[587.] The trace of a fire trench or of a system of fire trenches depends upon the ground and the proposed density of the entire firing line. The trenches are laid out in company lengths, if possible.
Adjoining trenches should afford each other mutual support. The flanks and important gaps in the line should be protected by fire trenches echeloned in rear. ([Fig. 6]).
[588.] To locate the trace, lie on the ground at intervals and select the best field of fire consistent with the requirements of the situation.
A profile should be selected which will permit the fire to sweep the foreground, require the minimum of labor and time, and permit the best concealment. No fixed type can be prescribed. The type must be selected with due regard to the terrain, the enemy, time, tools, materials, soil, etc.
[589.] Hasty cover. With the intrenching tool, troops can quickly throw up a low parapet about 3 feet thick which will furnish considerable cover against rifle fire, but scarcely any against shrapnel. Such cover is frequently of value to an attack that is temporarily unable to continue. In time, and particularly at night, it may be developed into a deep fire or cover trench.
[590.] Fire trenches should be placed and constructed so as to give a good field of fire and to give the troops protection behind a vertical wall, preferably with some head or overhead cover. They should be concealed or inconspicuous in order to avoid artillery fire or to decrease its accuracy. They should have natural or artificial communication with their supports, but in establishing the trace this is a secondary consideration.
The simplest form of fire trench is deep and narrow and has a flat, concealed parapet. ([Fig. 1.]) In ordinary soil, and on a basis of two reliefs and tasks of 5 feet, it can be constructed in about two hours with intrenching tools.
This trench affords fair cover for troops subjected to fire, but not actually firing. When it is probable that time will permit elaboration, the simple trench should be planned with a view to developing it ultimately into a more complete form. (Figs. 2 and 3.) Devices should be added to increase the security of the trench and the comfort of the men.
Where the excavated earth is easily removed, a fire trench without parapet may be the one best suited to the soil and other conditions affecting the choice of profile. ([Fig. 4.]) The enemy's infantry, as well as his artillery, will generally have great difficulty in seeing this type of trench.
In very difficult soil, if the time is short, it may be necessary to dig a wider, shallower trench with a higher parapet.