Manual of military field engineering for the use of officers and troops of the line

Front matter

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Preface

The necessity existing at the Infantry and Cavalry School for a text-book on Field Engineering, including the various military expedients recognized in our service, is deemed sufficient reason for the following pages.

Most of the subjects treated of in this volume may be found in various military works published in our country during the past twenty-five years, but the fact remains that no one book has covered the required ground, nor has their revision been of very recent date; while, at the same time, the new field gun and small calibre rifle have necessarily modified previously existing profiles of Field Works and Shelter Trenches.

Access has been had to corresponding publications of the Germans, French, English and Austrians, as well as to our own Official Rebellion Records and many other available sources, native and foreign.

It has been the endeavor to limit the scope of this work to subjects considered indispensable as a part of a line officer's education.

The following Assistant Instructors in the Department of Engineering — viz.: 1st Lieut. E. A. Root, 19th Infantry; 1st Lieut. W. C. Wren, 17th Infantry, and 1st Lieut. T. H. Slavens, 6th Cavalry, have been associated with the undersigned in the preparation of this volume.

WM. D. BEACH, Captain, 3d Cavalry.

U. S. Infantry and Cavalry School,

Fort Leavenworth, Kansas, July, 1894.

Headquarters of the Army, Adjutant-General's Office,

Washington, March 25, 1895.

Circular No. 4.

With the approval of the Secretary of War, the special study of the books, pamphlets, orders, etc., hereinafter named, by officers of the army subject to examinations for promotion, is

recommended:

* * * * * * * *

Manual of Field Engineering— Captain W. D. Beach, 3rd

Cavalry.

****** * *

By command of Lieutenant-General Schofield.

[Signed] GEO. D. RUGGLE& Adjutant-General.

Preface To Fifth Edition

A fifth edition of this manual having been called for, the revisers have made certain alterations and additions which seem to them warranted by experiences which have fallen to their lot during the Spanish-American War and Philippine insurrection.

The peculiar difficulties confronting a rapidly advancing army are such as to render familiarity with various military expedients only a degree less important than an intimate acquaintance with one's weapon. The efficiency of a command may be paralyzed by the lack of a practicable road or by reason of a broken bridge as effectually as by tactical blunders or failure to reconnoiter.

Hasty intrenchments are more than ever important, and there seems little reason to doubt, judging from our experience before El Caney and at San Juan Hill, that odds of ten to one will hereafter be necessary in order to successfully assault trenches defended by good infantry armed with a magazine rifle. This deduction was originally made by Colonel A. L. Wagner, A. A.-G., in a report on El Caney, andr is borne out by a critical study of the fight at San Juan Hill and various actions during the Boer War in South Africa.

Recent experiences but render more certain the assertion that the assailant will usually find it necessary to intrench, the defender always.

Upon the line officer in the future as in the past will devolve the responsibility of tracing and directing the construction of shelter trenches as well as making intelligent use of expedients in bridging, rafting, road-building or camping.

As modern civilization tends to diminish the number of men skilled in handicraft, so much the more important does it become that all officers should require greater familiarity with what are ordinarily termed "military expedients." Results may be reached in many ways, but their attainment with economy of life and treasure marks a soldier skilled in his art, a leader worthy the best traditions of the military service.

W. D. B.

Fort Leavenworth, Kansas, August, 1902.

List of Books Consulted in the Preparation of this Work.

A Move for Better Roads L. A. Haupt.

Civil Engineering. Wheeler.

Cours de Fortification Passagere De Guise.

Ecole de Fortification de Campagne French.

Elements of Field Fortification Wheeler.

Engineering News. Magazine.

Engineer's Pocket-Book. . . Trautwine.

Field Fortification. . . . . . Turner.

Field Fortification Hutchinson.

Field Works Brackenbury.

Field Works Used in War. (Translation

from the German) Wilson.

Good Roads Magazine.

Gun Powder and High Explosives Walke.

Journal of the Military Service Institution

of the U.S.

Journal of the U. S. Cavalry Association.

Manual for Engineer Troops Duane.

Manual of Military Engineering Ernst.

Manual for Railway Engineers G. L. Vose.

Manuel de Fortification de Campagne. . . . Brialmont. Manuel des Travaux de Fortification de

Manuel de P'ortification Plessix and Legrand.

xVlanual of Heavy Artillery Jidball.

Military Bridges Haupt.

Military Bridges Chester.

Military Engineering, Instruction in Chatham Course

Military Land Mines Mercur.

Military Transport Furse.

Modern High Explosives Eissler.

Notes on Military Hygiene A. A. Woodhull.

Official Records of the Rebellion, U. S . . . . War Department.

Organization and Tactics Wagner.

Report of Chief Signal Officer, U. S., 1893. . War Department.

Roads and Railroads Chester.

Roads and Railroads Gillespie*

Roads, Streets, and Pavements Gillmore.

Temporary Fortification e Chester.

The American Railway Scribner.

V. S. Bridge Equipage and Drill . . , . War Department*

Table Of Contents

Chapter. Page. Plate.

I. General Principles 7

II. Fire, Projectiles and Penetration 11

CHAPTER I.— General Principles

1.— Military Field Engineering may be denned to be the art of utilizing the materials at hand for the attainment of the security, effectiveness, health and comfort of an army in the field.

The modern rifle has vastly increased the value of cover, both ia attack and defense, and rendered necessary the application of the principles of fortification to an army in the field. The result to be obtained in all fortification is to so strengthen a position, by artificial means, that a force occupying it may successfully resist or subdue another attacking it.

2. — Fortification is divided into two general classes, viz.:

(a)— Permanent.

(h)— Temporary or Field Fortifications.

With the former this manual has nothing to do.

3.— The latter division includes three quite distinct classes.

The first comprises all works devised for the temporary protection of important points, such as cities, arsenals, bridges, fords, positions, etc., and are technically known as Field Works.

8 General Principles.

The second comprises the various devices of the engineer for reducing a fortified place by means of parallels and approaches, called Siege Works.

The third division relates particularly to the quickly made defenses by which an army in the presence of an enemy protects itself; these are known as Battle Intreneliments or Hasty Intrencliments.

4.— A Defensive Position is one affording protection from the shot and observation of an enemy and, at the same time, commanding the ground in front, within range.

A position of perfect defense is not possible, but the following general principles are to be fulfilled as nearly as circumstances will permit.

(1) The defenders' position should conform to the special tactical requirements of the occasion* and should be such as to favor the use of their relatively strongest arm.

(2) It should be made impossible for the enemy to obtain natural cover during his advance. In other words, the position should have a free field of fire.

(3) The defenders should be protected from the fire and view of the enemy by cover so arranged as not to interfere with counterattacks.

(4) The advance of the enemy should be hindered by obstacles so arranged that he may be checked while under the fire of the defenders.

(5) Communications should be such that the defenders may freely move from one part of the position to another, while the contrary should obtain with respect to the enemy's ground in front.

The chief requisite of a defensive position is a free field of fire, especially at short and mid ranges. If the position is judiciously selected the field of fire will generally be obtained without much difficulty, but the advantages of the position and the effect of the fire may be enhanced by temporary fortifications. The cutting down of slight ridges which might afford cover for the enemy

*A nure 1 v defensive nosition, for instance, might have its flanks resting r»n impassable'obstac'es, and thus be secure from a turning movement, but this same position »~ight be found to be a faulty one were a quick offensive movement, by the defenders, contemplated.

General Principles. 9

within effective range or the removal of hedges, fences, etc., may sometimes be of more benefit than the actual preparation of defenses.

In the present advanced state of efficiency of fire-arms, artificial cover is, however, of greater importance than ever before. Constructed in the right place, at the proper time, field fortifications may render indispensable service, while their neglect may insure defeat.

5.— While formerly it was the special province of the Engineer to lay out and supervise the construction of defensive works, it has now, under the changed condition of warfare, become the work of the Line as well, and it may be laid down as an accepted rule that the defensive arrangements for a given position are to be made by the troops which are to occupy it.

These changes have affected the art in many ways. The field works now constructed are simpler, ruder, less regular, and less angular than before. An army in the presence of an enemy always fortifies, whether in camp, in bivouac, or in line.

6.— Rapidity of execution renders necessary the adoption of fixed types of works in the exercises in time of peace; but these types will sometimes be susceptible of modification in their real application. However, even in war, the endeavor should be to approximate to the regulation forms, for they are deduced from experience and observation, and realize, as well as possible, for each particular case, the best conditions of resistance compatible with rapidity of execution.

The advantage of regulation types is understood at once when it is borne in mind that, upon the battle-field, there should be no hesitation; everyone should stick to his individual role in order to unite efficiently in combined action.

Thorough study and frequent practical exercises, conducted methodically, are indispensable in order to escape feeling one's way, with the loss of time that an insufficient instruction renders inevitable. Upon the battle-field a few minutes may decide the fate of armies in each other's sight.

7.— Fortification, which at first glance may appear to dominate, as representing the "security" and "effectiveness" of an army,

10 General Principles.

the other and apparently less important subjects relating to health and comfort, is, however, so intimately connected with them that neglect of one may render all the others useless. Thus, "bridges," "roads," and "railroads" may, under certain conditions, relate particularly to the effectiveness and security of an army, in connection with Fortification, while under other circumstances they may be as important as various "camping expedients" in the attainment of "health" and "comfort."

CHAPTER II.— Fire, Projectiles and Penetration. 8.— Fire as regards its direction is classified as follows

(1) Frontal, when it is delivered at right angles to the front of the enemy's line, and sometimes so termed when delivered straight to its own front.

(2) Oblique, when the direction of the fire is at an oblique angle to the front of the enemy's line.

(3) Enfilade, which is delivered from positions on the prolongation of the enemy's line. In this case, the line of fire sweeps the enemy's front. When fire is used to sweep along the front of a defensive line and thus enfilade the assailants as they approach the position, it is known as flanking fire.

(4) Reverse, when delivered so as to strike troops or lines of defense from the rear.

(5) Cross, when the lines of fire from different positions cross on or in front of the enemy's line.

As regards its trajectory it is classified as

(1) Direct, when .delivered at seen objects at moderate angles of elevation— in the case of artillery when delivered at seen objects, with service charges at elevations not exceeding 15°.

(2) Indirect or Curved, when delivered with small-arms against an unseen object protected by a seen covering obstacle— in the case of artillery, as above, or with guns, howitzers or mortars with reduced charges at angles not exceeding 15°. Thus firing over an intervening hill at troops sheltered behind it would be an example of indirect fire.

(3) High Angle, when used at angles exceeding 15°.

(4) Grazing, when the projectile travels approximately parallel to the ground.

(5) Plunging, the muzzle is required to be depressed.

9.— The Artillery Projectiles used in the U. S. Army are shell, shrapnel and canister.

12 Fire, Projectiles and Penetration.

Shell.— Shell may be classified as common shell and torpedo shell. The common shell is "a hollow cast-iron or steel cylinder with an ogival head closed at one end and filled with powder." The torpedo shell is filled with gun-cotton, or other high explosive. Either shell may be characterized as a flying mine, the chief object of which is to destroy material objects at a distance, though the common shell may also be effectively used against troops.

10.— Shrapnel differs from common shell in being filled with bullets, and having only a sufficient bursting charge to rupture the envelope and release the bullets, which then move with a velocity which the projectile had at the moment of bursting. The bullets are assembled in circular layers and held in position by "separators," which are short cast-iron cylinders with hemispherical cavities into which the bullets fit. The shrapnel for the 3.2 inch gun contain 162 bullets % in. in diameter, and weighing 41 to the lb. The total number of bullets and individual pieces in the shrapnel is 201 when assembled, and many more after bursting.

11.— Canister, which is practically obsolete, is made of sheetiron or tin in the shape of an ordinary can, and is filled with bullets held in place by filling the interstices between the bullets with saw-dust, sulphur or rosin; the can is ruptured and its contents dispersed by the discharge of the piece.

12.— The charges in the shell and shrapnel are exploded by means of a combination fuse; by combination fuse is meant one that may be arranged to explode the charge either on impact, by percussion, or at a given time by certain arrangement of the parts of the fuse.

13.— Field Guns range up to 6000 yds., but will be seldom used at a range greater than 2500 yds.

14.— The U. S. Magazine Rifle, when used as a single loader, has fired 21 aimed shots in one minute, and when used as a magazine rifle, 23 shots in one minute; its range is over 3000 yds. and it is sighted to 1900 yds.

The average heights over which fire may be delivered are as follows: Man standing, 4 ft. 4 in.; kneeliug, 3 ft.; lying down, 1 ft.; field guns, 3 ft.

Fire, Projectiles and Penetration. 13

15.— The following thickness of material may be considered as proof against small-arm projectiles at all ranges:

Sand 30 in.

Earth 39 in.

Boggy or turfy ground 60 in.

Gabion filled with earth 1

Well-made fascines 3

Sand bag well packed, header 1

" " " " stretcher 2

Stacked sod 79 in.

Packed snow 79 in.

Soft wood 40 to 49 in.

Oak or other hard wood 24 in.

Grain sheaves piled 16 ft.

Iron plate 7-16 in.

Steel plate % in.

♦Masonry brickwork with broken joints 20 in.

Crib of broken stone 8 in.

Against field artillery.

Sand 10 ft.

Earth 13 ft.

Clay 17 ft,

Snow well packed 27 ft.

Masonry (for a short time) 40 in.

*A wall two bricks thick, breaking the joints, will stop any one bullet, but after a time the bricks will be smashed and some bullets get through.

A well-built wall with fine joints set in cement mortar, 9 inches thick, is practically bullet-proof.

A 24 -inch sun-dried brick wall is fairly bullet-proof a short time after setting.

15a. — The mean penetration of shells from siege guns, with a striking velocity of about 800 feet, is:

6-inch 8- and 10-inch Guns. Guns.

Feet. Feet.

Sand, mixed with gravel 9.84 11.48

Light earth 13.12 16.73

Light earth, loose (newly stirred up) 14.76 20.34

Clay (argillaceous earth) 21.33 27.89

14 Fire, Projectiles and Penetration.

Dimensions of craters made by 6- and 8.5-inch shells:

6-lnch. 8.5-inch.

Diam. Depth. Diain. Depth.

Feet. Feet. Feet. , Feet.

Calcareous sandstone 13.1 3.3 16.4 3.9

Rough masonry 9 2 6.6 3

Note. — With delayed-action fuses, shells burst after gaining a

certain penetration. The maximum effect of the bursting charge

will be found on the line of least resistance.

CHAPTER III.— Field Geometry

16.— Before proceeding to that portion of field engineering which involves geometry some of its simplest applications will be explained.

17.— Slopes. The usual description of a slope is by a fraction, the numerator being the height and the denominator the base. Thus, in PL 1, Fig. 1, the vertical height is l-6th part of the base, and the slope is read as 1 on 6. In Fig. 2, the slope is G on 1.

18.— To lay out a Right Angle: First Method. Let A be a point in the line BC, Fig. 3. Lay off from A the equal distances AD and AE. With a radius greater than AD, and with D and E as centers, describe arcs cutting each other at X. Join X with A. Then is XA perpendicular to BC.

Second Method. Find a point such that the distances are in the proportions of 3, 4 and 5: then will the angle included between the two shorter sides be a right angle. Thus (Fig. 4) with chain or tape measure the distance AD equal to 4 yds. Place one end of tape at D, the other at A, pulling it out and making XD equal to 5 yds., XA equal to 3 yds.

Third Method. At extremity of line, as A (Fig. 4), assume any point as C. Measure distance CA, set a stake on line BA at a distance from C equal to CA, as D. Set a third stake on line CD at X, making CX equal to CD. Then will XA be perpendicular to BA.

19.— To erect a perpendicular to a line from a point without. Let X (Fig. 5) be the point without, then, with X as a center, and a distance greater than XA as radius, describe an arc cutting BC at D and E. With D and E as centers, and with a radius greater than DA, describe arcs cutting each other at Y. Join X and Y. Then will XY be perpendicular to BC.

20.— To bisect a given angle. Let BAC (Fig. 6) be the angle. With A as a center, and with any convenient radius, as AD, -2-

B \D A zV

IE C x

A

3 \

E ^^B

Figure 7 y yV\^

A ' C '

1 D ' B

B D N A EC

B D C

Y

Field Geometry. 17

describe an arc cutting AB and AC at E and D. With D and E as centers, describe arcs cutting each other at X. Join X with A. The line XA bisects the angle BAC.

21. — To lay out an equilateral triangle constructing adjacent angles of 60° and 120°. Let AB (Fig. 7) be a given line. Lay off from B any convenient distance, as BE. Then, with B and E as centers, and a radius equal to BE, describe arcs cutting each other at D. Join D with E and B. The angles DEB, DBE and EDB are each equal to 60°. The angle AED is equal to 120°. Combining this method with that of slopes an angle of almost any number of degrees can be laid out.

22. — To lay out an angle equal to a given angle. Let X (Fig. 8) be a point in the line AB, from which it is required to lay out an angle equal to OEC. Fix the points O and C at convenient distances from E. From X lay off Xg equal to OE. Then, with X and G as centers, and EC and OC as radii respectively, strike arcs intersecting at F. Join X and F. The angle FXG is equal to the angle CEO.

23. — To draw a line parallel to a given line and at a given distance from it. Let AB (Fig. 9) be the given line. From any two points, as C and D, erect perpendiculars. On these lay off the required distance CE and DF. Join E and F.

24. — To find the distance between any two points when it cannot be measured directly. First Method. To find AO, take a point B in line with AO and from this point (Fig. 10) lay off any convenient angle, as ABC. At D make EDC equal to ABC. Measure BC, DC and DE, putting E in the line CO. From similar triangles

BC X DE

BO : BC :: DE : DC .'. BO =

DC

From the result thus found, subtract the distance AB. The remainder is the distance AO.

Second Method. (Fig. 11.) Mark B in prolongation of the line AO. Assume any point as C. Lay off AF, making AC equal to CF: also BE, making BC equal to CE. Prolong EF until a point K is found in line with CO. Measure FK. This is the required distance.

18 Field Geometry.

25. — Areas. To find the area of a rectangle. Multiply the base by the height.

To find the area of a trapezoid. Multiply the sum of the two parallel sides by the perpendicular distance between them and take half the product.

To find the area of a triangle. Multiply the base by the altitude and take half the product. Or,

Area = \/ s (s — a) (s — b) (s — c) in which s is the half sum of the three sides a, b, and c. Or,

Area — % a b sin G in which a and b are two sides and C the included angle.

26.— The Field Level (PL 2, Fig. 1) consists of three strips of wood, A, B and C, each % in. thick and 2 in. wide. A being 62 in. long, B and C each 44.42 in. The distance between centers on A is 60 in., on B and C 42.42 in. This makes a right angle between B and C. There is a thumb nut at E ciamping the arm B to the arm A when the level is used. The screw at F projects, holding the arm B, when folded, up. There is a stud at H, affording an attachment for a plumb bob. There are permanent joints between B and C, and A and C.

Fig. 1 shows the level and its joints, plumb bob for reading slopes, and spirit level. Fig. 2 shows side for protracting angles.

27. — Uses of Level. The level may be used as follows:

(1) As a spirit-level, the level being on the edge G.

(2) As a square for setting out a right angle.

(3) As a protractor.

(4) For setting off slopes.

(5) As a mason's level with a plumb bob.

SHELTER TRENCH.

- LYING.-

SHELTER TRENCH.

-STANDING-

SPLINTER PROOF,

(earth cover omitted).

Fig. 5 . SHELTER m SUPPORTS <m RESERVES.

HEAD LOG and BRUSH-WOOD LOOPHOLE

LOOP-HOLE

CHAPTER IV.— Hasty Intrenchments, Gun Pits and Epaulements

28. — The intensity of fire made possible by the fire-arms of to-day renders some form of shelter on the field of battle imperative. Circumstances may occur when advancing lines of skirmishers will find natural shelter, but in many cases artificial cover will have to be constructed on the spot.

Fortifications used on the field of battle depend, as to their position, extent and use, on the ground; in conformity to this idea they are constructed at the time of the battle, and not before.

They are called "Battle" or "Hasty" Intrenchments, and should fulfill the following conditions:

(a) The thickness of earth embankment should be such that it will not be liable to penetration by small-arm projectiles or shrapnel fragments.

(b) The intrenchments should conform to the average heights over which men can fire in the various positions,— viz. : lying prone, 1 ft.; kneeling, 3 ft.; standing, 4 ft. 4 in.; and at the same time the height of earth embankment above the natural surface of the ground should be small, for the reason that the trenches can thus be more easily concealed and are less liable to be struck by artillery projectiles.

Hasty or Battle Intrenchments consist of cover for

(1) Skirmishers, lying, kneeling or sitting.

(2) Firing Line, Supports and Reserves, kneeling, sitting or standing.

(3) Gun Pits and Epaulements.

'.id. — The shelter trench for skirmishers lying down is shown in Tl. 3, Fig. 1. It gives earth protection of a thickness of 2% feet; this thickness of loose earth will stop small-arm projectiles under ordinary circumstances. The average time required by one man to make 5 ft. (2 paces) of this trench is, with large pick and shovel, 15 minutes; with small intrenching spade, 20 to 25 minutes.

The number of skirmishers that can use this trench is usually computed as two for each five feet of length, although three may occupy this space by lying partially on their left sides. In firing, the left elbow rests on the berm.

30. — For men kneeling in two ranks, cover is gained by deepening the trench already dug to 1 ft. 8 in. and making it 5 ft. wide

22 Hasty Intrenchments, Gun Pits and Epaulements.

with an embankment in front having a height of 1 ft. 4 in. and a resulting thickness of about 5% ft. (Fig. 2.)

The average time required by one man to transform 5 ft. of the trench "lying" into the trench "kneeling" is, with large pick and shovel, 25 minutes; with small intrenching spade, 45 minutes.

Infantry in double rank kneeling can fire from this trench, the number of rifles being computed at 4 for each 5 ft. length of trench. The kneeling trench affords protection to men sitting, but horizontal fire from this position is impossible. The step at a would only appear in the converted trench.

31. — Cover standing is obtained by deepening the kneeling trench to 4 ft., leaving a step 20 in. high and 3 ft. wide next the front wall, so as to facilitate leaving the trench to the front and at the same time allowing a protected passage in rear. The step serves as a banquette for men firing over the embankment.

The embankment is given a height of 2 ft., the resulting thickness being about 5% ft. (Fig. 3.)

The average time required by one man to transform 5 ft. of the trench "kneeling" into the trench "standing" is, with large pick and shovel, 1 hour.*

32. — When isolated trenches for single skirmishers lying are desired, they should be made with the same section as that shown in Figure 1 and have a length of 1 pace. Isolated kneeling trenches for two men should also have a length of 1 pace, but the rifle pit or isolated shelter standi no should be 5 ft. in length on account of difficulty in constructing a smaller one. The last readily accommodates three men, two of whom can fire over the embankment, while the third, standing in the 4 ft. trench, protects the flanks.

33. — When necessary to intrench supports and reserves, the cover

:: 'The ordinary form of Spanish hasty intrenchment for fire, standing and cover sitting, kneeling or crouching is a trench about two feet wide and four feet deep, having vertical sides and no embankments. This type of trench does not readily permit relieving or re inforcing the occupants an^. as it is too narrow to permit carrying the wounded out, there results extreme suffering, since the injured must lie at the bottom and run the risk of being trampled upon. This trench has the advantage however of affording a small target for the enemy's artillery.

The Hermans claim that the surface presented to shrapnel bullets by a man lying down is practically the same as a man standing in the open; therefore they do not believe in broad shallow trenches They advocate the making of trenches in the natural soil with the fresh earth carried away, or u?ed for dummy trenches so as to deceive the enemy's artillerists and prevent execution from shrapnel fire.

Hasty Intrenchments, Gun Pits and Epaulements. 23

kneeling or standing should be used in parallel rows close to one another. (Fig. 5.)

In the construction advantage should be taken, where possible, of plows for loosening the earth. Two or three plows following each other at intervals can be used to great advantage.

34. — The trenches here illustrated are all made on level ground and are simply types showing the best forms and giving general ideas as to the time required to construct cover.* On slopes they must be modified so that the tops of the embankments are, in general, parallel to the ground; they may also be varied according to the kind of earth, sand requiring less thickness of embankment and gentler slopes than clayey soil, while sod mixed with earth allows greater penetration than earth without it.

35. — The location of trenches depends primarily on tactical situations, and secondarily on the nature of the ground.

Primarily, they should always occupy a position giving the greatest development of fire, and hence are generally located near the crest of tlie most abrupt slope— i. c., near the "military crest." (See par. 153.) The exact position is determined by placing the eye at a distance above the ground equal to the proposed height of embankment, and then selecting that line which gives a clear field of fire to the front. ^

As to the secondary consideration, it is desirable in locating trenches to avoid stony ground and that close to the edges of woods, the former on account of the liability to flying fragments should the embankment be struck by an artillery projectile, and the latter by reason of the difficulty in constructing the trenches.