Attack of Fortified Places. Including Siege-works, Mining, and Demolitions. / Prepared for the use of the Cadets of the United States Military Academy

by James Mercur

EXECUTION OF PARALLELS AND APPROACHES.

17. Simple Trench.—A trench made by excavating the earth and forming a parapet without revetment of any kind is known as a “simple trench,” or as “simple trench-work.”

Flying sap or flying trench-work.—When, in order to obtain cover more quickly, gabions are used to hold the earth first excavated, and subsequently to serve as a revetment to the interior slope of the parapet, the trench is known as a “flying sap” or “flying trench-work.”

18. Construction by Simple Trench.—The first parallel and the distant approaches are usually constructed by the use of the simple trench, as follows, viz. ([Pl. I], Figs. 9 and 10): The men having been posted along the tracing-tape at five-foot intervals, as previously described, and their positions verified by the engineer officers, the command “Commence work” is given. Each man marks the left and front of his task by a line dug with his pick, and, commencing at the left of his task, at once excavates a trench 3 feet long, 1½ feet deep, and 6½ feet wide, throwing the earth to the front, and making a parapet 1½ feet high, leaving a berm of 1½ feet. Then, commencing at 1½ feet from the front of his trench, he deepens it to 4 feet, making the parapet 3 feet high. When the task of a party is finished each man cleans off his pick and shovel, places them at the rear of the trench, and leaves them there for the use of the second relief.

By excavating in this way, partial cover while at work and a defensible parapet are rapidly obtained, and, at the completion of the task, the parapet admits of a strong defence, and affords cover sufficient to allow the first relief to be withdrawn and the second to be posted without exposure. Special care must be taken during the work to make the men face toward the parapet while digging, in order to avoid striking their neighbors with the pick when raising it for a blow.

The second relief widens the trench 4 feet; forms a bottom step 18 inches wide with such materials as are available; heightens the parapet to 4½ feet, and throws the rest of the earth to the front to thicken it ([Pl. I], Fig. 8).

The third relief widens the trench 2½ feet at the bottom and slopes off the reverse as steep as the earth will stand. The earth is used to thicken the parapet, additional shovels and shovellers being provided if found necessary.

The approaches ([Pl. VIII], Fig. 82) are extended in a similar manner; the tasks of the reliefs are marked on the sections.

Variations from these sections are made when rendered necessary by the presence of rock or water in the soil ([Pl. I], Fig. 11); when a wider trench is required for a tramway or for free communication; or, in special cases, when a narrower trench will answer the purpose and save work. Should a specially heavy fire make additional cover necessary, it may be obtained by deepening the ditch and thickening the parapet, leaving its crest at the same height. The sections above given have been found best for ordinary cases.

19. Construction by Flying Sap.—The construction of the first parallel having indicated to the defence the front of attack, further operations will usually be subject to a more destructive small-arm and machine-gun fire. This will, as the siege advances, render the losses experienced in constructing a simple trench too extravagant, and a quicker method of obtaining cover must be used. This method is found in the flying sap ([Pl. I], Figs. 8 and 13), which is executed as follows, viz.:

The men are posted and the gabions placed as previously described. The engineer officer having marked the lines, the order “Commence work” is given. Each man marks with his pick the front and left of his task (which in this case is 4 × 6½ feet, leaving a berm of 1½ feet), and proceeds at once to dig on its left, filling first the left gabion, next the right, and then throwing the earth over and in front of the gabions. Each gabion, when it is half filled, is tipped outward until it has a slope of about 4 on 1. The filling is then completed.

As each man of the first relief occupies only 4 feet of front (2 gabions), his task is four-fifths as great as it is in executing the simple trench.

The second and third reliefs have the same tasks as in the simple trench. When the first relief finishes its task, every fifth workman (indicated by the sapper of the section) retains his pick and shovel and returns them to the depot when he marches past it.

The others leave their tools for the use of the second and third reliefs. In good soil the gabions may be filled in from 7 to 15 minutes.

The English sap-shield is designed for use when the fire is so severe that the flying sap with gabions becomes impracticable. Owing to its weight (80 lbs.) a man can carry but one; hence a carrying party equal to the working party assists in placing the shields and then retires. This gives to each workman a task of 3½ × 6½ feet.

The shields are placed as shown in [Pl. I], Fig. 6; the trench is executed as previously described, the earth being thrown over and beyond the shield. The shields are removed after the task of the first relief is finished.

The sap-shield is designed to be used in special cases for covering the head of a full sap (described further on), in which case it is placed as shown on [Pl. I], Fig. 4; and also as a body cover for a man moving for a short distance in the face of a heavy fire, as is necessary at times in sapping and mining operations. It has not yet stood the test of service in a siege.