles
added; protected shelters for personnel are provided, etc.
H 16. Use of Deliberate Fortifications. — a. In the case of
defensive positions organized in rear areas out of contact
with the enemy, the fortifications constructed normally will
be' of the deliberate type with no attempt being made first
to provide hasty fortifications and then develop these into
deliberate fortifications. The initial work will consist at
least of standard fire trenches, open-standing or splinterproof
weapon emplacements, complete double-apron entanglements,
adequate antitank obstacles, and clearing extensive fields
of fire.
b. Description of various defensive areas given in the fol¬
lowing paragraphs is based on use of deliberate fortifications
in a rear position. The work indicated is the minimum con¬
sistent with an adequate defense. These descriptions may
be applied to defensive areas utilizing hasty fortifications by
allowing for the fact that fox holes will be used in place of
fire trenches, that the belts of obstacles will be comparatively
incomplete, and that weapon emplacements will all be of
the open shallow or standing types.
■ 17. Battle Position. — a . Definition. — In defense of a posi¬
tion, troops are distributed along the front within a zone
which is called the battle position. This is the position of
principal resistance in defense consisting of a system of
mutually supporting defensive sectors and areas disposed in
breadth and depth. Each sector and area has a definite
assignment of troops and a definite mission.
b. Organisation. —(1) Depth .—A battle position should be
organized in depth for two reasons:
(a) A single line of defense could be penetrated easily and
the attacker would then be able to destroy vital installations
in rear on which defending troops depend in order to fight.
Instead of a single line the defense must present a series
of resisting areas which the attacker will have to penetrate
successively with increasing resistance before he can reach
these installations.
25
17
ENGINEER FIELD MANUAL
(b) Depth in the battle position makes possible dispersion
of the defending force so that the enemy cannot cover all
parts of the position with fire at one time.
. (2) Occupation .—A battle position cannot be occupied
economically with equal density throughout its length. One
of the major advantages of defense is that it permits holding
extensive fronts with relatively weak forces. This is accom¬
plished by actually occupying only certain portions of the
front and covering gaps between occupied portions with fire.
Occupied portions are called defensive areas. They are
placed so that they can support each other with fire so that—
(a) Frontal fire from any one defensive area can be rein¬
forced by flanking fire from neighboring areas.
<b) If a defensive area is occupied by the enemy, fire can
be directed into it from the neighboring areas.
c. Influence of terrain .—(1) Factors .—Location and layout
of a battle position depend largely upon the tactieal situa¬
tion and upon the terrain. The influence of terrain may be
summarized under headings corresponding to the five terrain
factors (See par. 9) as follows:
(a) Observation .—Since effectiveness of defensive fire de¬
pends primarily on observation, the defense selects ground
which affords good observation. The battle position should
then be located so as to protect that observation.
(bl Fields of fire .—Existing and available fields of fire
determine the strong areas and weak points and thus dictate
where to put troops and in what strength.
(c) Cover .—All elements Of a battle position must use cover
for protection against enemy fire and observation as an
essential means of conserving the fighting capacity of troops
in prolonged occupation of a position, and to attain effect
of surprise through concealment of location of principal
works and reserves. Each element of a battle position should
take advantage of natural cover wherever possible in order
to reduce the need for constructing artificial cover.
id) Obstacles .—A battle position should be located so as
to take advantage of natural obstacles particularly against
mechanized attack to help stop the enemy, thus reducing need
for artificial obstacles. Not infrequently the commander
selecting the main line Of resistance may have to decide
26
FIELD FORTIFICATIONS
17
whether a natural obstacle or observation is the most im¬
portant consideration.
(e) Routes of communication. —A battle position should be
located so as to use existing roads, railroads, and other routes
of communication as far as possible to meet its supply and
evacuation requirements, and thus reduce need for new con¬
struction.
(2) Critical features. —Whether it is a point affording com¬
manding observation or whether some other element of ter¬
rain, there may be some terrain feature whose possession by
us or denial to the enemy will result directly or indirectly in
accomplishment of an important step in the plan of opera¬
tions. Such a feature is termed a critical point. For ex¬
ample, a critical point may be a feature which is especially
important for subsequent parts of the maneuver, or it may
be a sensitive point the capture of which by the enemy may
lead to breaking up our whole defense.
(3) Features vital to success of mission. .—If a critical point
is of such outstanding importance that its possession by us
or denial to the enemy will result in accomplishment of the
mission or its loss in failure of the mission, then it may be
said to be vital or essential to success of the mission. Such a
tactical locality is termed a key point. The importance of
such a feature may result from its having been named in the
mission or in orders, or the importance may be deduced from a
study of the terrain. On the other hand, there may be no
such feature. When there is such, it is frequently a logical
objective of the enemy’s main attack either directly or by
outflanking, or it may be the terrain feature which is the
object of the main effort of the defense.
d. Composition. —A battle position is subdivided into de¬
fensive areas in such a way as to conform to subdivision of
the defending force into tactical units. This permits each
defensive area to be occupied by a single tactical unit of
appropriate size, and provides for unified control of groups
of defensive areas through the normal chain of command
(see fig. 11).
Cl) The platoon defense area is usually the smallest de¬
fensive area. The company defense area, the next largest
defensive area, is a group of platoon or smaller defense areas
27
17
ENGINEER FIELD MANUAL
under control of the company. A battalion defense area is a
defensive area composed of a group of company defense areas
under control of the battalion commander. A regimental
defensive area is a group of battalion defense areas under
regimental control, and is called a regimental sector.
(2) A line at the forward boundary of the battle position
designated to coordinate the defensive fires of all units and
supporting weapons is called the main line of resistance
(MLR) (see fig. 11). In rear of company defense areas
on or near the main line of resistance may be company
defense areas forming battalion reserves. A line designated
to coordinate the locations and actions of the regimental
reserves in the battle position is term the regimental reserve
line (RKL).
(3) The defense areas assigned units in defense will vary
with mission of the command, natural defensive strength
of the ground, importance of the sector to the defense as a
28
FIELD FORTIFICATIONS
17-18
whole, degree of control required, and number and strength
of units available for the whole defense.
■ 18. Platoon Defense Area. — a. A platoon defense area is
organized for all around defense. It may consist of one Are
trench for each squad in the area and a belt of obstacles,
usually wire entanglements, entirely surrounding the area.
It is not normally organized in depth, the squad trenches
being roughly in line rather than one in rear of the other
(see fig. 12).
b. The frontage which can be defended depends on many
factors including mission, size of the unit assigned thereto,
and terrain. See Table I which is furnished as a general
guide only.
Table I. — Frontage (in yards )
Minimum (heavily wooded |
terrain)
Maximum (flat, open
| terrain)
Size of unit defense l
areas
Interval
between
defense
areas
Frontage
actually
occupied
Total
: front
defended
Interval
between
defense
areas
Frontage
actually
occupied
Total
front
defended
1 squad (12 men)-
25
30
55
100 j
50
150
Platoon, less 1
squad ( 2 squads)..
50
75
125
150 .
100
250
Full platoon (3
1
squads)_
100
100
200
200
200
400
c. Each defense area should be able to cover by fire its own
front, the fronts of adjacent defense areas, and the unoccu¬
pied intervals between them. Hence, squad trenches on the
flanks of a platoon defense area may be faced partly toward
adjacent areas to facilitate such flanking fire. Each squad
trench should provide at least 5 lineal feet per man. The
simple standing fire trench, preferably with the octagonal
trace, represents minimum protection to be provided by
squad trenches when time and conditions permit.
d. The purpose of the belt of obstacles surrounding a
platoon defense area is to prevent the trenches being rushed
or surprised by foot troops. Such an obstacle must be close
262375'
29
18-19
ENGINEER FIELD MANUAL
enough to the trenches to be under observation and fire
at all times, especially at night, but must not be so close
as to permit an enemy to approach within hand grenade
range of the trenches. A distance of from 30 to 100 yards
from the trenches generally will be suitable. The obstacles
should at a minimum consist of the four-strand fence or
its equivalent. When the interval between defensive areas is
small the obstacles on the flanks will be less than 30 yards
distant from the trenches. Obstacles should be concealed
so that the enemy is surprised by them.
e. Squad trenches should be provided with clear fields of
fire throughout the width of their assigned sectors at least
as far forward as the belt of obstacles and preferably to a
greater distance.
S 19. Company Defense Area. — a. The platoon defense areas
of a company defense area are located with a view to resist¬
ance to the front and flanks and if necessary to the rear. A
company defense is normally organized for a protracted all
around defense.
i>. A company can defend a front of from 400 to 600 yards,
depending on terrain. The portion of this front which is
actually occupied is generally not less than 200 yards in
width by 100 yards in depth in order to minimize losses from
enemy fire, and not greater than 400 yards in width by 300
yards in depth in order to avoid undue dispersion.
e. Supporting weapons. —(1) Light machine guns are
placed within platoon defense areas in accordance with the
battalion plan of fire (see par. 20 e.) The guns may be sited
individually for employment on separate missions. They
should be provided with open standing type emplacements
carefully concealed, with alternate emplacement for each
gun from which its primary Are mission can be accomplished.
(2) The 60-mm mortars may be located in a single group
within a company defense area far enough forward to have
observed fire and at a point where they have natural defilade
from the front, or they may be placed in platoon defense
areas in accordance with the battalion plan of fire. Where
no natural defilade exists, they should be put in open em¬
placements and provided with an approach trench for am¬
munition supply. In any case they should be concealed. It
30
FIELD FORTIFICATIONS
19-20
is not essential that they be within a platoon defensive area
but they should either have front and flank protection by
fire from nearby platoon defensive areas or should be sur¬
rounded by obstacles.
d. The belts of obstacles around the platoon defensive
areas are connected so as to form a belt of obstacles com¬
pletely around the company defensive area effective against
foot troops and located so that it can be covered by fire
throughout, particularly by flanking Are from the light
machine guns.
e. The company command post should be located in a
sheltered position such as a ditch or ravine providing pro¬
tection from small-arms fire and concealment from view.
It must be easily accessible to all elements of the company and
to battalion headquarters and is therefore generally located
in the rear part of the defensive area. Both the command
post proper and all routes of approach thereto should be
concealed.
/. The company observation post should be located near
the command post and should afford a view of all or the
greater portion of the defensive area and the ground in
front thereof. Both the observation post and the approach
thereto should be concealed.
■ 20. Battalion Defense Area. — a. The company defense
areas in a battalion defense area are usually disposed laterally
and in depth.
b. The rifle battalion ordinarily is the smallest unit whose
defensive plan involves all infantry weapons and supporting
artillery fire. Therefore in determining the frontage which
a large command can defend effectively, the rifle battalion
becomes the unit of measure. A battalion organizing a front¬
line defense area can defend the following frontages:
(1) For defense of vital tactical locality with limited ob¬
servation and fields of fire such as in heavily wooded terrain,
not to exceed 800 yards.
(2) For defense of vital tactical locality on average terrain,
not to exceed 1,500 yards.
c. A front-line battalion usually will locate two companies
on the main line of resistance and one company in battalion
reserve (see fig. 12),
31
20
ENGINEER FIELD MANUAL
(1) Companies on the main line of resistance organize
company defense areas capable of mutual support.
(2) The function of the battalion reserve is to expel the
enemy by Are and movement from any portion of the bat¬
talion area which may have been occupied by the enemy.
Should the tactical situation prevent counterattack, the re¬
serve must stop or delay further advance of the enemy and
must accordingly be prepared for defense to the front, flanks,
and rear.
d. A battalion defense area on the regimental reserve line
ordinarily must cover the combined frontages of two battalion
areas on the main line of resistance. A battalion in regi¬
mental reserve usually prepares positions for its companies
abreast along the regimental reserve line; any organization
in depth of the regimental reserve position must be accom¬
plished by the companies in their dispositions of rifle platoons,
light machine guns, and 60-mm mortars.
e. Supporting weapons .—A battalion plan is drawn up for
use of all supporting weapons.
(1) Heavy machine guns. —(a) The heavy machine guns
are disposed in width and depth throughout the battalion
defense area. Their fire forms the framework with which all
other fire directed against foot troops is coordinated, other
weapons being used to fill in any gaps in such fire. They
should be able to cover the front of the position with con¬
tinuous interlocking bands of grazing fire, and to cover the
most likely avenues of enemy approach "with enfilading fire.
Other weapons are used to fill in any gaps in fire of machine
guns. The primary mission for each gun normally includes
placing an extended band of grazing fire along the wire or
other obstacle on the front or flank of a defensive area, such
fire to be delivered under any conditions of visibility. The line
along which this fire is delivered is called the final protective
line.
(b) It is necessary to provide depth to machine-gun de¬
fense so that the guns will be in position both to prevent
enemy penetration to the reaT part of the position and. to
support counterattacks.
(c) Heavy machine guns are sited in pairs, the guns of
each pair having identical sectors of Are. They are located
32
FIELD FORTIFICATIONS
20
either within or under protection of platoon defense areas.
Each gun should have primary and alternate emplacements
from which their primary mission can be accomplished, and
a supplementary emplacement from which a secondary mis¬
sion can be accomplished. The guns In a pair should be
spaced 20 to 50 yards apart, close enough for control by
one man but far enough apart so that one shell burst can¬
not put both guns out of action. Primary and alternate
emplacements should be a* least 50 yards apart so that fire
directed on one location will not through natural dispersion
cover both. All emplacements should provide splinterproof
shelter and should have covered routes of approach or
approach trenches for supply and communication.
(2) Mortars .—The 81-mm mortars are assigned a posi¬
tion area within the battalion defense area far enough to
the rear of the main line of resistance so that they can
direct fire into platoon defense areas on that line without
displacing to the rear. Within the position area each mor¬
tar is provided with primary and alternate emplacements
so located that from them fire can be directed on any gaps
in fire of the flat trajectory weapons. Each mortar requires
an observation post and a covered route of approach. All
emplacements should be the open standing type carefully
concealed from air and ground observation. They should
preferably be located in defiladed positions.
(3) Antitank guns .—The mission of the battalion anti¬
tank guns is to provide protection for the battalion sector
from tank attacks. The guns should therefore be located so
as to cover probable routes of approach for tanks. They
should be far enough forward to engage a tank attack be¬
fore it reaches the main line of resistance, but in any case
should not be forward of the front line of platoon defense
areas. The guns are distributed laterally rather than in
depth, and where possible should be able to fire in support
of each other. They should be provided with primary, alter¬
nate, and supplementary positions, all of the open stand¬
ing type, well concealed. As the guns operate by direct
observed flre only, all emplacements must be located and
constructed to permit this.
33
20
ENGINEER FIELD MANUAL
f. A battalion defense area is organized for all around
defense against both foot troops and tanks.
(1) Belts of obstacles surrounding the company defense
areas are connected so as to form a continuous belt effective
against foot troops around the battalion defense area located
so that all points can be covered by fire. Particular atten¬
tion must be paid to siting of obstacles so they can be
enfiladed by fire of heavy machine guns. Wire entangle¬
ment placed primarily for this purpose is called tactical
wire, as distinguished from wire entanglement used primarily
to protect a platoon defense area from being rushed, which
is called protective wire. Wherever possible, both functions
should be performed by one belt of wire or other obstacle in
order to conserve materials and labor.
(2) Tank obstacles should be provided to block all possible
avenues of approach for tanks Into the battalion defense area.
Locations of tank obstacles and battalion antitank guns should
be coordinated so that the guns can cover the obstacles by fire,
if possible. Otherwise the antitank guns are located to cover
the most favorable approaches for tanks not covered by
obstacles.
g. Battalion command and observation posts are located in
accordance with provisions enunciated for the company, the
command post usually being in the battalion reserve area.
h. Company and platoon defense areas should be assigned
sectors or positions which will most effectively protect the key
point of the battalion defense area by covering routes of ap¬
proach to the key point with planned fire. Development of
the fire plan involves siting weapons and assignment of sectors
and areas of fire so that all area In front of the position can be
covered by some type of destructive fire, and that fire can be
delivered against the enemy if and when he succeeds in pene¬
trating any part of the position. It is essential to coordinate
fire of subordinate units to avoid and eliminate duplication, to
assure that all areas are covered, to see that fire gives mutual
protection to adjacent units and weapons, to assure maximum
development of flanking fires, and to see that fires are
capable of being switched and shifted to meet unexpected
developments.
34
FIELD FORTIFICATIONS
21
■ 21. Regimental Sectors. — a. A regimental sector is tlie larg¬
est defensive area in a battle position and is the only one which
completely covers the battle position in depth. It normally
consists of three battalion defense areas, of which two are on
the main line of resistance and one on the regimental reserve
line. They are disposed so as to occupy and defend keypoints
Figure 12.—Battalion defense area.
Note. —Locations and fires of all weapons of the battalion to In¬
clude light machine guns of rifle companies and locations of their
60-mm mortars are shown. Primary target areas for the 81-mm
mortars and normal barrages of supporting artillery are shown
Note that some of the 60-mm mortars are attached to front-line
platoons and that the caliber .30 light machine guns are employed
in the defense in the same manner as heavy cal. .30 machine guns.
within the regimental sector, and so as to protect keypoints in
rear of the regimental sector, especially those affording obser¬
vation to the supporting artillery. Their fire must be coordi¬
nated both within the regimental sector and with that of
adjoining sectors.
35
21-23
engineer field manual
b. 37-mm antitank guns. —(1) Antitank defense of a regi¬
mental sector consists of two coordinated echelons, the bat¬
talion antitank guns for defense of forward areas and the regi¬
mental 37-mm antitank guns to give depth to the defense and
to protect the flanks. They should either be placed within or
under protection of company defense areas or be assigned
troops to furnish local protection.
(2) Due to its low mount, the antitank gun cannot be dug
in at its firing position. When not in action the guns are
usually held in positions of readiness near their firing posi¬
tions in areas defiladed from flat trajectory fire, shelter or
concealment being provided for gun and crew. Firing posi¬
tions should be located on commanding ground with wide,
clear sectors of fire covering probable routes of tank approach.
They are not prepared except for providing fox holes for oper¬
ating crew and digging in ammunition.
c. In organizing the regimental sector, no obstacles against
foot troops are ordinarily provided beyond those included in
the battalion defense area. However, additional antitank ob¬
stacles normally will be necessary. They usually will include
the more readily constructed types such as mine fields and
heavy abatis. Their location should be coordinated with fire
of antitank guns and caliber .50 machine guns so that all such
obstacles are covered by fire.
d. Antiaircraft protection within the regimental sector is
limited to prescribing areas of air responsibility for the vari¬
ous subordinate units and assigning antiaircraft fire missions
as primary missions to certain units or weapons, principally
to heavy machine guns.
■ 22. Additional Organization. —The preceding paragraphs
do not describe a complete organization of the ground;
rather, they indicate the minimum that should be done for a
rear position. The defensive strength of a position can be
increased materially by connecting squad trenches, provid¬
ing approach trenches, strengthening tank obstacles, pro¬
viding shellproof weapon emplacements and protected shel¬
ters for personnel, and such other additional work as the
situation demands or time and labor permit.
■ 23. Artillery Support.— In addition to organic weapons,
the infantry normally receives direct support from artillery
36
FIELD FORTIFICATIONS
23-24
in the form of barrages and concentrations covering such
areas and delivered at such times as the Infantry desires.
Pertinent data on such Are are given in table II.
Table II .—Placing of barrage fired bp a battery of field
artillery
Cali¬
ber
(mm)
Type
Burst
of I
shell
Area of barrage
!
Diam¬
eter of
concen¬
tration
Minimum safe dist¬
ance from in-
1 fantryin—
1
Normal j
Emer¬
gency
Open
Treuehes
75
Gun.
5x30
100 x 200
100 x 300
100-300
200-500
200-400
105
How.
9x40
100 X 300
100 x 400
I 200-400
300-400
200-400
155
[Gun.
[How.
J9x 70
! 200-400
600-700
300-400
Note.—A ll dimensions given are in yards. Safe distances -will vary
with ranges and nature and efficiency of observation. In general,
barrages should be placed at distances of 300-500 yards from infan¬
try. If protection furnished is exceptionally good, minimum dis¬
tances given may be reduced by about 50 yards.
■ 24. Outpost Area.— a. The enemy situation permitting,
every battle position should.be covered by an outpost to the
front. The area in front of the main line of resistance of
the battle position occupied by the outpost is called the
outpost area.
b- When the outpost troops have only the usual outpost
missions, that is, to protect troops in rear against surprise,
to prevent an attack upon them before they can be prepared
to resist, and to prevent or restrict enemy reconnaissance and
ground observation, little effective resistance to a general
attack is expected. The outpost consists of squads, platoons,
companies, or battalions, depending on depth of the outpost
area, sent forward by units holding the battle position. Little
ground organization is contemplated. An outpost line of
resistance (OPLR) should be designated along which the
outpost is disposed so as to carry out its mission. The fires
of elements of the outpost and its supporting artillery are
coordinated along this line.
37
24
engineer field manual
c. When in addition to the above missions the outpost is
charged with absorbing shock of an attack, depriving it of
momentum and breaking up the enemy organization, battle
ensues over a zone of considerable depth. The outpost area
is organized as thoroughly as conditions will permit, and
battle positions and organized outpost area collectively are
called a defensive zone (see fig. 13).
38
DEFENSIVE ZONE
FIELD FORTIFICATIONS
24
d. Organization of the outpost area is coordinated by des¬
ignation of a line of resistance for the outpost which should
be located not less than 1,500 nor more than 4,000 yards in
front of the main line of resistance in order to give sufficient
depth to the outpost area and in order that artillery located
behind the battle position can support the outpost and place
fire from 1,500 to 2,000 yards beyond the front line. A line
of observation is established in front of the outpost line of
resistance. Outpost forces generally consist of battalions or
companies with attached machine guns sent forward by regi¬
ments or brigades holding sectors of the defensive zone. The
organization consists of mutually supporting tactical locali¬
ties similar to those heretofore described for the battle posi¬
tion. Units of the outpost usually are required to cover
more extensive fronts than similar units on the battle position.
This results in greater intervals between tactical localities.
Companies may be required to cover 800 to 1,500 yards of
front and a battalion 2,000 to 2,500 yards. The important
tactical localities in an outpost area are those which furnish
important observation into the enemy territory and control
probable routes of enemy advance into the defensive system.
These are organized usually as company defense areas or
if their importance warrants as battalion defense areas.
Intervals between these important localities should not exceed
3,000 yards. Where necessary, intervals between these de¬
fense areas are covered by the organization of detached
platoon defense areas. Because of the longer time they can
hold out and the greater effect of larger organizations in
breaking up enemy attack formations, provision should be
made in case of a general attack for withdrawal of garrisons
of small organized localities into the larger and more
important ones.
e. In some situations in zone defense the importance of the
front held and quantity and power of enemy artillery require
that the main line of resistance be placed far enough in rear
of the OPLR to be beyond effective range of the bulk of the
hostile guns, 6,000 to 8,000 yards. This depth of outpost
area is too great to permit adequate support of the outpost
from the battle position, and some troops (infantry and ar¬
tillery) must then be located between the front line and the
39
24
ENGINEER FIELD MANUAL
battle position to support the forward outpost elements and
supplement their action in delaying and disorganizing hostile
attacks. Such an outpost area is in effect a delaying area
(fig. 14).
Figure 14. —Defensive zone with deep outpost area (square division).
/. Continued occupancy of a defensive zone results in de¬
velopment of organized tactical localities of the outpost area
into a more or less well-organized position consisting of—
(1) A line of observation (the front line) in front of the
line of resistance occupied by outguards to provide observa¬
tion and defense of the foreground.
40
FIELD FORTIFICATIONS
24-25
(2) A line of resistance occupied by company defense areas
to cover the front by fire and to hold important tactical
localities.
(3) A reserve area occupied by company defense areas to
assist the units on the line of resistance (fig. 15).
■ 25. Reserve Battle Position.— a. In any defensive situa¬
tion the commander must always consider the possibility of
Figtjke 15.—Elements of an outpost position for a defensive zone
(square division). (Distances shown are approximate.)
defeat in the selected battle position and the necessity of a
continuation of the defense farther to the rear. Depending
upon time available and importance of the front held, an
additional battle position may be—
(1) Selected from the map.
(2) Reconnoitered and a plan of organization prepared.
(3) Staked out and partly or wholly organized.
41
25-27
ENGINEER FIELD MANUAL
b. Such a position is designated as the “reserve battle posi¬
tion” (fig. 16). On highly important fronts additional posi¬
tions in rear of the reserve battle position may be selected,
reconnoitered, and staked out. Selection of the reserve battle
position and other positions in rear is based on placing them
at such a distance in rear of the battle position next in front
that the enemy after having successfully attacked a forward
position would have to advance the bulk of his artillery be¬
fore undertaking attack of the next battle position. The
requirement calls for a distance of at least 6,000 yards; con¬
figuration of the terrain may require some modification of
this rule.
■ 26. Switch Positions. —In addition to the several positions
or organized areas of a defensive zone paralleling the front,
additional positions are provided oblique to the front and con¬
necting the forward position or areas with those in rear.
These oblique positions, designated “switch positions” (fig.
16), are established on the flanks of localities in the defensive
system where due to lack of natural defensive strength or
for other reasons there is a probability of an enemy penetra¬
tion. Attacks against a defensive zone in order to break
through the defensive organization on a front broad
enough to insure success must be exploited to the flanks as
the advance progresses to counteract narrowing of the Initial
front of attack by resistance of the defense. Switch positions
are planned to resist exploitation by the enemy to the flanks
of a penetration, to insure continuity of the front when
forward defenses have been broken through, and finally to
provide a line from which general counterattacks against the
flank of a penetration may be started.
Section IV
EFFECT OF PROJECTILES
0 27. General. —Penetration and effect of small-arms, ar¬
tillery projectiles, and aircraft bombs are extremely variable.
Fortifications must therefore be designed with large safety
factors. However, maximum probable penetration and effect
must be kept in mind so that works may be strong enough
to resist projectiles without unnecessary expenditure of labor
and material.
42
6000 Minimum ■ ■ ■ . . —- < - 1500 to 4000 Yds.
Outpost Area
:,ne of Resistance ^ ] ' -,wr- - ^ T
--**=V / 1 /SI 7^ ( \ --
o o o
^security
X° ° ° ° o
_ ^ I -JlpJp Line |
K ■!stance _1^1 __
X
X
^--l-^-CZ^^a/ Lejer^«.t'2S-— __f-
1 — -i- A* - 1 -
Front Line
(Line ofObser-
vation of Outpost)
Organized
tactical Localities.
-A> * .V 1
L Battle
Position
Artillery
X
and
■ aV ?T
X
X
Local
-Brigade Sector —i
/
/
Reserves
Brigade Sector -
•i vis ion Sector —>
Wff/Tf Division
y-
w
a«V
Jyf
£*
f*
1 rt_r
-Regimental
Sector
p- Regimental -«X
Sector X
-Division Sector
(Triangular Division)
_ Reserve Battle
Position
Figure 16. —Defensive system showing battle position, outpost area,
reserve battle position, and switch positions (square division).
262375—40 (Face p. 42)
FIELD FORTIFICATIONS
28
■ 28. Infantry Weapons. — a. Penetration of cal. .30 rifle,
automatic rifle, and machine-gun nonarmor-piercing bul¬
lets. —(1) The United States Army rifle, M1903, the new Ml
semi-automatic rifle, cal. .30 Browning automatic rifle, and
the cal. .30 machine gun all fire the same ammunition and
may be taken as a fair example of the small arms of the
various nations. The 174-grain boat-tail bullet fired by these
weapons has a flat trajectory and wide danger zone at all
probable ranges.
(2) Table in below gives approximate maximum penetra¬
tions in various materials of the 174-grain nonarmor-pierc¬
ing bullet fired from the rifle at range of about 200 yards,
together with thickness of armor necessary to give adequate
protection. Prolonged concentrated fire (for example, from
a machine gun) will penetrate these thicknesses. Though
this effect is not likely to occur often, it may be necessary
in special cases to provide extra thickness for protection.
Table III. — Safe thickness of material to protect against the
174-grain, nonarmor-piercing bullet
Material
Maximum
1 penetra¬
tion
Thickness
to be pro¬
vided
Remarks
Armor plate_
Inches
0.3
Inches |
0.5 1
Concrete (plain).
2.0
3.0 |
Brick masonry..
5.0
7.0
Greater penetration when
Oravel_____
8.0
10.0
bullets strike in soft m ortar.
Sand:
Dry.......
12.0
14.0
Moist_-...
34.5
18.0
Solid oak... .
20.0
24.0 |
Earth loam-
30.0
30
60.0
72 !
Varies greatly. This is maxi-
Greasy clay.^
!
!
mum.
b. Armor-piercing caliber .30 and caliber .50 bullets .—
(1) Table TV below gives some of the characteristics of
special armor-piercing caliber .30 and caliber .50 bullets.
43
28
engineer field manual
Table IV
Caliber 1
Projectile 1
weight |
| Armor penetration at—
Thickness
of armor
to provide
protection
100 yards
300 yards
.30, M6_
Grain* 1
174
753 |
Indies
Inches
Inches
1
2
.50, MO...
1
(2) It may be taken as a general rule that specially de¬
signed armor-piercing rifle and machine-gun bullets at their
most favorable ranges can penetrate special steel (tank)
armor twice their caliber in thickness and that armor
four times the caliber in thickness gives adequate protection
against them.
e. Special armor-piercing. —(1) Table V gives character¬
istics of infantry weapons specially designed for their
armor-piercing qualities. They are typical of such weapons
likely to be found in the various modern foreign armies.
Table V
Antitank gun
Projectile
weight
Muzzle
velocity
Armor penetration
600 yards,
normal
impact
1 ,000 yards,
20° impact
Pounds
0. 72
1.85
3.50
Feet per
second
3,000
2,600
2,000
| Inches
1. 95
2.20
1.90
Inches
1.5
(2) Penetration of armor cannot be counted upon when
the angle of impact is greater than 30° from normal. At
these larger angles of impact the projectile either ricochets
or breaks up.
d. Mortar shell .—Infantry mortar shell similar to the
60-mm and 81-mm mortars have a high angle of fall and
can reach defiladed objects. Due to their low velocity they
44
FIELD FORTIFICATIONS
28-29
have little penetrative power. On the other hand, they carry
heavier bursting charges than artillery projectiles of corre¬
sponding weight and caliber. They are consequently espe¬
cially effective when used against light obstacles, and produce
serious destructive effects when they fall within trenches.
They are effective against surface and light shelters but
are relatively ineffective against shell-resisting or bomb-
resisting structures.
■ 29. Artillery and Aircraft.— a. General. —(1) This para¬
graph covers penetration and crater effect of projectiles in
natural earth and their effect upon various types of fortifi¬
cations. Effect of single artillery shells or bombs is stated
with a view to establishing in the mind of the engineer a
conception of what constitutes safe construction and the
resistance to fire various type of fortifications will present.
(2) Penetration and crater effect of projectiles are highly
variable. Fortifications intended to resist continued bom¬
bardment must be designed with very large safety factors.
Probability of such continued bombardment and probable
power and direction of projectiles likely to be used by the
enemy must be fully considered.
b. Shell or bomb. —(1) If the projectile has an instanta¬
neous fuse, the detonation of the filling and bursting of the
case will occur so quickly that the projectile will not have time
to penetrate the object it strikes to any great extent and
depth of penetration will be indeterminate. If a fuse with
sufficient delay is used and the angle of impact is not such
as to produce a ricochet, the projectile will penetrate the
object it strikes and detonate at a depth dependent on sev¬
eral factors, among which are nature of materials penetrated,
terminal velocity of projectile, weight and cross section of
projectile, strength of projectile case, and type of delay fuse
used. Deep penetration in connection with long delay fuses
where the projectile has attained its full penetration before
it explodes serves to place the projectile where it is well
tamped and the explosion probably does the most damage,
that is, deep in parapets of trenches, inside casemates, and
nearer chambers of underground shelters than the surface.
Destructive effect is due to impact of the unexploded shell.
262375 °— 40 -
-4
45
29
ENGINEER FIELD MANUAL
impact of the explosion, and shattering or disruptive effect
of the explosion, including fragmentation and cratering.
(2) It is emphasized that these several factors which
produce destructive effect of projectiles are defined only for
general information purposes. Sufficient experimental data
are not available to conclude that in any given case the vari¬
ous effects are cumulative or selective, and it is not possible
to state against which effect or combination of effects pro¬
tection must be provided. It is important to realize the
limitations imposed both by inherent uncertainties of warfare
and difficulties surrounding research.
c. Ricochets ,—The course followed by a projectile after
impact on level ground depends on angle of impact, terminal
velocity, weight and dimensions of the projectile, its rota¬
tional motion (if any), and resistance of the ground. In
general, action is as follows:
(1) If angle of impact is less than 7°, projectile ricochets.
(2) If angle of impact is between 7° and 25°, shell either
ricochets after traveling a short distance in the ground, or
remains in the ground at slight depth. Long pointed shells
usually show a greater tendency to come out of the ground
than short pointed ones.
(3) If angle of impact is between 25° and 40°, projectile
has a tendency to turn toward the surface and deep pene¬
trations are not generally secured.
(4) If angle of impact is greater than 40°, projectile fol¬
lows a straight course and reaches maximum depth consistent
with its terminal velocity, weight, form, and hardness of the
ground. This is the class of fire against which protected
shelters must be designed, and is in general characteristic
of aircraft bombs on horizontal surfaces.
d. Penetrations. —(1) Formula .—Experimental data with
respect to penetration are inadequate.' The following em¬
pirical formula has been developed to give maximum pene¬
tration of projectiles in various materials, impact being
normal. It applies both to artillery and aircraft projectiles.
46
FIELD FORTIFICATIONS
29
Results given by the formula will be considered only as
approximations.
p_ 0.23 W A K
D=
in which P=Penetration of projectile in feet.
W=Weight of projectile in pounds.
D=Dlameter of projectile in inches.
A=A constant depending on striking velocity ac¬
cording to the following table:
Velocity, feet per
second
A
Velocity, feet per
second
A
Velocity, feet per
second-
A
130
0.33
657_
4. 77
1,180..
8.76
197..
0. 72
720___
5. 34
1,250_
262..
1. 21
788...
5.89
1,320..
9.54
328 . . . . _
1. 7G
854...
6.41
1,375__
9.92
394
2. 36
920..
6.92
1,445__
10.29
460 - .
2. 97
985_
7.40
3,510..
10. 61
525-..
3.58
1,050__
7.87
10. 98
592.
4.17
1,113..
8.31
1,640__
11.20
K=A constant depending upon nature of resistance shown in
the following table:
Material
K
! Material
K
0.64
ii
2.94
0.94
3.80
1.63
5.87
II '
W and D are obtained from characteristic tables of the
projectile under consideration.
(2) Field artillery projectiles .—The following table gives
penetration of field artillery projectiles in ordinary compact
soil:
47
29
ENGINEER FIELD MANUAL
Table VI .—Penetration of field artillery projectiles
Caliber
Striking
Angle of
| Penetration
velocity ^
impact
Vertical !
Horizontal
Feet per
second
Degrees
Fed
Feet
75-mtn.- ..- ..
730
45
4 1
4
105-mm____
800
45
6
5
155*nnii--_ _
770
45
7
7
8-inch____
790
45
9
9
240-nim-.____
S06
45
14
I
14
(a) Figures 17 and 18 give data secured by firing sand-
loaded shell, observing point of fall, and then digging up the
shell. These illustrations, together with the more extensive
experiments upon which they are based, indicate that when
angle of impact is greater than 40° the projectile moves
in a straight line until a considerable proportion of its
velocity is lost. Then, particularly if it meets a somewhat
harder stratum of material, it tumbles forward and comes
to rest with its nose to the rear.
(£»> To secure deep penetrations, large caliber howitzer
projectiles which strike the target with an angle of impact
in excess of 40“ are required. Weapons smaller than the
155-mm howitzers and flat trajectory weapons are in general
ineffective against heavy fortifications. Large howitzers or
mortars are required to destroy well-constructed cut-and-
cover and cave shelters.
(c) For thickness of cover required to provide protection
against various artillery projectiles see table XXVI, para¬
graph 53.
(3) Airplane bombs. —(a) Data .—Experimental data per¬
taining to the penetrations of airplane bombs in earth are
inadequate.
(b) Striking velocity .—The trajectory of a bomb is a curve
which would be a parabola in vacuo but in air is somewhat
steeper. The bomb loses speed in its initial direction but
gravity gives it velocity toward the earth which increases
to a certain maximum limited by air resistance. Maximum
48
FIELD FORTIFICATIONS
!9
Figure 17.—Penetration of 4,7-inch shell.
49
29
ENGINEER FIELD MANUAL
50
FIELD FORTIFICATIONS
29
striking velocity depends on weight, shape, and dimensions
of the bomb, and prevailing atmospheric conditions. From
the point of view of structural protection, angle of impact,
that is, the angle which the bomb makes with the vertical
at the moment of impact, is important. Table VII gives
approximate striking velocity and angle of impact for bombs
heavier than 100 pounds released from an aircraft flying
horizontally at 200 miles an hour.
Table VII
Height of
release
A nglc of
impart
with vor¬
tical
Striking
velocity
Fed
Degrees
Feet per
second
1,000
40
390
3,000
33
520
5,000
20
010
7. 500
22
710
10. 000
19
800
1*2. 50(1
17.5
880
15, 000
10
950
Taking all factors into consideration, it is safe to assume
that the maximum striking velocity of a high-explosive
bomb that need be taken into account in structural defense
is 1,000 feet per second. The corresponding velocity for a
2-pound incendiary bomb is about 400 feet per second.
(c) Types and dimensions .—For convenience types of HE '
bombs are classified with reference to weight of the container
as heavy-case, medium-case, light-case, and antipersonnel, as
noted in table VHI.
Table VIII
Ty|n‘ of Unnli j
Charge 4-
weight per-
cchi age
Gross weight
(pounds)
SmalL .
250 to 2,000.
50 to 1,000.
60 to 2,000.
20.
Medimn-case...
25 to 40_
15 to 20.
51
29
ENGINEER FIELD MANUAL
Table IX gives typical dimensions for a few bombs. The
figures in brackets in the second column give the length of
bomb-carcass, that is, the part enclosing the charge. The
tail portion is often only a stream lining designed to give
good ballistics and to carry the fins.
Table IX .—Typical diviensions of aircraft bombs
Bomb
Length
Diameter
Sectional
pressure 1
2 ,000-pound, light-case---
Feet
H (9)
6 (4)
Inches
24
12
Pounds per
square inch
4.4
9.7
15
3 1
4^(2)
10
2.8
4 (2)
9
1.6
iJOpound, antipersonnel. ..
2 (1)
5
1.0
i Weight—maximum cross section area.
<d) Depth .—Figure 19 gives approximate depth of pene¬
tration at zero angle of impact, for unit sectional density of
aircraft bombs. To obtain total penetration multiply value
for penetration taken from the figure, by sectional pressure
given in table IX. The formula in paragraph 29 d conforms
in general to the curves in figure 19.
(e) Referring to the formula given, in paragraph 29tl, the
following computations are made:
550-pound bomb, D=15 inches (from table IX).
W=550 pounds (approximate).
Assume sandy earth, K=2.94; conditions giving 1,000
feet per second velocity of impact, then let A=7.50.
p _ .23WAK .23X550X7.50X2.94
D2 15 2
12.4 feet.
These results can only be accepted as approximations of pene¬
trations to be expected in a sandy soil.
(/) Lighter bombs have little penetrative power, even in
natural earth. Larger bombs have sufficient penetrative
power to attack successfully protected shelters and buildings,
52
DEPTH OF PEf\|ETRAT fON 1 FEET
FOR UNIT SECTIONAL PRESSURE (LS/m 2
FIELD FORTIFICATIONS
29
APPROXIMATE DEPTH OP PENETRATION
FOR UNIT SECTIONAL PRESSURE OF BOMB
Figure 19.—Penetration of aircraft bombs.
53
29
ENGINEER FIELD MANUAL
and the largest bombs may even penetrate heavy concrete
and armored structures.
(gr) Table X gives penetrations of bombs of various sizes in
earth, gravel, concrete, etc., as well as thicknesses required to
be proof against them. These data are compiled chiefly
from foreign sources and are, in general, based on theoretical
analysis, since there is very little experimental information
on the subject.
e. Crater dimensions. —(1) Light artillery projectiles make
relatively small and ineffective craters for destruction of
54
FIELD FORTIFICATIONS
29
proof thickness to perforation and explosion , compiled
sources
Thickness just per¬
forated by pene¬
tration without
explosion
Thick ness just
perforated,
explosion
only
Proof thickness penetration and explosion
o
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Feet
Feet
Feet
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Lb! ini
Lb/in !
Lbfini
2.8
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2.0
1.5
19.5
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11.5
5.5
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11.5
6.3
1.0
6 . 6
4.9
Columns 18 and 19 apply to thickness of slab just “perforable” by explosion of a
bomb placed on the slab.
tt=Mininium crushing strength of concrete, at age of 28 clays, in pounds per square
inch.
trenches and shelters. These projectiles are effective how-
ever against exposed personnel and light obstacles designed
to stop foot troops.
(2) Medium and heavy artillery projectiles are used for
destruction of shelters, trenches, and other types of fortifi¬
cations. Table XI below gives some idea of their relative
effectiveness but it must be borne in mind that craters vary
greatly with nature of the soil and depth to which the
shell penetrated before explosion. This penetration is de¬
pendent upon both kind and setting of fuse and also upon
the soil. Tabular dimensions must be considered as average.
55
29
ENGINEER FIELD MANUAL
Table XI.— Probable size of maters in ordinary compact
virgin soil
(Light loam may double, loose soil triple these values)
Caliber
Slight penetration
Medium penetration
1 Diameter
Depth
Diameter
Depth
1 Fat
Feet
Feet
Feet
75-nrni_
, *
1 .5
5
3
lOR-mni _
0.5
2.5
7.5
3.75
155-imn- --
10
4
1*2
5
8 -inch ___
11.5
4
13.5
5
240-mni. ...- .
14
1
4
15.5
5.5
(3) Table XII gives average crater dimensions for aircraft
bombs with instantaneous and delay-action fuses falling
on sandy loam.
Table XII.— Crater dimensions , sandy loam
Bomb
1 Viih i n&ta tit a nco us fuse:
Fat
Fed
Cubic yards
100 -pound------
o
9
4
300-pound.---
3
13
10
600-pom id..
5
17
17
1 ,100-pound-
6
20
28
2 ,000-pound......
1 \'ith delay-action fuse:
'
*
47
100 - pound..__
5
20
30
300-pound--
7
27
70
600-poun(l--
10
37
170
1 ,100-pound_
13
«
320
2 ,000-pound__
17
50
600
f. Trenches. —Artillery fire upon trenches can overthrow
parapets and interior slopes, obstruct loopholes, and displace
earth to the extent that the trenches are filled up or ob¬
structed. Only direct hits by light artillery shells, 75-mm to
105-mm, produce appreciable results and fire of these guns
is effective only when single hits can be observed, or when
56
FIELD FORTIFICATIONS
29
the trenches are crowded with troops. Calibers of 155-mm
and greater are normally required for destruction of trenches;
155-mm or 6-inch howitzers usually available in large num¬
bers are the most important weapons used for such purposes.
g. Shelters. —(1) By use of protected shelters, it is pos¬
sible to maintain the physical condition and morale of troops
held near their combat positions. Modern artillery fire,
especially that of howitzers which can destroy troops in open
trenches, makes protected shelters an essential and possibly
the most important element in a stabilized fortified battle
position.
(2) Projectiles fired against protected shelters cause the
most damage when they have penetrated deeply and explode
either against or near the structure itself, the overlying ma¬
terial serving as a tamping so that the full force of the
explosion is expended upon the shelter. Conversely, if deep
penetrations are not secured and the projectile explodes
nearer the ground surface than the shelter, this tamping
effect is decreased and the force of the explosion escapes
upward. Damage to the shelter is thereby greatly reduced.
Howitzers and mortars are very effective weapons for attack
of shelters as their fire produces few ricochets and nearly
vertical impact with deep penetration.
h. Obstacles and entanglements. —(1) Light and medium
artillery and trench mortars are sometimes employed to cut
gaps in wire entanglements. The object would be to adjust
a beaten zone of fire upon the entanglement so that enough
shells will actually fall within its limits to effect destruction
at the point desired.
(2) Requirements in rounds of 75-mm caliber to open a
gap about 25 yards in width and 30 yards deep are approxi¬
mately as follows;
Range, yards
Number of
rounds
Range, yards
Number of
rounds
2, .500_ _
600
5,000....___
1,000
3,000 ..-_
700
7,000__
1,200
4,000...__
SOO
57
29
ENGINEER FIELD MANUAL
These requirements apply only to bands of Wire not greater
than 30 yards in depth located on practically level ground.
Expenditure of such great amounts of ammunition on such a
relatively nonremunerative target would rarely be justified.
More ammunition must be expended to destroy deeper en¬
tanglements. Forward slopes increase and reverse slopes de¬
crease accuracy of fire and respectively decrease and increase
ammunition requirements. In particular, reverse slopes serve
to make cutting entanglements by gunfire a difficult and ex¬
pensive operation.
(3) Successive bands of wire should be sufficiently separated
so that artillery fire must be adjusted on each band. A series
of separate bands are much more difficult for artillery to cut
than the same amount of material in a single band.
(4) Artillery firing upon entanglements uses high-explosive
shell with instantaneous fuses except when the angle of fall
results in ricochets. Then short delay fuses are used to secure
bursts as the projectiles rise,
(5) Types of obstacles other than wire entanglements are
attacked by artillery in the same manner but in general do
not resist fire as well. Antitank obstacles however require
tremendous expenditures of artillery ammunition for effective
destruction and except in the ease of fire against mine fields
on hard ground, such fire may leave the ground as difficult for
tanks to traverse as before,
i. Concrete .—< 1) Existing information on resistance of con¬
crete to shell fire has been assembled from observations made
during the World War and from experiments in the case of
heavy, permanent fortifications and primarily from wartime
observations alone in the case of field fortifications.
(2) When large shells are exploded against concrete forti¬
fications without penetration, severe vibrations are set up in
the structure. At times these vibrations cause a separation
of the steel and concrete in the case of reinforced structures
which lowers resistance to subsequent shots. The heavy ex¬
plosions coupled with the severe vibrations have a very ap¬
preciable destructive effect on morale of fortification garri¬
sons,
(3) Large caliber weapons firing base-fused projectiles with
solid heads arid thick walls are required for destruction of
58
FIELD FORTIFICATIONS
29
concrete works. Against horizontal slabs or those sloping
very little from the horizontal, high angle fire must be em¬
ployed.
(4) Concrete splinters badly under shell fire. Steps must
be taken to protect the troops occupying concrete works from
flying fragments of concrete.
(5) Concrete fortifications protected by sand or earth are
much more effective than those without such protective cov¬
ering. A layer of earth or sand tends to cause shells striking
at an angle of less than 40“ to begin to turn toward the sur¬
face or to ricochet and hence reduces the angle of impact
with the concrete. The covering must first be blown off by
shell fire before the concrete itself becomes vulnerable. When
a bomb or shell with a delay-action fuse is used however,
there is danger that the earth cover will help to confine or
tamp the explosion and increase its effect. A concrete burster
course or detonating slab is valuable for this reason, since its
function is to detonate an instantaneous projectile or to pre¬
vent penetration of a projectile with a delay-action fuse.
Walls which are perpendicular or nearly so are usually more
easily destroyed by artillery fire than horizontal slabs. Not
only is it easier to secure angles of impact approaching 90°
against such walls, but the sand, earth, and debris in front of
them fall away quickly and expose the bare concrete.
(6) On the whole, concrete, either plain or reinforced, may
be considered as a most effective shell-resisting material,
It is highly valuable for field fortifications whenever diffi¬
culties fn connection with its use can be overcome. It is
particularly useful for machine-gun emplacements, observa¬
tion posts, shelters for machine-gun and other personnel
that fire from open emplacements, in burster courses made
up of many small, easily handled slabs, and for shelters that
must be built above ground because of ground water or hard
rock near the surface. See PM 5-10 for details of mixing
and placing concrete in the field.
j. Masonry .—Stone and brick masonry show many char¬
acteristics similar to those of plain concrete but usually
have much less power of resistance. The quality and strength
of the mortar, interlocking bond used in laying courses, size
and nature of stone or brick, all have a great deal to do with
the resistance of masonry.
59
29-31
engineer field manual
k. Armor plate .—Armor has been used extensively hereto¬
fore only in permanent fortifications. Steel turrets have
been found to resist even the most intense artillery bom¬
bardment very well. Due to their small size it is difficult to
secure direct hits on them. If they are properly designed
even direct hits by heavy shell frequently ricochet without
doing serious damage. Penetration is not essential to disable
a turret however since it may become jammed by a direct hit
or the supporting concrete so cracked or displaced that it
cannot function.
l. Buildings and villages. —(1) Light and medium artillery
and small aircraft bombs are used against wooden buildings.
White phosphorus smoke shell, incendiary bombs, and low-
bursting shrapnel are effective in starting fires.
(2) Medium and heavy artillery and medium aircraft
bombs are used against stone and brick buildings. The
cellars of masonry buildings are difficult to reach with artil¬
lery fire and frequently serve as effective shelters. The pos¬
sibility of being trapped or buried by debris may make such
locations dangerous.
Section V
TRENCHES
■ 30. General. — a. Trench construction may be considered
under three heads, planning, tracing, and execution.
b. No work except that of the greatest urgency, notably that
in connection with hasty defenses, should be undertaken with¬
out careful planning. Even when some delay results from this
procedure, the time lost is usually regained in execution of
the project.
c. It may be enunciated as a general rule that no work
should be done at night that can be performed by day.
B 31. Trace, Tracing, and Profile.— a. The detailed location
of a trench on the ground or on a map is called its trace, and
the operation of laying out this location on the ground with
suitable marking devices is called tracing.
b. A profile is the cross section of the trench upon a vertical
plane perpendicular to its direction. In order that discussions
following may be dear, figure 20 shows nomenclature of
various trench features.
60
field fortifications
32-33
■ 32. Necessity for Standard Types. —Trenches constructed
in contact with the enemy are usually the result of hasty and
frequently poorly coordinated efforts to obtain cover from
enemy fire. Such trenches except in the most mobile situa¬
tions must be developed into more complete and well-coor¬
dinated defensive systems. Both in this development and in
construction of deliberate defenses out of contact with the
enemy, a ready knowledge of certain standard trench traces
and profiles assists materially. Established standards, work¬
able and well-known with standardized material provided,
give an objective toward which to work, though this ideal in
many cases may never be reached. Experience teaches that
practice may be restricted to a limited number of simple forms
Figure 20,—Trench nomenclature.
that will meet practically all situations. Standardization of
types and materials and familiarity therewith by ail combat
personnel in such detail as their duties require result in uni¬
formity in training and practice, and economy in planning,
execution, and supply of tools and materials.
■ 33. Classification. — a. Direction (fig. 21). — With reference
to direction, trenches are classified as—
(1) Parallel, or those with general direction parallel to the
front, primarily for fire purposes but also providing for lateral
communication.
(2) Approach, or those with general direction perpendicular
to the front, for fire or communication purposes or for both,
depending upon location and use in the defensive system.
262375°—40-5 61
33
engineer field manual
(3) Switch, or those with general direction inclined to the
front; these connect two parallels for the purpose of preserv¬
ing continuity of the front in ease of capture of a portion of
the line. Switch trenches are primarily fire trenches but may
also serve as communication trenches.
b, Employment .—With reference to employment, trenches
are classified as—
(1) Fire, designed primarily to provide cover for personnel
when delivering rifle fire.
(2) Communication, designed primarily to provide cover
for personnel moving from one part of an intrenched zone
to another.
C. Construction .—With reference to construction, trenches
are classified as hasty and deliberate. (See sec. I.)
(1) The principal works employed in connection with hasty
defenses are hasty fox holes, trenches, and emplacements for
automatic weapons and light mortars constructed on the
62
FIELD FORTIFICATIONS
33-34
battlefield during mobile situations and usually under enemy
fire.
(2) Deliberate trenches, constructed in connection with
deliberate defenses, are those not included in hasty trenches
on the one hand, and not pertaining to permanent fortifica¬
tions on the other. Such trenches are usually more carefully
designed than hasty types, are of stronger profile, and have
greater defensive power.
d. Sap .—A sap is a trench which is constantly prolonged
in the desired direction by digging away-the earth at its
head from within the trench itself. The earth is usually
thrown up as a parapet on the exposed flank and end,
thereby giving additional protection to the working party.
Construction of saps is referred to as sapping.
■ 34. Hasty. — a. General .—The normal inclination of the
soldier pinned to the ground by enemy fire is to “dig in”
to hold the ground he has gained. The result of his efforts
is a hasty intrenchment or fox hole which he constructs with
the intrenching tools carried on the infantry pack. The
hasty trenches discussed and illustrated in this section are
given as examples of good practice rather than as standards
to be followed rigidly. They represent excavations to gain
hasty cover and to be developed eventually into forms approxi¬
mating deliberate trenches.
b. Skirmisher .—Figure 22 shows the simple lying-down or
skirmisher trench dug by the soldier while under small-arms
fire. It is ordinarily excavated from the prone position by
single soldiers or by small groups working side by side. In
soft ground the soldier can mask himself from view from
the front and secure appreciable protection from rifle and
machine-gun fire in from 10 to 12 minutes. The trench
should be completed to the dimensions shown in less than
1 hour when dug from a prone position with the infantry
intrenching tools. It gives excellent protection against rifle
and machine-gun fire, and to some extent against high-
explosive shells. It is too shallow to protect satisfactorily
from shrapnel fire. Additional protection may be gained by
placing the pack, if carried, on the parapet.
c. Fox hole .—A more usual form of hasty intrenchment is
the fox hole shown in figure 23. It is started from a prone
63
34
engineer field manual
or crouching position 0 and may be developed successively
to kneeling © and standing©, which gives satisfactory pro¬
tection against small-arms and light artillery Are. If en¬
gineer tools are used, not under harassing Are and in medium
soil, the standing type can be completed by one man in
slightly less than 1 hour. If infantry intrenching tools are
used, the time to complete the standing type in medium
soil is about IV 2 hours. If under harassing Are, time required
for completion will be increased, amount of increase depend¬
ing on degree of enemy interference.
d. Shell hole positions .—Hasty intrenchment may take the
form of an improved shell hole shown in Agure 24. In this
case the improvement consists of a recess hollowed in the
forward slope to give an inconspicuous but effective Aring
position with lateral protection from shell fragments and
enfilade fire. In a shell-pitted area, shell holes afford quick
protection and a high degree of concealment for a small
expenditure of labor.
e. Slit trenches .—Narrow trenches are used during mobile
situations for communication purposes and particularly for
immediate protection of personnel from artillery fire. Figure
64
34 ENGINEER FIELD MANUAL
25 shows creeping © and standing © types. Figure 26 shows
a deeper trench revetted with struts resting against opposite
walls and especially designed to protect against shell fire.
Such a trench would be constructed only in short lengths
adjacent to the firing position; it is particularly applicable
for use as a shelter for machine-gun personnel. When spoil
Figure 24.—-Improved shell hole.
from slit trenches is wasted at a distance from the excava¬
tion, the trenches become nearly invisible to ground observa¬
tion, Slit trenches are liable to be caved in by concentrated
artillery fire thus becoming blocked as lines of communica¬
tion, and deeper types are more dangerous to their occupants
than trenches of wider profile.
66
FIELD FORTIFICATIONS
34
67
34-35
ENGINEER FIELD MANUAL
f. Shallow connecting trenches .—In order to provide for
lateral communication along the line of fox holes, shallow
connecting trenches are dug. The trench shown in figure 27
may be taken as typical. It has an average width of less
than 2 feet and a depth of 1 to l'/ 2 feet, and requires about
2 cubic feet of excavation per running foot of trench. With
a parapet about 9 inches high such a trench provides cover
for a man crawling on his hands and knees. When men
are to occupy fox holes overnight, they usually extend the
fox holes longitudinally, merging them with the shallow
connecting trench so as to lie prone while sleeping. If
occupied for several days, recesses with overhead cover may
be prepared. Care must be taken to dispose of the spoil
from such digging so as not to attract enemy attention to
the work.
■ 35, Deliberate. — a. General .—In deliberate organization
of a position cohstruction of trenches is inseparably related
to the other elements of field fortification. The entire de¬
fensive organization must conform to the tactical plan. The
completeness of the system will depend on the time,
materials, equipment, and labor available.
b. Requirements .—A deliberate trench must fulfill the
following requirements.
(1) If a fire trench, it must have a good field of fire and
permit the maximum use of the defender’s weapons. It
must lend itself particularly to development of flank or cross¬
fire as well as fire to the front.
(2) It must provide protection against enfilade fire by
means of frequent abrupt general changes in direction.
Both parallels and approaches that may be enfiladed by
68
FIELD FORTIFICATIONS
35
observed enemy fire should make a general change in direc¬
tion at least once in 100 yards <see fig. 30).
(3) It must limit the effect of a bursting shell through
subdivision by means of frequent changes of direction.
(4) In order to reduce the number of projectiles that may
fall directly into it, it must be as narrow as practicable, but
it must not be so narrow as to make circulation within it
unduly difficult or to render it easily blocked by caving.
(5) It must be sited to take advantage of natural drainage
of the terrain and so constructed as to be adequately drained.
(6) It must be simple and easy to lay out and construct.
(7) In addition to the foregoing requirements, if a com¬
munication trench, it should be wide enough to permit two
columns of men in single file to pass readily. If it is imprac¬
ticable to provide such a width, throughout the length, it
must be widened at intervals to provide passing places.
Where sharp changes of direction occur, the trench must be
widened so that a bulky piece of equipment such as an open
litter can be carried through easily.
18) When the various platoon, company, and battalion
areas of a defensive position are connected by parallels
and approaches, the defensive lines thus created should as¬
sume bold curves in making major changes in direction addi¬
tional to those prescribed in (2) above in order to increase
the enemy’s difficulty of organizing bombardments and bar¬
rage fire.
c. Camouflage and concealment. —*1) All trenches should
be camouflaged or concealed from both air and ground ob¬
servation Insofar as practicable or, falling that, rendered as
inconspicuous as possible. It is impossible to camouflage
extensive trench systems except in woods. Individual posi¬
tions within the trench system, for example machine-gun
emplacements, shelters, and trenches approaching these posi¬
tions, can be camouflaged effectively and with great benefit.
Such camouflage combined with continuity and uniformity
in trace and profile of the uncamouflaged elements of the
position serves largely to withhold from the enemy knowledge
of the detailed location and numbers of the garrison. See
PM 5-20.
69
35
ENGINEER FIELD MANUAL
(2) It is possible to conceal trenches from ground observa¬
tion to a large degree. Thus attacking infantry, although
knowing the general location of trenches by means of air¬
plane observation, are confused as to their detailed location
and relative position and hampered in the attack. Pull ad¬
vantage should be taken of woods and brush. Freshly exca¬
vated earth that contrasts strongly with the surroundings
should be covered with topsoil, sod, weeds, or brush. In gen¬
eral, avoid sharp or regular crest lines along parapet and
parados; these lines should have the characteristics of sur¬
rounding terrain, and dimensions shown in illustrations
should be modified as required to bring about this condition.
Depressions in the parapet serve as firing embrasures, and
the parados should be sufficiently high to prevent the heads
Figure 28.—Trench with irregular parapet and parados.
of the defenders of the trench from being seen in silhouette
(see fig. 28).
d. Clearing field of fire. —(1) To comply with the con¬
dition that a field of fire of at least 100 yards is available
in front of each fire trench, a certain amount of clearing
may have to be done even in the most open country. Clear¬
ing a reasonable field of fire will ordinarily precede digging
complete fire trenches or will at least be done concurrently
with such organized excavation. In areas organized for
close defense it is best to commence clearing work at the
trench and work forward. In delaying actions where fire
effect at long range is desired, any effort that can be directed
toward clearing should be employed on areas distant from
the trench. Before commencing any work the question of
just how much may be accomplished in the time available
should be determined since a field of fire only partially
field fortifications
35
cleared may give more cover to an attacking enemy than it
did in its original condition. It is desirable to leave a thin
natural screen to hide the position. A thin line of small
trees or brush left standing may impede the enemy’s observa¬
tion and at the same time not hinder fire of the defenders.
(2) Large scattered trees, if left standing, give less cover
to an attacker than if cut down and are sometimes useful as
range marks. Unless they can be entirely removed or con¬
verted into dead abatis (see par. 43), only the lower branches
should be cut off. Thick brushwood left standing may some¬
times serve as an obstacle but Infantry can usually pass with
ease any but the thickest growth of this kind. Therefore it
is imperative to clear such growth. It is rarely possible or
desirable to undertake the wholesale clearing of woods, and
the work usually is restricted to clearing undergrowth and
removing lower branches of the larger trees. Narrow lanes
running obliquely in front of a line to be defended may be
entirely cleared through woods and swept by automatic
weapon fire.
(3) The tools found useful in clearing woods are axes,
brush hooks, canthooks, hatchets, machetes, mattocks, and
crosscut and power-driven saws. In situations where ex¬
tensive clearing is necessary those classes of the above tools
not found in quantity in equipment of combat units should
be procured from the larger engineer supply establishments.
Large trees may be cut down by the use of explosives but
lack of large quantities of explosives for this purpose ordi¬
narily makes it impracticable.
(4) The following table for an area of 100 square yards
will be found useful in estimating time required for a given
clearing.
71
35
ENGINEER FIELD MANUAL
Description of area
Method
Man-hours
required
Covered wiih brush under 6 inches in di- '
ameter and containing 25 trees 6 inches to
2feet in diameter (heavy clearing).
Chopping or sawing trees j
and clearing brush.
7
Covered with undergrowth and some trees
cot exceeding 12 inches in diameter
(medium clearing).
_do......
3,5
Covered only with small brush (light clear¬
ing).
1*6
(5) Walls should be demolished only when resulting
debris will not give more protection than the wall itself.
Sufficient gaps must be made in the wall in any case to give
the defenders a clear view. Walls may be knocked down by
use of a tree trunk or rail as a battering ram, or by use of
power machinery or explosives if available and situation
permits their use.
(6) The same principles apply to buildings as to walls.
Low buildings may be knocked down. Larger buildings should
be burned and walls left standing. This will usually prevent
the upper stories from being used as observation posts and at
the same time will not provide piles of debris in which the
enemy may take shelter.
(7) Grain crops require somewhat the same treatment as
woods. If ripe and dry, they may of course be burned. In
the absence of mowing machines or in situations where such
machines cannot be used, lanes must be cut with scythes.
e. Standard traces. —(1) General .—Standard trench traces
must conform to the requirements of b above. Of the fol¬
lowing standard types the one most suitable to the tactical
situation and the terrain should be employed, special combi¬
nations and modifications being made to meet requirements of
special conditions. It is understood that the actual trace
will approximate only the geometrical precision shown in
figure 29. Note in figure 29 that the fire bay for the octagonal
trace trench is given as 16 paces or 40 feet. This is based on
72
FIELD FORTIFICATIONS
35
an 8-man squad with allowance of 5 feet per man. If a larger
squad such as one of 12 men is to occupy the trench, the fire
bay may be correspondingly increased to 24 paces or 60 feet.
(2) Octagonal. —The octagonal trace is an excellent type
for fire trenches in most situations. It provides excellent
protection and localizes results of bursting shells.
(a) Advantages are that it—
1. Affords easy communication.
2. Facilitates oblique fire along the front.
3. Is an economical type to construct both as to labor
and material.
4. May be provided with a continuous fire step.
(b) Disadvantages are that it—
1. Lacks simplicity of detail.
2. May be identified readily from the air as a fire
trench.
(3) Zigzag. —This trace gives less protection from enfilade
fire and shell bursts. Effectiveness may be increased by em¬
ployment of short tangents and by occupation of alternate
tangents. Advantages are that it—
(a) Is the simplest and easiest to trace, construct, revet,
and maintain of all types.
(b) May be adapted readily to terrain.
(c) Is equally suitable for use as a fire or communication
trench.
( d ) May be provided with a continuous fire step.
(e) Provides for both frontal and flanking fire.
(4) Wavy. —Communication through this type is freer than
for any other type, and it is therefore specially adapted for
use for communication purposes. Advantages of the zigzag
trace largely apply to the wavy trace. Slightly more labor
and materia] are required for its construction than for the
zigzag.
(5) Echelon.- —The echelon trace is used for fire trenches
when ground must be gained either toward or away from
the enemy without subjecting the fire bays to enfilade fire.
It may also be regarded as a zigzag trace fronting obliquely
toward the enemy. It has advantages and disadvantages of
that type of trace.
73
38
ENGINEER FIELD MANUAL
ZIGZAG
WAV/
tCHELON
Figure 29.—Standard trench, traces.
74
FIELD FORTIFICATIONS
35
(6) Modifications .—By modifying angles of the zigzag or
echelon into curves, those traces assume characteristics of a
wavy trace. Such a modified or wavy trace, with the intro¬
duction of tangents at advantageous points, is a natural
development during hasty intrenchment.
f. Tracing. —(1) General.—(a) Method .—Tracing trenches
should always be supervised by an officer. If possible they
should be traced with tape before arrival of digging parties.
It is preferable to trace during the daytime. If enemy action
prevents daylight tracing, favorable times are twilight periods
at dusk or dawn. When a trench cannot be completely traced
during the day it is usually possible to go over the ground
during the daytime and locate controlling points with stakes,
stones, or nails to which pieces of paper or rags may be
fastened. This work should be done by an officer accom¬
panied by qualified noncommissioned officers who later will
be invaluable as guides for working parties. The salient
points of the trench should be located with respect to easily
recognizable landmarks, by azimuth and distance. If tape is
not available, the trace may be marked with wire, twine,
stakes, or stones, or the working party may be extended along
the line thereby indicating it. The last method is most diffi¬
cult to put in execution in the vicinity of the enemy and
should be used only as the final resort. In case this is at¬
tempted it will usually be necessary to have noncommissioned
officers pass along the line pacing off the distances and ver¬
ifying and correcting the position of each man.
(b) Controlling lines .—Parallels are traced along the edge
nearest the enemy and approaches along the right hand edge
when facing the enemy. Width of the trench is then meas¬
ured from the trace line by means of a gage stick, known
lengths of intrenching tools, or by pacing.
(c) Controlling dimensions .—Figure 29 shows dimensions
in feet and paces of the various standard traces. One pace
is taken as 2.5 feet. It is habitual to trace trenches by pacing
the various distances, greater refinement in making measure¬
ments being unwarranted,
(d) General irregularities .—General changes in direction
should be introduced at intervals of from 75 to 125 paces as
75
35
ENGINEER FIELD MANUAL
indicated by figure 30 in parallels that may be enfiladed by
observed enemy fire, and in approaches.
(e> Tracing party .—The tracing party consists of 1 officer,
1 noncommissioned officer, and 3 privates. It is equipped
with tracing tape, 6-inch nails or stakes, and hammers or
hatchets.
(2) Octagonal. —(a) The officer lays the tape along the
general trace line established by the front edges of the fire
bays.
(b) The noncommissioned officer accompanied by one pri¬
vate follows the general trace established by the officer, roll¬
ing up the tape as he goes. At the same time he paces off
distances alternately of 16 and 6 paces and indicates to the
private the ends of the fire bays. The private then drives
stakes at these points. The noncommissioned officer also indi-
Figure 30.—Genera! irregularities introduced in wavy and zigzag
traces.
cates to the private points where offsets are to be taken. The
private then paces 6 paces to the rear from these offset points
and drives a stake, thus marking the ends of the rear bay.
The noncommissioned officer makes minor adjustments as
required.
(c) The two other privates stretch a tape along the line
thus established, making a round turn at each comer. They
provide additional stakes if required.
(3) Zigzag. —(a) The officer lays the tape along the general
trace line which Indicates general direction of the trench,
providing slack for the zigzags.
(b) The noncommissioned officer accompanied by one pri¬
vate follows along the general trace pacing off distances of
16 paces and . indicating these points to the private. The
private takes an offset of 6 paces to the rear at every other
point so indicated, and drives a stake. He drives a stake on
76
field fortifications
35
the general trace at the other 16 pace points. The noncom¬
missioned officer makes minor adjustments as required.
(c) The two other privates stretch the tape taut between
stakes, taking a round turn over each stake. They provide
additional stakes as required.
(4) Wavy. —( a ) Duties of the officer and noncommissioned
officer are as indicated for zigzag in (3) above.
(b) One private drives a nail or stake at each corner lo¬
cated by the noncommissioned officer. The other two privates
drive stakes to convert the straight portions of the zigzag
into curves, and stretch the tape along the line of stakes thus
established. About five or six stakes are required between the
corner stakes located by the noncommissioned officer to pro¬
vide the requisite sinuosity,
(5) Echelon .—The echelon trench may be traced as a zig¬
zag trench running obliquely to the front or the officer in
charge may. indicate the various comers, the detachment
then stretching the tape between them.
(6) Tracing without tape .—The same methods may be fol¬
lowed when tracing without tape as when tracing with tape,
corners and intermediate points being marked by stakes or
stones.
(7) Variations .—Other methods may be devised by an
officer tracing trenches either with or without tape. A rigid
conformity to forms and dimensions shown by figure 29 may
not always be desirable. Pull utilization of the defensive
value of terrain and of all terrain features that may serve
to conceal the trenches constructed, constantly observing
the general provisions given in b above is of much greater
importance. In case of a variation from standard dimensions,
the officer specifies to the noncommissioned officer the exact
length of fire bays, traverses, or diagonals he desires to use.
g. Standard profiles .— ( 1 ) General .—Trenches constructed
in the face of the enemy and under difficult conditions neces¬
sarily vary in profile in order to meet those conditions.
Standard profiles are prescribed for use except where unusual
conditions require modification or do not admit complete
execution. The standard profiles shown should be used in all
instruction and training. They give a comparatively wide
trench. Experience teaches that the advantage of additional
262375
77
35
ENGINEER FIELD MANUAL
protection offered by a narrow trench is more than offset by
the freedom of circulation provided by the wider trench and
by the fact that the wider trench is not easily blocked by
cave-ins. Officers and men should familiarize themselves with
these profiles so as to avoid confusion and loss of time in
constructing trenches under difficult conditions. It is under¬
stood that the exact geometrical preciseness shown in the
illustrations will never be attained in actual practice. Refer¬
ence is made to figure 20 showing terms adopted for the vari¬
ous parts of 1 a standard trench.. Standard profiles are shown
in figures 31 to 35. Development of a simple standing trench
into the A or B standard type is shown by figure 36.
(■2) Simple standing trench (fig. 31) is Unrevetted and pro¬
vides no room for circulation in rear of the firing line. It is
Figure 31.—Standard profile, simple standing trench.
the first profile sought when trenches must be constructed
rapidly, but should be developed to type A in order to provide
better cover and communication as soon as time, labor, and
material permit,
(3) Type A (figs. 32 and 33) is unrevetted (except for para¬
pet of fire trench and fire step) and therefore is the standard
for hard ground where excavated slopes will stand without
revetment. In soft ground this profile should be developed
to type B as soon as time, labor, and material permit.
(4) Type B (figs. 34 and 35) is deeper and wider than type
A, with lower half of the trench revetted throughout with
A-frame supports and the upper half unrevetted. It has
wide berms and easy slopes so as to avoid blocking of the
trench by cave-ins under shell fire.
78
FIELD FORTIFICATIONS
35
(5) Figure 37 shows a wide communication trench (type C)
used in special cases for main thoroughfares in order to pro¬
vide ample room for increased traffic, litter bearers, etc.
(6) The berms at top of A-frames (see figs. 34 and 35) may
be omitted at first if necessary to save time and labor, and the
ground above sloped as shown by broken lines in the drawings.
These berms however should be constructed as soon as oppor¬
tunity affords in order to prevent the eartli above from falling
into the bottom of the trench and blocking the drain.
Figure S3.—Standard profile, communication trench, type A.
(7) The height of parapet adopted as a standard for fire
trenches should be varied to suit conditions of terrain requir¬
ing more command, or where ground water or hard rock is
encountered making excavation of the trench to full depth
79
81
FIELD FORTIFICATIONS
35
impossible. In such a case profile of the trench except for the
parados will remain the same, all parts of the trench bearing
the same relation to each other, although the distance of any
particular part above or below ground level will vary from the
distance shown in figures.
ft. Intersections. —An approach trench should enter a par¬
allel at the rear of a bay. If it continues beyond the parallel
Ficore 37.— Standard profile, wide communication trench, type C.
it should leave it at a point not less than 25 yards from
the point of entrance. Trace of the parallel between these
two points should be modified to provide easy communication
(see fig. 38). A better solution is to provide two independent
Figure 38.—Approach crossing parallel, octagonal trace.
crossings as shown in figure 39 in order that traffic may
not be blocked if one of these crossings is destroyed.
i. Intrenchment in presence of enemy .—When deliberate
trench construction is subject to interference by enemy fire,
the work must necessarily take on some aspects of hasty
81
35
engineer field manual
intrenchment. The trench will frequently have to be traced
and dug at night. If it cannot be completed during a single
night it should be dug to sufficient depth during the first
night’s work to afford adequate shelter for day working
parties. Some work should be done all along the line and
sections at intervals should be completed. The day working
party extends these sections by sapping. As only one man
can work in a heading at one time, he should work as rapidly
Figure 39.—Approach crossing parallel, octagonal trace, alternate
method.
as possible and should be relieved frequently. Progress may
be expedited by excavating in two stages, the first stage being
construction by sapping of a trench similar to the standing
slit trench (see fig. 25 ©) and the second stage involving
completion to profile dimensions. Trenches may be advanced
by sapping methods from 1 to 4 linear feet per hour. When
combat is imminent in order that working parties may not
82
FIELD FORTIFICATIONS
35
be too fatigued to engage the enemy with effect, reliefs of
not more than 4 hours should prevail.
j. Planning and reconnaissance. —(1) Deliberate intrench-
ments should always be carefully planned. Planning exten¬
sive intrenchments presupposes prior determination of their
general location, extent, and mission. These points are
properly covered in orders issued from higher headquarters
of combat units to the officer responsible for construction
of the works.
(2) The officer directed to take charge of construction of
intrenchments is normally an engineer for extensive or
complicated trench systems out of contact with the enemy,
and an officer from the combat unit that is to do the work
for lesser undertakings. Upon receipt of orders to proceed
with the work he should visit and reconnoiter the site by day
if possible. He should take with him a party to assist in
tracing the works and later to serve as guides for working
parties, if required. Preliminary reconnaissance having been
completed, the intrenchments should be traced. When they
cannot be traced completely by day it is frequently possible
to go over the ground and mark controlling points. Loca¬
tions of these points and length and direction of major
lengths of the trenches to be constructed should be tied in to
landmarks by azimuths and distances.
C3) Data determined as a result of reconnaissance should
be—
(a) Sketch or overlay on an existing map or a position
sketch showing general location, length, and azimuth of the
principal straight stretches of the trench and relation of
controlling points along it to existing works.
(b) Decision as to profile to be used.
(c) Estimate of nature of soil and proper proportion of
tools.
(d) Estimate of total amount of material to be excavated.
(e) Estimate of time, men, tools, and materials required.
(/) Decision as to necessity for a covering force to protect
the laboring force.
k. Estimates of time, labor and tools. —(1) If length
of the general trace of a trench line is known, actual length
83
35
ENGINEER FIELD MANUAL
of trench to be dug may be found by multiplying by the
appropriate coefficient, as follows:
Type of trace
Coefficient
1.09
1.07
1 1.08
1.11
1 This will vary considerably with tho actual trace.
(2) Table XIII gives figures for use in estimating time,
labor, and tools required for trench construction. It is appli¬
cable to day work by inexperienced men using pioneer tools.
The figures given represent the best performance that can be
anticipated from large groups of soldier labor.
Table XHI.— Day work, single relief, using pioneer tools 1
Number cubic feet of excavation per man in—
Soil
1 hr. |
2 hr.
3 hr.
4 hr.
5 hr.
6 hr.
7 hr.
8 hr.
Hard »___
15
24
32
40
47
54
61
67
Average____
23
37
49
00
71
81
91
100
Light 1 .--
30
30
66
80
U4
108
121
133
1 This tabic contemplates a rest of 10 minutes every hour after the first hour.
2 All must be loosened with a pick. Requires 2 picks to 1 shovel.
2 Requires little or no picking. Requires 1 pick to 2 shovels.
(3) Estimates must be reduced from the totals indicated
by table XIII for night work, rain or other unfavorable
weather conditions, and annoyance by enemy fire, in the
discretion and based upon experience of the estimating offi¬
cer. Night work i$ about two-thirds as effective as day
work. Night work should be planned to take full advantage
of moonlight hours and duration and phase of the moon and
probable cloud and weather conditions should be considered
carefully when preparing estimates.
84
FIELD FORTIFICATIONS
35
(4) A 5-foot length of trench (measured along the front
edge of the trench) is a standard task for one man; the men
should be spaced so that they can work efficiently and safely.
In special cases when speed is essential and daylight work
is possible, men may be spaced at 3.5-foot intervals. The
men then should work at right angles to the axis of the
trench as much as possible.
(5) If ample tools are available each man should be issued
a pick and shovel. Otherwise, picks or shovels should be
issued to men in the ratios indicated by table XIII, depending
on nature of the soil.
(6) Experience teaches that soldier labor and untrained
men in general become excessively tired after more than
4 hours’ continuous work, and their output thereafter dimin¬
ishes very rapidly. Men may be worked in one or in two
or more reliefs. In the first case men work continuously
with 10 minutes’ rest out of every hour. In the second case
there are two or more complete crews which relieve each
other at intervals of several hours. A given piece of work
may be completed more quickly using two reliefs, but com¬
pared to a single relief is not economical in manpower. The
method of “doubled crews" may also be used. Here the men
work in pairs; one man works as hard as he can for 4 or 5
minutes and then is relieved by the alternate man of the
pair. The efficiency of a double crew is about 10 percent
more than for one relief in light soil for 1 hour’s work and
about 30 percent more in hard soil for 8 hours. When work¬
ing without enemy interference, in a standing position engi¬
neer tools are about 30 percent more efficient than infantry
tools.
(7) With soldier labor, assignment of a definite task and
provision for relief of the working unit upon completion of
the task usually produce the best results. Under task
assignments the output for any given period of time usually
exceeds that indicated by table XTEI. Tasks should be as¬
signed by individuals, squads, platoons, or companies. As¬
signments should be such that each unit can return to camp
or quarters when its task is completed, Great care must be
exercised and the best experience available utilized in assign¬
ing tasks; once established, they should not be altered unless
85
35
ENGINEER FIELD MANUAL
found to be extremely unfair or entirely impracticable of
accomplishment. With sloldier labor, one-relief task assign¬
ments requiring approximately 4 hours for their completion
are recommended. Such a procedure results in a maximum
volume of excavation per man and permits employment of
two reliefs, for example, one in the forenoon and one in the
afternoon if work can be done during daylight hours.
(8) Experienced laborers using commercial tools which are
larger and heavier than the pioneer tools can exceed by from
50 to 100 percent the quantities given in table XIII. Such
labor should be worked an 8- to 10-hour day. Trained labor
troops to be used continuously for pick and shovel work
should be supplied with commercial tools and worked on
an 8- to 10-hour daily schedule.
(9) While deliberate intrenchments can be constructed with
the infantry tools carried on the pack, these tools are rela¬
tively ineffective as compared with pioneer tools. The use
of infantry tools on deliberate works should be avoided if
possible, but they are normally employed in connection with
hasty intrenchments.
(10) Table XIV gives time required for completion of
Type A and B trenches. It is computed from table XIII.
Table XIV.— Hours required to complete Vfe-loot and 5-foot
tasks; or if not completed, percentage finished in 8 hours
of day work, using pioneer tools
Nature of
soil
Hard_
Average..
Light.
Type A iinrevetted
Fire trench
One relief
3'A
feet
85%
5. 75
3.75
5
feet
60%
00 %
6.0
Two reliefs
3 A
feet
5
feet
7.25
5.0
3.5
Communication trench
One relief
feet
7.0
4.0
2.75
Two reliefs
5 j VA
feet I feet
75% 4.0
0. 75 2. 75
4.5 2. 0
5
feet
5.75
4.0
3.0
Type B revetted
Fire and communi¬
cation trench
One relief
3)4
feet
65%
05%
6.00
5
feet
45%
65%
85%
Two reliefs
3A
feet
7.00
|4. 5
3.5
5
feet
80 %
6.5
5.0
86
FIELD FORTIFICATIONS
35-36
(11) When preparing estimates the following points should
be kept in mind:
(a) Do all work by daylight if possible.
(5) When night work is required, do some excavation
along the entire length of the trench the first night; at least
complete section C, figure 36, which should require from 2
to 6 hours’ work for 5-foot tasks. Complete the type A
profile the first night if possible.
(c) In general, when using soldier labor, use the one-relief
task assignment system. The tasks should require approxi¬
mately 4 hours for their completion and the standard 5-foot
assignments should be employed. Use two such reliefs with
task assignments when speed is essential and plenty of men
are available.
(d) Use the 354-foot assignment only in case of emergency.
(e) Increase estimates for labor and tools by at least 5
percent to allow for contingencies.
(/) Request covering force, antimechanized, antiaircraft,
and gas warning personnel for protection of laboring force
when presence or activities of the enemy require it.
■ 36. Drainage. —a. General, — (1) Drainage of trenches is of
the Utmost importance. Improper drainage not only may
have serious effects on health of the occupants but may
interfere so much with use of trenches as to contribute ma¬
terially to the failure of military operations.
(2) The question of proper drainage must be considered
when siting trenches, during their construction, and also
during their occupation. A small amount of forethought may
later save a large amount of labor.
(3) Drainage plans should be prepared before construction
is commenced. If this has not been done they should be
prepared by the officer commanding the occupying troops
as soon as possible. Drainage plans should be prepared and
the work these plans call for completed during dry periods
so that the trenches may be of use during wet weather.
(4) Surface water must be excluded. It is much easier to
prevent its flow into the trench than to remove it once it
has gained access.
87
36
ENGINEER FIELD MANUAL
(5) Water may be disposed of by—
(a) Drainage ditches that conduct the water by natural
flow into nearby drainage lines.
(b) Sumps that either penetrate permeable soil and dis¬
pose of the water by percolation or collect the water for
disposition by pumping, siphoning, or bailing. Combination
of sump for collecting the water over a drill hole penetrating
permeable soil, and disposing of same by percolation through
the drill hole.
(6) Sumps should never be installed when the water can be
disposed of by drainage ditches of reasonable length.
<7) A moderate slope of 1 percent, that is, 1 foot in 100
feet or 0°35', is desirable. If the slope is excessive, damage
by erosion may result.
(8) The eye cannot be relied upon to determine differences
of level in connection with drainage. A carefully adjusted
clinometer, spirit level, or in important cases, a surveyor’s
level should be used.
(9) Presence or absence of underground water when in
doubt should be determined by digging test pits before
starting construction of extensive field works. In some lo¬
calities it is so abundant or so .close to the surface as to
require use of high parapets or breastworks instead of
trenches. Except in such localities ground water is unlikely
to interfere with the works discussed in this section. With
respect to construction of deep cave shelters, however, the
subject is more important.
b. Sites. —(1) Trenches should not be located in marshy
or boggy ground unless it is absolutely unavoidable.
(2) Communication trenches should not be located in the
bottom of valleys or gullies but should be placed along the
side slopes.
(3) Special care must be taken to site trenches so that
all parts can be easily drained toward naturally low points
in the line, avoiding any unnecessary sag which will be
difficult to drain later (see fig. 40).
(4) Never site a trench exactly along a contour, but give
it enough slope to carry the water in the desired direction
as shown in figure 40.
88
FIELD FORTIFICATIONS
36
c. Construction. —(1) Plan for drainage from the begin¬
ning. First, start the drainage ditch or sump, always keep¬
ing progress of its excavation in advance of that of the
trenches draining into it.
Figure 40.—Siting trenches to eliminate low spots.
(2) Complete excavation progressively upgrade when prac¬
ticable so as to avoid formation of undrained pockets in the
bottom of a partially completed trench.
(3) Gutters or intercepting ditches should be provided
on the uphill side of trenches for collecting surface water
and conducting it to a natural watercourse. The parados
sometimes may be made to serve this purpose. If necessary,
these surface ditches are carried over or under the trenches
(see figs. 41 and 42).
d. Maintenance of system. —(1) Maintenance of the drain¬
age system is the duty of the troops occupying the sector.
Responsibility for maintenance of drainage of all trenches
89
36
ENGINEER FIELD MANUAL
within a given area devolves upon the commander of that
area. , This responsibility includes communication trenches,
the drainage of which is as important as that of occupied
trenches and is more apt to be neglected.
12 ) In a sector which is to be occupied for sometime, a small
force should be detailed to inspect and maintain the drain¬
age system, It should be charged with keeping all drains
and sumps clear, including the intercepting ditches for
surface drainage.
t3) AH caving.surfaces must be repaired, and earth thrown
into trenches by shell explosions must be removed at once.
Constant attention to this matter will save a large amount
of labor later, as water backing up in a trench will cause
additional caving.
Figure 42.—Surface water carried under trench.
(4) In level country sign boards marked drain and bear¬
ing an arrow showing direction of flow should be placed at
appropriate points in trenches in order that this direction
may be known when cleaning Out ditches.
(5) An excellent way to determine measures necessary for
maintenance of the drainage system is to inspect it during
or immediately after a rain.
e. Flooring. —(1) Trench boards <flg. 43) when available
should be placed in the bottom of all trenches, except on
rock. In firm ground they may be laid without special
supports, or merely with the ends supported on short boards
laid transversely across the bottom of the trench. In soft
ground they should always be supported on trestles (fig. 44)
or on the A-frames used for revetting (see figs. 47 and 48).
Trench boards are lightly fastened down as they must not
90
FIELD FORTIFICATIONS
36
be continually getting out of position on the one hand, and
must not be too difficult to raise when cleaning out drains
on the other.
r
-/'-S'
Outer edge of board
at 4" from fnd p}_
niiEi
1,„
2‘*4
:J*
1
^t.l £ I mgamca rgrn m ga ™
Figure 43.—Trench board.
(2) Serviceable trench flooring may be improvised from
planks or boards of usual dimensions or from trunks of
small trees.
Figure 44,—Trench board trestle.
(3) In rock or very hard ground where trench boards are
not required, the bottom of the trench should as far as prac¬
ticable be crowned as shown in figure 45. Undercutting of
Figure 45.—Trench without flooring.
the side slopes in order to secure space for side ditches should
never be permitted.
). Sumps ,—Sumps should if possible reach porous strata.
Their efficiency for disposal of water by percolation can
sometimes be greatly increased by drilling a hole and shooting
91
36-37
ENGINEER FIELD MANUAL
a small charge of explosive some distance below the bottom
to loosen up the ground. They can be located underneath
the trench boards or in an offset.
g. Pumping and bailing. —(1) Pumps are sometimes neces¬
sary for removing water from sumps. Classes of pumps fur¬
nished complete with suction and discharge hose usually
available are—
(a) Hand-force, to be operated inside the trench by one
or two men.
(hi Hand-suction, to be operated outside the trench by
one or two men; limit of suction approximately 16 feet.
(c) Gasoline-driven force, for use in special cases; limit
of suction approximately 20 feet.
(2) Bailing scoops or galvanized iron buckets may be used
for bailing out sumps.
■ 37. Revetments. — a. General. — (1) A revetment is a re¬
taining wall or facing for maintaining earth slopes at a
Steeper angle than their natural angle of repose.
(2) Revetments may be classified as—
(a) Retaining tvall type, which is self-supporting and acts
on the gravity principle for retaining walls. It is largely
used in connection with fills, parapets, and breastworks.
<b) Surface or superficial type, which must be supported
and acts largely by protecting revetted surface from disinte¬
grating and erosive effects of weather and from abrasion due
to occupation of trench. When strongly constructed it may
also serve to retain loose materials and prevent settlement. It
is used principally in cuts.
(3) A good revetment must possess—
(a) Simplicity of detail.
(b) Adaptability to available materials.
(c) Ease of fabrication and erection.
<cf) Low fragmentation under shell fire.
(e) Ease of removal from obstructed trenches.
(4) In general, revetments should be constructed only
when obviously required and should not be more extensive
than necessary. They require a great deal of labor and
material and should therefore be avoided if possible. On the
other hand, during long occupation of trenches and when
92
FIELD FORTIFICATIONS
37
time, labor, and materials are available, extensive revetments
make the garrison more comfortable and reduce maintenance.
(5) If sides of the excavation are carefully sloped the
amount of revetment required is greatly reduced. Unevenness
causes rain to lodge in or erode the surface, to soak into the
earth, and results in rapid disintegration of the slope. The
steeper the slopes of a trench the greater is the need for
revetment to prevent them from caving.
(6) The interior slope of the parapet of fire trenches (not
the entire front slope) is always revetted. In addition, only
the lower 2 or 3 feet of a trench usually require revetment.
Pbr standard profiles as shown in figures 32, 34, and 35, this
involves revetment to the level of the fire step or lower berm.
Such revetment supports portions of the trench subject to
the most wear, preserves drainage, is seldom injured by the
enemy’s fire, and preserves the profile of the bottom of the
trench for clearing after a cave-in.
b. Retaining watt types .—As these revetments must be self-
supporting they should always take the form of a properly
built retaining wall. The thickness at any level should be
at least one-half the remaining height and the average thick¬
ness not less than one-third of the total height. Amount of
excavation required for retaining wall types is considerably
greater than for surface types.
(1) Sandbag .—This type is easily and quickly constructed,
does not splinter from shelling, and is especially useful for
emergency work, for repairs, crowning, and revetting the
interior slopes of parapets.
(a) The standard sandbag is 14 by 2614 inches flat, with
an attached tie string 3 inches from the top of the bag.
When filled three-fourths full, each bag weighs from 45 to
75 pounds depending upon material and whether it is wet or
dry, averaging approximately 65 pounds, and fills a space
approximately 4% by 10 by 19 inches. Thus 10 linear feet
of parapet revetment for type A fire trench, as shown in
figure 32, requires 24 sandbags, and 10 linear feet of revet¬
ment for front slope and parapet of Type B fire trench, as
shown in figure 46, requires 132 sandbags.
262375°—4<
7
93
37
ENGINEER FIELD MANUAL
(b) When laying sandbags attention should be paid to the
following points:
1. Fill bags uniformly about three-fourths full.
2. Build revetment at slope of from 3 on 1 to 4 on 1.
3. Lay bags perpendicular to Slope.
4. Lay bottom row headers on prepared bed. Alter¬
nate intermediate rows as stretchers and headers
and complete with a top row of headers.
5. Lay bags with seams and choked ends inward.
Figure 46.—Sandbag revetment.
6. Break joints. Beat bags into a rectangular shape
with the back of a shovel and tuck corners of
bags in when placing.
7. A sandbag revetment will last much longer if wire
netting, preferably doubled, is placed oveT the face.
(2) Sod is more durable than sandbag revetment, and
its use is recommended where sods can be obtained In suffi¬
cient quantity. Sods are cut 18 by 9 inches, laid grass down
except the top layer, and pinned together with wooden pegs.
The provisions given above for sandbag revetment apply.
94
FIELD FORTIFICATIONS
37
(3) Stones and bricks may be used foi - revetment in the
form of retaining walls laid dry, in which case a slope of
not steeper than 4 on 1, laid at right angles to the face, and
broken joints are especially important. Due to danger from
flying splinters in case a stone revetment is hit by a shell, its
use is not recommended where other material is available.
If used in a parapet it should always be crowned with earth-
filled sandbags.
c. Surface types. —(1) General .—This form of revetment
consists of two parts, the revetting material which retains
—
O
STRAP IRON Cx |V‘x 4* 7"
IF AVAILABLE
END ELEVATION
Figure 47. —Standard A-frame.
the earth, and the supports which hold the revetting ma-
terial in place. It is most useful in retaining slopes of
trenches since little additional excavation is needed.
(2) Supports may be standard A-frames, pickets, or struts.
(a) A -frames .—Standard A-frames are used in the bot¬
tom third of the trench, placed 3 feet center to center and
support material described above (see figs. 47 and 48).
Methods of placing these frames around corners in octagonal
trenches are shown in figure 49.
95
37
96
FIELD FORTIFICATIONS
37
An estimate sufficiently accurate for all practical pur¬
poses of the number of A-frames and trench boards required
for any given frontage of trench may be made as follows:
1. A- frames.
Octagonal trace—
[frontage (In yards) +10 percent (to convert frontage to
length of trench) +10 percent (to provide for angles in
trench) ]
Zigzag, wavy, and echelon traces =
frontage (in yards) +10 percent
2. Trench boards.
Single walkway=
Number A-frames
2
Double walkway=
Number A-frames.
Thus a frontage of 1,000 yards of octagonal fire trench type
B, using standard A-frames and single walkway requires:
A-frames = [1,000+1001'+10 percent = 1,100+110 =
1 , 210 .
Trench boards= ^y^=605.
Por the same frontage of wavy, communication trench, type
C (wide communication trench), using special A-frames and
double walkway:
A-frames=l,000+100=1,100.
Trench boards= 1,100.
(b) Pickets .—If A-frames are not available and the soil
permits, the revetment may be supported by means of pickets
driven into the bottom of the trench and braced, as shown in
figure 50. Anchored pickets are used when revetting front
slopes of fire trenches (fig. 51), or in other special cases where
high revetment is required. Revetting pickets should be from
2 to 3 Ys inches in diameter, straight, pointed at the small end,
and driven into the ground from 1 to 1% feet. Light angle-
iron pickets may also be used. Lateral spacing of pickets
varies with soil and character of working materials. Por
example, burlapped chicken wire requires more closely spaced
supports than sheets of corrugated iron. Similarly, sand
requires more support than firm clay. Spacing may vary
97
37
ENGINEER FIELD MANUAL
Figube 50.—Braced revetting pickets.
Figure 51.—Method of wiring and anchoring revetment.
98
FIELD FORTIFICATIONS
37
from V/2 to 6 feet. Anchor pickets should be driven firmly
into solid ground 8 to 10 feet from the edge of the trench,
staggered so as to avoid forming a plane of weakness parallel
to the trench, and inclined so as to be perpendicular to direc¬
tion of pull on the anchor wire. Anchor wires, preferably No.
14 American wire gage, should pass at least four or five times
between picket and stake. Each strand should take a round
turn around the head of the anchor picket and then the
strands should be twisted together with a short stick to tighten
them.
(c) Struts resting against opposite walls of a trench are for
use only in narrow, deep trenches in which there is little cir¬
culation as in shell slits for protection against shell fire as
shown in figure 26.
(3) Revetting materials may consist of expanded metal,
wire netting (chicken wire), corrugated iron sheets, brush¬
wood hurdles, burlap, canvas, poles, brush, or lumber, or com¬
binations of these materials, depending on the materials
available and nature of the soil.
( a ) Expanded metal and wire netting are used most effec¬
tively either alone or in combination with burlap, canvas, or
similar materials. The burlap checks evaporation and pre¬
vents disintegration and erosion, and the metal or netting
supports the burlap. If burlap or similar material is not avail¬
able, grass, leaves, twigs, etc., may be substituted with good
results. In placing expanded metal or wire netting revetment
(fig. 52) the following operations are necessary:
Cut vertical grooves for the anchor pickets throughout
length of bay at spacing decided upon.
Drive the two end pickets of each bay first and anchor
them back loosely.
Stretch the metal or a double thickness of netting behind
the two end pickets, holding it taut until these pickets are
pulled into their grooves by tightening the anchor wires.
Drive remaining pickets and anchor them back, thus draw¬
ing revetting material tight against surface to be revetted.
(b) A continuous brush revetment of the superficial type
may be constructed by driving pickets from 2 >4 to 3 inches in
diameter at about l-pace intervals along the face of surface
to be revetted and about 4 inches from it. Tops of pickets
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37
ENGINEER FIELD MANUAL
should project above the ground. The space behind the pickets
is then packed with small straight brush laid parallel to the
surface and held in place by the pickets which are drawn back
firmly by means of wire and anchor pickets.
d. Brush work. —(1) In practically all wars brush has
been used extensively in revetment and improvement of
earthworks. While its value in modern warfare is not always
commensurate with labor and skill involved in its use, its
flexible utility and ready availability in forested areas will still
result in its extensive use in the future. At times it may be
the only material available. It may be used as continuous
revetment already described, or as hurdles, gabions, fascines
as described below; or any combination of these.
Figure 52. —Constructing wire mesh revetment.
(2) Almost any kind of brush reasonably straight, tough,
flexible, and free from refractory branches, dangerous thorns,
or other objectionable characteristics is suitable, but willow,
birch, ash, hickory, hazel, and similar woods are desirable.
Split bamboo of pliable dimensions, reeds, and vines are also
valuable. Brush for weaving should not be more than an
inch in diameter at the butt. That to be used without
weaving may be of larger size. When cut, brush should be
assorted in sizes for the various uses and made up in bundles
weighing 40 to 60 pounds, the butts in one direction. Poles
of 2 Vz inches diameter at the butt or larger end are not
bundled but are piled together. They are used for posts.
100
FIELD FORTIFICATIONS
37
pickets, struts, binders, grillage, and similar purposes. It is
frequently best to fabricate hurdles, gabions, or fascines at
the point where the brush is cut, later transporting finished
brush work to the point of use.
(3) A brushwood hurdle is a woven revetment unit, usually
6 feet long and of the required height (see fig. 53). It is
constructed on sharpened pickets which are driven 18 inches
into the ground and then moved to location in the trench.
PICKETS ABOUT
plain wire With
A TURN AROUNO
EACH PICKET
RFWN IM ANO TWISTED
EWN N UNTIL TIGHT
AW
BRUSHWOOD V TO l" OIA. BUTTS ALTERNATING
TIME TO CONSTRUCT UNIT SHOWN I* MAN HOURS
Figure 53.—Brush hurdle.
(4) A gabion (fig. 54) is a cylindrical basket with open
ends made of brush woven on pickets.
(5) (a) A fascine (fig. 55) is a cylindrical bundle of brush
closely bound. The usual length is 18 feet and the diameter
9 inches when compressed. Lengths of 9, 6, and 3 feet when
needed are conveniently obtained by sawing a standard
fascine into pieces.
(t>) When a large number of fascines is required and brush
is plentiful at one point, a portable frame as shown in figure
56 may be constructed.
101
Figure 54.— Brush gabion.
wear much better than sandbags. Fascines may also be used
in connection with footings for gabions and as a crown for
gabion, hurdle, or other types of revetment. Figure 57 shows
a gabion and fascine breastwork topped with sandbags.
e. Revetting materials carried in engineer supply establish¬
ments. —Sandbags, corrugated iron, expanded metal, metal
FIELD FORTIFICATIONS
37-38
lath, wire netting (chicken wire), pickets (including light
angle-iron pickets), and binding wire are normally carried
in the engineer sections of army depots, corps parks, divi¬
sion dumps, and division distributing points.
■ 38. Breastworks. —If need for additional command or
presence of water, rock, or very hard material makes con¬
struction of standard trenches impracticable, breastworks
must be constructed. Profile of the breastworks should ap-
Figure 55.—Brusli fascine and cradle for fabrication.
proximate the standard trench profile as nearly as possible.
A parados should be constructed to protect against the back
blast and fragments from shells. In some cases the entire
protection above the fire step may be constructed in full
and be revetted as shown in figure 58. Angles are provided
as in standard trenches. Careful provision for drainage in
wet soil may greatly reduce the height of the breastworks
and consequently the labor required for their construction.
103
38
ENGINEER FIELD MANUAL
FIGURE 56.—Portable frame for compressing fascines.
104
FIELD FORTIFICATIONS
39
■ 39. Accessories.— a. General .—Any special work or facility
designed to increase usefulness of an intrenched position or
to provide for the safety, comfort, health, or convenience of
the garrison is called a trench accessory. Continued occupa¬
tion of a trench system results in a great demand for and
rapid multiplication of such accessories. Types of frequently
figure 57. —Gabion and fascine breastwork topped with sandbags.
constructed accessories are described below. Trench acces¬
sories are ordinarily built by the troops occupying a position.
b. Observation posts. —(1) Observation posts vary widely
from very simple ones in open warfare to rather elaborate
structures in position warfare. In any defensive position
they also vary in simplicity, depending on their location with
105
39
ENGINEER FIELD MANUAL
106
FIELD FORTIFICATIONS
39
respect to the enemy. They may be located well forward for
surveillance of a limited portion of the enemy’s position or
well to the rear where a considerable area may be kept under
observation.
(2) The first requisite of a good observation post is a
clear view of the area or position it is desirable to keep
under observation. Immunity from enemy fire is the next
consideration. This may be secured by concealment which
is always of paramount importance. If time and material
are available it may be desirable to secure additional im¬
munity by providing cover so that personnel may occupy
the post in safety during a general bombardment. Impor¬
tance of nullifying the defenders’ observation is so great
that the attacker will use every means to attain this end.
If an observation post is discovered it is almost sure to be
shelled prior to an attack. Unless it affords sufficient cover
to withstand such a shelling its usefulness ceases. Inasmuch
as it will not often be practicable to provide such cover,
importance of concealment becomes still more evident.
(3) In the first stages of a defense, observation posts will
generally be located in existing structures or terrain features
where concealment and as much protection as possible may
be secured. Thereafter as time is available for such work,
such locations may be strengthened and shelter built to
increase the comfort and security of the personnel occupying
the post. Suitable means of signal communication between
observation posts and command posts must be arranged. As
complete stabilization takes place, special structures for use
as observation posts may be erected. These are of two general
classes, periscope and direct view, some of which are described
below.
(a) Periscopes may be disguised as trees, stumps, telephone
posts, etc. For small trench periscopes advantage is taken
of any natural object such as a stump or broken stick pro¬
jecting from the parapet to provide concealment. Periscopes
should be installed at night and all precautions must, be taken
to prevent the enemy suspecting their existence. Serviceable
periscopes may be improvised when necessary by using two
mirrors and pieces of light box material.
107
39
ENGINEER FIELD MANUAL
(b) Small direct view observation posts may be located
either in the trenches themselves or in saps leading from
them. There are two general types, cabin and portable.
A cabin observation post is shown in figure 59.
Figure 59.—Cabin type observation post.
(c) Portable observation posts may be either armored or
unarmored. Figures 60 and 61 show two bullet- and splinter-
proof types. Unarmored types may be readily improvised
from various kinds of material. They are frequently made
FIELD FORTIFICATIONS
39
to be set into the parapet of a trench. Affording no protec¬
tion. they must be entirely inconspicuous.
(4) Observation posts are frequently located to give an
oblique view to the front which makes it more difficult for
am enemy observer directly opposite to locate them. Usual
reasons for discovery of a post of this kind are either trails
to it or presence of a number of men in its immediate vi-
Figdre 60. —Portable armored observation post.
cinity. It may be advisable to camouflage the entrance to a
parapet observation post so that the observer will not be
caught by raiding parties and will have a greater feeling of
security and therefore function more efficiently.
(5) In addition to these forward observation posts, very
elaborate posts to the rear are sometimes employed in suit-
Ficohe 61 . —Roll-top steel observation post,
able terrain. They are located so that they command an ex¬
tensive view of the enemy's position and are usually far
enough to the rear to be safe from minor attacks. They are
provided with excellent observing instruments and have tele¬
phonic connection with the higher headquarters. Such em¬
placements are built very solidly, often in the manner of con-
262375 0 —40-8
109
39
ENGINEER FIELD MANUAL
Crete machine-gun emplacements (see par. 48e (4)). Care
should be taken to prevent their location becoming known
to the enemy.
c. Command posts. —(1) In a hasty position a command
post may consist merely of a trench to contain personnel if
shell holes, natural protection, or buildings are not available
(see pars. 48d (9) and 64).
(2) A simple trench may be made splinterproof for use
as a command post by covering it with a layer of logs and
at least 1 foot of earth as shown in figure 62.
(3) The trench itself is not of sufficient size for continued
occupancy as a command post and in order to facilitate
Figure 62.—Splinterproof trencli for command post.
movement at the command post splinterproof compartments
may be constructed in the side of the trench (see fig. 63).
d. Sniper’s posts. —(1) Sniper’s posts are located and used
by specially detailed men. The posts should be marked
plainly and all persons except snipers forbidden to use them.
As a rule they are located within 400 yards of the enemy.
(2) Requirements of a sniper’s post are—
(o) Concealment from enemy observation.
(b) Good view and field of fire over a designated section
of the enemy lines.
(c) Two loopholes are desirable but not essential, one
for the rifleman and one for an observer.
110
Figure 64. —Sniper's post.
ill
39
ENGINEER FIELD MANUAL
( d ) Curtain behind each loophole and a stopper or “gag”
to be pushed into each loophole from the inside.
(3) Figure 64 shows a portable sniper’s post which can be
shifted readily from one location to another in a night.
e. Ammunition and grenade boxes .—Boxes let into the
trench walls to contain reserve ammunition and grenades
are desirable in fire trenches and machine-gun emplace¬
ments. Boxes should be placed in the front wall of the
trench or beside the loader of an automatic weapon, and
to avoid interference with use of the trench should be
flush with the surface of the ground. They should be
covered with roofing paper to make them watertight and
should be provided with gasproof curtains.
/. Signs. —(1) Direction signs are as necessary in compli¬
cated trench systems common to stabilized situations as
road signs in unfamiliar country. The engineers are charged
with naming trenches and preparing and posting signs. A
good system is to name works, sectors, and their subdivisions
with the names of cities in the United States and have the
position of these elements bear the same general relation
to each other as these cities do.
(2) Main parallels and main approaches should bear the
same names throughout their length, and approaches should
be designated as in or out trenches.
(3) A good size for signs is 24 by 12 inches. The letters
should be large and heavy enough to be read at a distance.
A white background with black letters is most effective.
(4) For night use luminous paint should be employed
when available. Lighting trench signs is usually imprac¬
ticable.
(5) Beacons and red lanterns shielded from enemy ob¬
servation may be used on roads or main routes of approach,
but generally cannot be used in the forward areas.
g. Latrines. —(1) Latrines should be located in offsets from
trenches, usually not more than 50 yards away from and
in a place convenient of access for the men who will use
them. They should not be located in dugouts, except in
very large ones where special provision is made for ventila¬
tion. They should not be located near points which are
likely to draw fire.
112
FIELD FORTIFICATIONS
39
(2) Accommodation both in latrine and urinal facilities
should be provided for at least 4 percent of the command.
If a trench system is to be occupied a considerable time,
facilities should be provided for at least 8 percent of the
command.
Figure 65 .— Deep latrine, straddle type.
(3) Types. — (a) Bucket .—Any type of bucket or can pro¬
vided with a seat and cover may be used. The buckets are
placed in an excavation from which they can be removed
easily, and a routine arrangement made for emptying them.
113
39
ENGINEER FIELD MANUAL
This system is used only where deep latrines cannot be
constructed.
< b) Deep .—The straddle (fig. 65) and the boxseat <fig.
66) are the usual types. Pits should be dug 6 to 8 feet deep
and when filled to within 2 feet of the top should be com¬
pletely filled with earth and a new latrine dug.
Figure 66. —Deep latrine, boxseat type.
(4) Urine troughs or tubs should be provided in every
latrine; troughs may be made easily of standard corrugated-
iron sheets.
ft. Ladders and steps. —(1) Ladders. —(a) Ladders form
the most satisfactory means of exit from trenches for the
114
FIELD FORTIFICATIONS
39
purpose of attack. Permanent ladders 5 feet long and in
sufficient numbers for patrols are ordinarily fixed to the front
wall of a trench.
(b) In preparation for attack a large number of loose
ladders should be placed in the parallels of departure. When
the attack is about to start these may be supplemented by
taking up trench boards and using them as ladders after
knocking off alternate crosspieces.
(2) Steps .—To permit travel over surface of the ground
at night, ladders, steps, or ramps should be provided along
the communication trenches throughout the position. Steps
Figure 67. —Sortie ladder and steps.
may be cut into the earth and revetted. Figure 67 shows
simple types of sortie ladders and steps.
i. Telephone lines. —(1) Erection, maintenance, and repair
of telephone lines other than organizational wire are duties
of the Signal Corps. In trenches the two methods of install¬
ing wire generally used are—
(a) Wire trenches constructed for this purpose.
(b> Installation in regular fire and communication
trenches.
115
39
ENGINEER FIELD MANUAL
(2) Wire trenches vary in size from 10 inches wide by 10
inches deep to 36 inches wide by 30 inches deep, a usual
size being 18 inches wide at the bottom and 30 inches deep.
These trenches for stringing field telephone wire are left
open. They offer considerable protection from shell fire
and render wires easily accessible for repair. They have
the disadvantage of offering an obstacle to friendly traffic
and are easily visible to the enemy. The field wire may
be fastened to cross arms on short poles or the cross arms
may rest against or project into sides of the trench without
poles. Instead of using cross arms the wire may be fastened
to the sides of vertical poles. Field wire may be fastened
directly to these supports with wire or marline but it is
preferable to use insulators.
(3) When it is impracticable to construct wire trenches,
field wire may be installed in fire or communication trenches.
Ordinarily wire should be kept on the side of the trench
nearest the enemy and from 10 to 30 inches from the
bottom of the trench. The field wire may be fastened to the
side of a trench by—
(a) Stiff wire staples about 12 inches long driven into side
of trench.
(b) Insulators mounted on boards which are fastened to
sides of trench by rods or staples.
(c) Insulators on wooden stakes driven into side of trench.
(d) Insulators on posts driven into bottom of trench close
to the side.
(e) Attaching wire to revetment posts either directly or on
insulators.
(4) Lead covered cables are laid along the bottom of
specially dug trenches and the trench filled in. Such trenches
about 8 feet deep effectively protect against most shell fire
but involve a great deal of labor. Lead covered cables may
also be laid in communication trenches from 2 to 3 feet below
the bottom of the trench. As a temporary measure cables
may be strung along sides of trenches.
116
FIELD FORTIFICATIONS
40-41
Section VI
OBSTACLES
■ 40. Clasification. —a. Obstacles are natural or artificial.
(1) Examples of natural obstacles are mountain ranges,
streams, lakes, swamps, thick underbrush, and tropical jungle.
(2) Artificial obstacles are either—
(a) Fixed, examples being high, double-apron, and low wire
entanglements, abatis, and inundations or
(b) Portable, examples being concertinas, chevaux-de-frise,
and gooseberries.
b. From the tactical viewpoint obstacles are either tactical
or protective.
(1) Tactical obstacles may be either natural or artificial,
and are required to break up the enemy's attack formation
and hold him in areas which are covered by intense defensive
fires, particularly of automatic weapons. For this reason their
location is usually determined by the defensive fires, especially
the final protective line of machine guns.
(2) Protective obstacles are usually artificial and prevent
the enemy from delivering a surprise assault from areas close
to a position. They must be near enough for adequate sur¬
veillance by day or night and far enough away to prevent
the enemy from lying beyond the obstacle and bombing the
position with hand grenades, Thus they should be between
30 and 100 yards from the trenches. Skillful location may
permit an obstacle to serve both tactical and protective
missions.
c. Obstacles may also be classified with respect to what they
are expected to stop, as against foot troops, horse cavalry, and
track-laying and wheeled vehicles. Obstacles against track
and wheeled vehicles are covered in PM 5-30. This section
deals principally with obstacles against foot troops and horse
cavalry.
■ 41. Basis of Location, Design, and Construction.— a.
Cover throughout by fire of the defense.
b. Protective obstacles under observation at all times.
c. Deny enemy ground which might offer him shelter.
d. Concealed by taking advantage of natural irregularities
of ground and natural growth.
117
41-42
ENGINEER FIELD MANUAL
e. Provide for all around protection.
f. Avoid regular, geometric lay-out that discloses location
of position and its elements.
g. Use belts from 4 to 10 yards wide separated by intervals
of from 15 to 40 yards.
h. Barbed wire entanglements are the most nearly ideal
of the artificial obstacles.
■ 42. Barbed Wire.— a. Materials .— (1) Barbed wire is obtain¬
able in various styles. The standard is the familiar two-wire
type of No. 12, A. S. & W. gage wire with four-point barbs,
spaced approximately 4 inches apart. The length on a com¬
mercial reel as shipped from the factory is about 420 yards,
and the weight of a full reel about 100 pounds plus weight of
the reel, about 5 pounds. Reels of about one-half the fore¬
going size are also obtainable, and are desirable. Hand bob¬
bins are usually made up at the rear from the large size
reels, contain 30 yards of wire, and weigh from 8 to 9 pounds
each. They are always used when constructing entanglements.
(2) To make bobbins, secure 1-inch round or square sticks
approximately 2V 2 feet long. When necessary round off the
ends to facilitate handling. It is desirable to drive eight-
penny or tenpenny nails through the stick about 8 inches
from each end. This type of bobbin stick and an alternative
type, which takes more time and material to make but is
easier to handle, are shown in figure 68. Improvise trestles,
pass a pick handle, piece of pipe, or other suitable article
through the reel, and place in the trestle so that the wire may
be unreeled from the bottom (see fig. 68). Two men work at
making bobbins, one at the bobbin and one at the reel, alter¬
nating duties from time to time. The bobbin man fastens
the end of the wire to one of the nails in a bobbin stick and
draws the wire out over a measured distance of 30 yards. The
man at the reel controls movement of the reel by hand or by
braking with a stick. The bobbin man grasps the stick at the
center with both hands and keeps his hands in this position
while making the bobbin. Maintaining a constant strain on
the wire, he moves toward the reel at the same time winding
the wire on the bobbin by overhand movements so that the
wire is passed alternately over and around first one end of
the stick, then the other, coming to rest on the nails as shown
118
FIELD FORTIFICATIONS
42
in figure 68. When the 30-yard length of wire, about 45 turns
on the bobbin, has been wound the wire is cut and the free
end marked with white rag or tape and secured. The result
FIELD FORTIFICATIONS
42
(3) Wooden pickets are cut in nearby woods or are shipped
from the rear. They should range in diameter from 2Vz to
4 inches, and should be cut 5 feet long for the high entangle¬
ment and 2 feet 6 inches for the low type. Pickets split from
a log by quartering should be avoided as they increase mate¬
rially the visibility of an entanglement.
Figure 70. —Angle-iron pickets.
(41 figure 69 illustrates standard types of screw pickets,
the helix permitting them to be screwed noiselessly into the
ground.
(5) Figure 70 shows standard types of angle-iron pickets.
(6) Table of wire entanglement materials .—The following
table gives data relative to standard entanglement materials:
Table XV
Material
Weight
1
Length
Number
easily
carried by
1 1 man
Weight
of man
load
!
Wooden picket:
Pounds
| Ft. In.
Pounds
Long, 3- to 4-inch diameter-
12-16
5 0 i
3 |
3G-4S
Short. 2- to 3-inch diameter_
4-8
2 6
8
32-18
Screw picket:
Lunff.--—-
9
4 10 |
4
36
Medium.___
6
! 2 8 j
6
36
Anchor___I
4
1 9 |
8
32
Angle iron:
Long-!
10
6 0 ;
4 1
40
Short...
6
3 8
6
36
Full reel wire, 420 yards, approxi- |
mate..._
105
1,260
a
1 52.5
Bobbin, 30 yards-.- ---
8-9
90
4-6
32-54
1 Full-si7 t £*d reels are carried l<y two men upon their shoulders by means of a picket
passed through the hole in the reel.
121
42
engineer field manual
b. Types of entanglements .—The following obstacles may
be considered standard and methods for their construction
are outlined:
(1) High wire entanglement.
(2) Double-apron fence.
(2) Low wire entanglement.
(4) Belts of barbed wire or concertinas.
(5) Portable wire obstacles..
c. Wire entanglement drills. —(1) General .—The following
remarks and precautions are applicable to all entanglement
drills.
(a) The line of stakes toward the enemy for high wire and
the center line for double-apron and low wire should be
traced and marked in the manner described for trenches.
Use of tracing tape is highly desirable for night work in
presence of the enemy.
(b) The men are given numbers in the order in which they
first proceed to work, each having definite, limited tasks.
They start at intervals so that men doing different tasks will
not be bunched, exposed to fire, or interfere with each other.
One row of pickets is set by pacing. The others are placed
by eye, using the paced row as a guide. The elements nearest
the enemy should be placed first, and work continued to the
rear.
(c) Pickets should be carried under the left arm and placed
on the ground with the right hand so that end of the screw
or point of the picket faces the enemy, indicating the spot
at which the picket is to be erected. For carrying, all bundles
of screw and iron pickets should be wrapped with a sandbag
and secured in at least two places by a turn of plain wire
with the ends twisted together. Enough end to this wire must
always be left so that it can be untwisted by hand without
pliers. Bundles of long wooden pickets should be tied to¬
gether in at least two places with a plain wire. Short wooden
pickets are carried best in sandbags, a suitable number in
each bag. Two bags are tied together and slung over the
shoulder. Screw pickets must be screwed in so that the eyes
are parallel to the length of the entanglement, and the eye
points in the direction from which the men are working, that
is, toward the starting point. Compliance with this rule fa-
122
FIELD FORTIFICATIONS
42
cilitates placing wires. Wooden pickets used as hold-fasts
(anchors) should be driven in approximately at right angles
to the stay wire to be attached to them, but screw anchorage
pickets must be placed in prolongation of this stay wire.
( d> In running out barbed wire two men work together;
one man walks out with the bobbin, unwinding as he goes,
and the other stretches the wire and fastens it to the pickets.
(e) Men fastening the wires must always work facing the
enemy.
(/) The carrying parties indicated in the drills can carry
at one trip all the material required for 50 yards of entangle¬
ment. If a round trip to the dump requires more time than
construction of 50 yards of obstacle, strength of the carrying
party must be increased accordingly.
(2) Fastening wire to pickets. —(a) To fasten wire in top
eye of screw picket (see fig. 71 (T)),' pull the fixed wire (the
one leading toward the starting point) taut and slip the
wire up into the eye, turn the running end up over the
eye, thus threading the wire in the eye. Then take a turn
with the running end over the standing end and around
the picket below the eye.
(b) To fasten the wire in a lower eye of a screw picket
when there is already a wire in the top eye—
1. Pull the fixed end taut and slip the wire up into the
eye. Then take a bight on the running end, pass
it around the picket above the eye, and take a
turn with the bight on the running end (see fig.
71 ®).
2. If one eye is on the opposite side of the pickets from
the others, the wire must be forced down into the
eye, and the bight on the running end passed
around the picket under the eye (see fig. 71 ©).
(c) The foregoing rules ((a) and (b) above) apply which¬
ever way the wiremen are working, from right to left or left
to right, and if carried out the wire will be fixed firmly in the
eye and cannot slip up or down the picket. Also, if one bay
is cut the wire in the bays on either side remains taut and
does not slip through the eyes.
(d) Wires are fastened to wooden pickets as shown in
figure 72.
123
FIELD FORTIFICATIONS
42
(e) To fix one wire to another a short length of smooth
wire may be used or the two wires may be twisted together
by means of a rack stick as shown in figure 73. This is the
better method and is known as “windlasslng.” Each mem¬
ber of a wiring party is equipped with a rack or windlass
stick.
(3) High wire. —(a) This entanglement consists of a se¬
ries of fences made of standard long pickets either wood,
screw, or angle iron, spaced 10 feet apart. A sufficiently
close approximation to the distance is 4 paces of an average
Figure 72.—Proper method of fastening wire to wooden pickets.
man. The spacing in depth between successive rows is the
same; thus five rows of pickets give an entanglement 40
feet in depth. In each row the pickets are placed opposite
the center of the intervals between pickets of the adjacent
rows. This arrangement is shown in figure 74.
(b) On each fence or row of pickets there is a bottom
horizontal wire about 6 inches above the ground and a top
horizontal wire at or near the top of the pickets; also, in
each bay there are two diagonals. This system of top and
bottom horizontals and diagonals also runs In a zigzag
262375'
125
TO BOBBIN
BACK STICK
•ALTtfi.NATEr METHOD
FiotmE 73.—Method of fastening two wires together by
“windlasstng.”
FIELD FORTIFICATIONS
42
direction between rows of pickets, giving a series of tri¬
angular cells. All wires are strung in the order and man¬
ner shown in figure 74.
(c) This type of entanglement may be constructed with
two rows of pickets, giving a complete unit. For more im¬
portant obstacles it is constructed in greater widths, five
rows of pickets generally being employed and being con¬
sidered a full belt. The full width of the entanglement
need not be built as the first operation; two or three rows of
pickets may be driven and wired across the front to be
covered to develop the defensive value of the position at
once. Inclined anchorage pickets may be placed opposite
each picket of the outside row and at a distance of about
5 feet from the picket. Guy wires are attached to these,
running to the head of the main pickets. This practice is
sometimes varied by placing the anchor pickets opposite the
center of intervals between main pickets and guying di¬
agonally in both directions. At least two strands of wire
should be used on each guy, preferably barbed wire as used
in the entanglement itself.
(d) In case sufficient barbed wire is not available for the
whole entanglement, the most efficient utilization of it would
be for the top and bottom fence wires as it is these that the
body encounters whether the soldier attempts to go over or
under the obstacle.
(e) Drill for erecting high wire entanglement.
Table XVI.— 50 yards high wire entanglement (two rows of
stakes )
Materials
Wiring party
Carrying party
8 bundles (total of 32)
long pickets.
32 30' yard bobbins
barbed wire.
I noncommissioned officer
(carries pliers).
16 men (each carries a rack
stick).
I man, carrier.
1 noncommissioned
officer.
18 men.
127
42
ENGINEER FIELD MANUAL
Table XVI. — 50 yards high wire entanglement (two rows of
stakes )
Xos.
First task
Second task
Third task
No n c o m mis¬
sioned
officer.
1_
Leads party to
head of work.
Paces front panel and indicates location of
pickets; supervises work.
Each man carries
out one bundle
pickets.
Place pickets of front
panel.
String bottom hori¬
zontal wire, zig¬
zag panel.
2_
4__
Place pickets of
rear panel.
String first diag¬
onal wire, zigzag
panel.
Screw in pickets of
front panel.
String second diag¬
onal wire, zig¬
zag panel.
G _ _
Screw in pickets rear
panel.
String top horizon¬
tal wire, zigzag
panel.
8 .--. .
0___
Each man carries
out three bob¬
bins barbed
wire.
String bottom hori¬
zontal wire, front
panel.
String bottom hori¬
zontal wire, rear
panel.
10. _
11_
String first diagonal
wire, front panel.
String first diago¬
nal wire, rear panel.
12_
13.
String seeond diag¬
onal front panel.
String second diag¬
onal wire, rear
panel.
14.._
IX __
lfi_
Siring top horizon¬
tal front panel.
String top horizon¬
tal wire, rear
panel.
17_
Carry out 5 bobbins
barbed wire.
Xotb.—N umbers 1 to 4 place pickets lightly in ground. In stringing, odd iium.
hers run out bobbins, even numbers tlx tvire to pickets.
128
FIELD FORTIFICATIONS
42
Figure 74.— High wire entanglement.
129
42
engineer field manual
Each additional row of high wire entanglement
Material
Wiring party
Carrying party
4 bundles (total of 16) long
pickets.
24 30-yard bobbins,
!
i noncommissioned offi¬
cer.
10 men.
1 noncommissioned
officer.
12 men.
(4) Double-apron fence. — (a) The double-apron fence,
4- and 2-pace, consists of a central row of long pickets spaced
10 feet, or 4 paces, apart, upon which are strung four hori¬
zontal strands equally spaced from top to bottom. Parallel
to and on each side of this row of pickets is placed a row of
anchorage pickets to which diagonally inclined wires forming
the apron are strung. The anchorage pickets are placed
opposite the intervals, and 5 feet, or 2 paces, from the line
of the long pickets. Bach of the two sets of diagonally
inclined wires carries three horizontal wires. This type of
entanglement is erected with great rapidity and resists de¬
struction by shell fire or bangalore torpedoes as well as any
other pattern. For very rapid work and to develop utility
of the barrier at once, the back apron is sometimes omitted.
If sufficient barbed wire is not available, smooth wire may be
used for the rear apron and if necessary for the diagonal
wire of the front apron. Value of the entanglement lies
chiefly in the front apron which should never be omitted.
AH wiring is done from front to rear, the diagonals in the
front apron being placed first. Three horizontal wires are
placed on the sloping apron and wlndlassed to the diagonal
wires. The four horizontal wires of the central fence are
next strung, following which the diagonals and apron wires
of the rear apron are placed successively (see fig, 75).
(b) This entanglement is economical in labor and mate¬
rial and can be developed into a broad barrier by construction
of successive bands. The space between bands can be filled
with portable obstacles or loose wire in the form of spirals.
As in the construction of the high wire type, the working
parties are covered by the first elements of wire placed, and
they work behind it toward their own lines.
13Q
(c) Drill for erecting double-apron fence.
Table XVII .—50 yards double-apron fence, 4- and 2-pace type
42
ENGINEER FIELD MANUAL
132
FIELD FORTIFICATIONS
42
133
anchor pickets.
42
engineer field manual
(d) The double-apron fence, 6- and 3-pace (fig. 76), is
exactly similar to the 4- and 2-pace type except that the
distance between long pickets is 6 paces instead of 4 and the
anchor pickets are placed 3 paces instead of 2 from the
center line of the obstacle. This results in a wider obstacle
but one that is probably not as strong as the 4- and 2-pace
type. As may be seen from table XXI, the number and
weight of long and anchor pickets is about 30 percent less for
the 6- and 3-pace type and there is no increase in amount of
wire required. The wiring party is reduced by one man and
hence the man-hours by V 2 man-hour for 50 yards of entan¬
glement. Also drill for erection of the 6- and 3-pace fence
is simpler. This reduction in amount and weight of material
and in man-hours required is a very important consideration
in many situations. However the 6- and 3-pace fence has
not had the benefit of actual war experience as has the
4- and 2-pace.
134
FIELD FORTIFICATIONS
(e) Drill for erecting double-apron fence.
Table XVIII.—50 yards double-apron fence, 6- and 3-pace type
42
ENGINEER FIELD MANUAL
136
Each carries out 1 Lay out and screw in Run out and fasten Run out and fasten Run out and fasten
bundle anchor rear anchor pickets. second horizontal third horizontal second horizontal
pickets. No. f> places pickets wire, front apron. wire of fence. wire, rear apron,
at end of work.
FIELD FORTIFICATIONS
42
137
42
ENGINEER FIELD MANUAL
(5) Low wire. —(a) The low entanglement (fig. 77) con¬
sists of a double-apron with the fence, except the top wire,
omitted. Medium pickets are used, preferably of the screw
type, spacing and arrangement being the same as for the
double-apron fence. The wire forming the inclined diagonals
toward the enemy is strung first. Across the diagonals are
then laid the three horizontal wires in the order indicated
in figure 77. A horizontal wire is next strung along the tops
of the pickets. The wire forming the rear diagonals is then
strung, followed by the three horizontal wires which it sup¬
ports in the order indicated in the figure. Horizontal wires
are windiassed to inclined diagonals at points of contact.
(t>) The chief advantages of the low wire type are its econ¬
omy of material and labor and its low visibility with respect
both to direct and terrestrial observation and to aerial pho¬
tography. It is preferably built in long grass where it becomes
almost entirely invisible. Its chief disadvantage is the obvious
one that a man with reasonable ability can pick his way
through it without much trouble. As a surprise entanglement
it is unquestionably effective.
(c) Low wire entanglement of either 6- and 3-pace or 4-
and 2-pace types may be constructed. Advantages and dis¬
advantages of the two types of double-apron wire (see (4)
(d) above) apply to low wire entanglements. Figure 77 and
table XIX give details and drill for the 6- and 3-pace type.
The 4- and 2-pace type is exactly similar except for substitu¬
tion of 4 and 2 paces for the 6 and 3 paces in laying out the
wire and for the changes noted in the drill table XIX.
138
WlXii
FlGUBE 77.—LOW '
6- and 3-pace.
(d) Drill for erecting
„jlement.
fABiE XIX .—50 yards low wire entanglement, 6- and 3-paci
Material
Wiring party
Carrying party
2 bundles jnedium pickets (1 of
1 noncommissioned officer
1 n o n c o in m I $
6, 1 of 5 each). 1
(carries pliers).
sioned officer.
4 bundles (total of 22) anchor
6 men (carry rack sticks).-
13 men.
pickets. 2
20 30-yard bobbins barbed wire.
•Changes in drill for 4- and 2-pace:
i 3 bundles medium pickets (1 of 6,2 of 5 each).
* -t bundles (32) anchor pickets.
139
42
ENGINEER FIELD MANUAL
Table XIX .—50 yards low wire entanglement, 6- and 3-pace*
— Continued.
2
First task
Second task
Third task
Fourth task
Fifth task
Sixth task
Noncommissioned officer 3 paces oil and indicates to Nos. 1 and 2 location for their
pickets. Supervises work.
1
2
3
4
5
Carries out
1 bundle
medium
pickets.
Carries o
…[truncated]