Frequently trenches designed primarily for other purposes (such as cover trenches, com municating trenches, approach trenches) are in places prepared for occupancy by a firing line, with appropriate banquette, etc. Sometimes approach trenches may be prepared for firing in both directions. These are not firing trenches, as their use for firing is secondary.
Cover trenches are from 10 to 100 yards in rear of the firing trenches, to protect men of the firing line (except firing trench guards) during all hut the infantry attack. It is mot advisable to occupy firing trenches in force ex cept when they are attacked by infantry. The bulk of the firing line should, therefore, be close at hand under cover, and to a certain extent the trenches which afford this cover facilitate lat eral communication. They are often not con tinuous, and there obtains with them also the tactical requirement of dispersal. Cover trenches are amply provided with cave shelters, bombproofs and the like, and in places with firing banquettes, overhead cover and loopholes, or head cover. From the cover trenches, ap proach trenches proceed forward at frequent intervals to the communicating trenches or fir ing trenches. Usually at less frequent intervals are approach trenches entering the cover trenches from the rear.
Communicating trenches are deep trenches used to afford proper lateral communication. They are frequently of short lengths, in front or in rear of the general line of the cover trenches, and generally opposite intervals be tween such trenches. Portions of the commu nicating trenches are prepared for fire and local supports may be sheltered in portions of them. Communicating trenches and cover trenches are connected by approach trenches; and where communicating trenches are in advance of cover trenches they are connected with the fire trenches by approach trenches relatively close together.
Approach trenches are those enabling one to pass within the intrenched zone from front to rear or vice-versa between firing trenches, cover trenches, communicating trenches and other parts of the trench system farther in rear. Por tions of approach trenches are prepared for fir ing, sometimes in both directions. They are deep, comparatively wide and winding, tra versed or zigzagged.
Local trenches extend to observation sta tions, command posts, latrines, machine-gun emplacements and the like from firing, com municating, cover or other trenches.
over the parapet with proper effect so he can hit the bottom of his own wire entanglement. Traverses must be adequate. A parados must be provided to give protection against the back blast of high explosive shell; the trace of the trench should be irregular, to provide flanking fire; and if the trench is to be held for any length of time, the sides must be revetted and the bottom of the trench must be floored and drained. The drawings show the trench pre Support trenches are designed to shelter all of the troops who form the local supports of the firing line during hostile artillery fire, and a portion of them during the period of the in fantry attack, if all are not then required far ther forward. They are ordinarily similar in type of construction to cover trenches, but are located farther to the rear. Toward the front
they merge into the cover trenches. Toward the rear they may be as remote from the firing trenches as 200 or 275 yards. Approach trenches 'provide communication from them to front and rear. Support trenches are some times provided with firing banquettes and the like, and machine-gun emplacements may be located among them. Often the ground is such as to permit the supports to occupy naturally sheltered positions, where support trenches may be much modified or omitted in places alto gether.
Reserve trenches are trenches designed to shelter the local reserves. They are in rear of the support trenches, but often merge into them toward the front. While the firing trenches hold, reserve trenches are not ordinarily sub jected to prolonged and severe artillery fire.
Every firing trench should fulfil the follow ing essential conditions: The parapet must be bullet proof ; every man must be able to fire pared for firing through and over the parapet and typical sections for long range searching or barrage fire. The narrower the trench, the bet ter the cover; but if too narrow it may hamper the movement of troops too much. Therefore, a firing trench is usually made broad enough to allow of movement behind the line of men manning the parapet. Every man must be able to use his rifle over the parapet, and the men moving behind must not have to stoop down in order to get their heads under cover.
The resulting section is, therefore, with a banquette or firing step 18 inches wide and four feet six inches (or five feet, if we allow for a small notch or hollow to hold the rifle) below the crest of the parapet, and behind this a deeper portion from 18 to 30 inches wide at the bottom and from six to seven feet below the crest line of the parapet. The firing step must have a level surface to give a firm foothold. It may consist of an earth step revetted with planks held in place by stakes.
The best type appears to be a trench with a firing banquette for five or six rifles in each bay, between two traverses, each six feet thick and with the back part of the excavation formed into a communicating trench for observation and manning of parapets. This trench is some what deeper and three feet wide.
Types of communicating and approach trenches are now becoming fixed. In France the following seem to have been the normal: Communicating and approach trenches well to the rear six feet wide, those farther forward three feet wide and both types six feet eight inches deep. A communicating trench less than three feet wide at the bottom is sure to become jammed. This is the minimum to be allowed, and the work should be commenced with a width of not less than three feet eight inches at the top, so as to get three feet at the bot tom, according to the stiffness of the soil.
Berms of one foot on each side of a sap or trench where earth is piled up are considered in dispensable. They prevent sliding of earth and furnish a little shelf on which to place tools, bags, guns and other articles, in case troops want to pass the occupants; the double berm also makes it possible for infantry to get out of a trench when the emergency requires.