(e) The communicating trenches (boyaux) should be zigzagged, wide and deep, and should follow the low ground. The longest straight trench should not exceed thirty paces. The angle made by each turn should be less than 140 degrees.
(f) The fire trenches should have salients and re-entrants so as to flank the wire entanglements. The bays are usually 27 feet long with 9 feet of traverse.
(g) There are two problems in the siting of trenches, one for those to be constructed under fire and another for those that will be constructed without any danger from fire. Trenches built under fire are usually made by connecting up individual shelters made by the front line when forced to halt. Trenches built under quiet conditions can be laid out according to the best possible plan.
Trench Construction.
Several kinds of difficulties face the trench digger: Sand, clay, water and bullets. In order to overcome them he must be familiar with the general arrangement of a trench, the principles which govern its construction and the standard trench as it has been worked out in the present war at the cost of thousands of lives.
General Arrangement.--A position is a combination of trenches, consisting of: The fire trench, or first line, nearest the enemy; the cover trench, just behind the first line, where all but sentinels of the fire trench garrison are held in dugouts or shelters; the support trench, from 150 to 200 yards in rear of the cover trench, and the reserve, from 800 to 1,200 yards still further to the rear.
The support trench is placed far enough from the first line to prevent the enemy from sh.e.l.ling both trenches at once. By a concentration of artillery fire and a determined advance of the hostile infantry the first line may be captured. The support trench must be so organized that it will then act as a line of resistance upon which the enemy"s advance will break. Lieutenant Colonel Azan of the French army says: "As long as the support trenches are strongly held, the position is not in the hands of the enemy."
[Ill.u.s.tration: Plate #16]
The reserve is usually a _strong point_, so organized that it can maintain independent resistance for several days if necessary, should the enemy obtain control of adjacent areas.
Where possible trenches should be on reverse slopes, with the exception of the first line; but usually the outline of a trench is determined in actual combat, or is a part of hostile trench converted.
Under these circ.u.mstances it cannot be arranged according to tactical ideals.
Artillery and the automatic gun are the determining factors in trench warfare to-day. The effect of artillery fire must be limited in its area as far as possible, and trenches are, therefore, cut by _traverses_, which are square blocks of earth not less than nine feet square, left every 27 feet along the trench. They should overlap the width of the trench by at least one yard, thereby limiting the effect of sh.e.l.l burst to a single _bay_, the 27-foot length of firing trench between two traverses. Sharp angles have the same effect as traverses, but angles of more than 120 degrees cannot be utilized in this way.
The sides of the trench are kept as nearly perpendicular as possible, to give the maximum protection from sh.e.l.l burst and the fall of high angle projectiles. The _parados_, the bank of earth to the rear of the trench, has been developed during the war to give protection from flying fragments of sh.e.l.ls exploding to the rear, and to prevent the figure of a sentinel from being outlined through a loop-hole against the sky. The _berm_, a ledge or shelf left between the side of the trench and the beginning of the parados, has come into general use in order to take the weight of the parados off the earth at the immediate edge of the trench, and so prevent the reverse slope from caving in easily under bombardment or heavy rain.
[Ill.u.s.tration: Plate #16A]
Automatic guns have made it necessary to break the line of the trench at every opportunity, in order to secure a flanking fire for these arms. Auto-rifles and machine guns have tremendous effectiveness only in depth, and flanking fire gives them their greatest opportunity.
Trench Construction.--The methods of building trenches are the same whether the work is carried on under fire or not. In an attack, upon reaching the limit of advance, the men immediately dig themselves in, and later connect these individual holes to make a continuous line of trench.
Most of the digging must be done at night, and must be organized to obtain the most work with the least confusion. There are three ways of increasing the efficiency of the men. In the first of these, squad shifts, the squad leader divides his men into reliefs and gives each man a limited period of intensive work. Reliefs may be made by squads or by individuals. The second way of increasing efficiency is to induce compet.i.tion among the man and squads, thus making the work a game in which each soldier"s interest will be aroused in the effort to do better than the others. The third method is to a.s.sign a fixed amount of work to each man. An average task, which all ought to accomplish in a given time, is found by experience, and those who finish before their time is up are relieved from further work during that shift, and allowed to return to their shelters.
Continual care must be used to check up the tools on hand, as the men are p.r.o.ne to leave them where they were working rather than carry them back and forth to work. Each unit must guard its property from appropriation by neighbors on its flanks.
System of Laying Out Trenches.--The trace of the trench is first staked out, particularly at traverses and corners when the work is to be done at night. Measurements should be exact, and the men should be required to line the limits of each trench so as not to exceed them in digging. All sod should be taken up carefully and used on the parapet for concealment or on the berm to make a square back wall for the dirt of the parapet. If possible this should be done with the parados wall, so as to make it as inconspicuous as possible from the front.
Men should begin to dig at the center of the trench and throw the dirt as far out on the sides as possible, so that as they go deeper the earth can be thrown just over the berm. The slope of the sides will be kept steep and the men prevented from widening the trench as they dig.
In sandy soil the sides of the trench should be allowed to reach their angle of repose (which is wider at the top than required), then the trench walls supported with _revettments_ to the proper width, which are filled in behind with sand. Always dig to full depth before beginning to revet, as it is impossible to dig deeper afterwards without loosening the revetting.
Revettments.--Every trench at points needs support, and this _revetting_ may be done with any of the following materials: Sod; corduroy of logs laid lengthwise; sand bags (size 20 in. x 10 in. x 5 in.); galvanized iron; chicken wire and cloth made in a frame about six feet long; _hurdles_, wicker mats made by driving three-inch stakes into the ground, leaving uprights as high above the ground as the depth of the trench, then weaving withes and slender saplings between the uprights; expanded metal; _gabions_, cylindrical baskets made like hurdles except that the stakes are driven in a circle; _fascines_, bundles of f.a.ggots about 10 inches in diameter by 9 feet long. The f.a.ggots are laid together on a horse or between stakes driven in the ground, then "choked," or bound tightly together, by a rope 3 feet 8 inches long with loops at each end, tightened with two stiff levers. The bundle is then bound with wire at intervals of two feet. The circ.u.mference of a fascine should be 25 inches.
[Ill.u.s.tration: Plate #17]
Capt. Powell of the C.E.F. found during 18 months" service in the trenches, that a separate construction for the bottom and firing step from that of the parapet made repair much simpler when the trench was damaged by sh.e.l.l fire. The upper part of the trench usually suffers most, while the bottom section, if unattached, often remains intact and the drainage system needs only to be cleared out. If the portion above the firing step is one piece with that below, however, the whole trench has to be reconstructed.
There is nothing more important than the supports used to keep revetting in place. With sods, sand bags, concrete and gabions, a proper arrangement in the first place will make other support unnecessary.
Sod should be placed carefully, with a slope of not more than one inch on four, with the vegetation uppermost. This type is least affected by rain.
Sand Bags should be used like brickwork, laid in alternate headers (binders) and stretchers. Their use should be confined as far as possible to emergency and repair work, because after a few weeks the bags rot and cannot be moved about. If the trench wall has been demolished by artillery fire, the particles of cloth make digging out the bottom of the trench a very difficult matter.
Concrete Work has been used extensively by the Germans, but the chips fly like bullets under sh.e.l.l explosion, and the concrete cracks and disintegrates in severe weather. It is used in the bottom of trenches for drainage and for the firing step.
Gabions may be set into the wall of the trench and filled with earth, or used at corners to prevent the wearing down of the edge, which reduces the protective effect of the trench. Set in at a slight angle they will hold the side without further re-enforcement.
With the other forms of revetting some secondary support is required.
This is usually furnished by sinking stakes into the bottom of the trench and securing their upper ends to a "dead man"--a stake or log sunk in the ground more than three feet away. The tendency is to sink the dead men too near to the trench, and to attach too many wires to one of them. It is important to sink the stakes at least one foot below the bottom of the trench. By digging holes for them instead of driving them in directly, the sides of the trench need not be disturbed by the concussion of driving the stakes. This is especially important in sandy soils. Stakes should be placed about two feet apart. Dead men should be buried deeply enough to prevent cutting by sh.e.l.l explosions.
Trench Armament.--A few machine guns are set in concealed emplacements along the trench to cover important salients. The automatic rifle is used over the parapet. Besides these there is the rifle grenade and trench mortar. The rifle grenade has a simple emplacement. After securing the proper elevation, the b.u.t.t of the rifle is placed between posts or blocks of wood and the muzzle rested against a log on the wall of the trench. A trench mortar emplacement is dug in the rear wall of the trench, or a sh.e.l.l hole is utilized, care being taken to conceal it from aerial observation.
Loopholes.--Loopholes are still much in use for observation, but they are employed less and less for firing, as they are difficult to conceal from the enemy and almost useless when the enemy is close.
They should cut the parapet diagonally, not directly to the front, and should be concealed by vegetation and by a curtain over the opening when they are not in use. Sheet steel plates with small peep holes are used on the parapet. They are set up with a slope to the rear to deflect bullets.
Trench Bottoms.--In clay or hard soil special arrangements must be made for drainage. Where possible the trench should have a convex surface and should be smooth. A rough bottom means delay in reliefs, and possible injuries. Where trenches are used for long periods board walks should be constructed. Under these drains or sink holes can be placed to collect water. A sink hole may be constructed by digging a pit filled with small stones, or a barrel may be sunk into the ground and filled with stone. Where there is not sufficient slope to carry off the water, or at the lowest point of a drainage system, a water hole should be dug in front of the trench large enough to handle the drainage water.
[Ill.u.s.tration: Plate #18]
Communication Trench (or Boyau).--Running to the rear and joining the different parallel resistance trenches are communication trenches or boyaux. These are for transportation of men and material as well as for communication. Communication trenches should be made wide enough to allow travel for men with loads, should be at least seven feet deep and as smooth as possible on the bottom. Rough places will delay traffic. They are dug with turns every 20 or 30 yards to prevent their being swept by gun fire. When boyaux are built by night sharp zig-zag corners should be used, or the angles will not be acute and protection will be lost. During daylight when the directions can be seen, the construction may be a serpentine curve, with no stretch of more than 30 yards visible from one point.
Boyaux are sometimes used for lateral defence and often emplacements for automatic guns are arranged to cover stretches of them. Bombing stations are placed near by to protect the guns and to clear the boyau of the enemy. At these points the boyau is left straight for a short distance in front. Where provision is made for lateral or frontal defence by rifle fire, firing steps are constructed. If this is inconvenient for movement along the boyau, individual emplacements must be made in the side wall for firing. Sentry posts are dug at right angles to the boyau.
Arrangements for pa.s.sage of men moving in opposite directions may be made by extending short spurs at the corners, enlarging the boyau at the bends, digging niches or pa.s.sing points here and there, or constructing island traverses with the boyau running around on each side.
[Ill.u.s.tration: Plate #19]
Every boyau should be marked where it meets a trench with a sign indicating the place and trench. Without this messengers, reliefs and re-enforcements may easily be lost in the maze of trenches.
Latrines should be run out about 20 feet from boyaux at points directly in rear of lateral trenches. If possible they should be placed so that men cannot enter them without pa.s.sing near the platoon leaders. This will prevent men from leaving the front line, under the pretense of going to latrines, during bombardments or mining operations. The trench leading to a latrine should be constructed like a boyau, and the pit should be close to the side nearest the enemy, to give the best possible protection from sh.e.l.l fire. There are three types of trench latrines: Deep boxes which are covered and have rough seats; short straddle trenches or trenches equipped with a single horizontal bar, and portable cans, used where the ground or the limited s.p.a.ce make it impossible to dig pits. These cans should be emptied daily into holes behind the trenches, which are covered after the cans are emptied into them.
Urinals should be separate from the latrines. They may be either holes about three feet deep filled with stone, troughs with a covered pit at the end, or portable cans.
Shelters. For the protection of men not actually on duty three forms of shelters are used. The _splinter-proof_ is a form of light shelter whose covering affords protection only against splinters. These are usually on the reserve line. About 12 inches to 20 inches of earth over a roof of logs or planks will afford protection from splinters and shrapnel. Curved sheets of iron may also be used. The _deep shelter_ or _bomb-proof_ is a chamber constructed by digging from the surface and constructing a roof. To protect against eight-inch sh.e.l.ls the top of the chamber should be twenty feet below the surface.
Heavy beams or sections of railroad iron are laid across the roof.
Above them is a layer of earth several feet thick; then another layer of timber extending to undisturbed ground on the sides with concrete, crushed stone, metal, etc., above to make a percussion surface for exploding projectiles that penetrate the upper layer of earth. This layer fills in the rest of the s.p.a.ce to the level of the ground.