Chapter III

TRENCHES, APPROACHES, PARALLELS, SAPS, SPLINTER PROOFS, AND PASSAGE OF THE DITCH.

=11.= =Trenches.=--A _military trench_ consists of a ditch and embankment affording cover from direct fire. Trenches are used for _approaches_ (or _boyaux_), _parallels_, and _communications_ with magazines, etc.

=12.= =Parallels= are trenches which take their name from the fact that they usually are located on lines approximately parallel to the general front of attack. In a regular siege at least three and frequently a greater number of parallels are used. The exterior one, which is first made, is known as the _first parallel_, the next one as the _second parallel_, and so on. They are used to cover the part of the besieging force known as “_the guard of the trenches_,” which protects the men making the approaches, etc., and also as “places of arms” for assembling troops for assault or for other purposes.

The trench of a parallel is usually 10 feet wide at the bottom and 4 feet deep, finished on the reverse with a slope and on the front with two steps and a berm, with treads of 18 inches and rises of 15-18 inches (Pl. I, Figs. 7 to 13).

The parapet of the parallel should not be higher than 4 feet 6 inches. Its upper surface, particularly in the second and third parallels, should be made approximately plain with a scraper, and its interior slope should be finished and if necessary revetted, so as to afford a good infantry-fire. To allow the troops to move out to the front in line, portions of the interior slope should be cut into steps of not more than about 18 inches rise, and be revetted with fascines or other materials (Pl. I, Figs. 12 and 13). These portions should be 25 or more yards long and near the approaches. If a general assault is to be made, the parallels must be similarly arranged for the necessary length of front.

=13.= =Approaches= are trenches leading up toward the fortification on the front of attack; they connect the parallels and give protection to the besiegers in moving back and forth. To avoid enfilading fire they usually run in zigzags (Pl. VIII, Figs. 80 and 81) across the capitals of the work, with branches seldom exceeding 100 yards in length at the first parallel, and growing continually shorter as they approach the work. Each branch is so directed that its prolongation will pass from 30 to 40 yards outside the most advanced position within effective range held by the defence. At each turning-point of the zigzag the more advanced branch is prolonged from 10 to 20 or more yards to the rear, to cover the angle of the approach. These returns are also useful for storing trench material, etc.

After the return is completed the sharp angle in the trench is rounded off to allow gun-carriages, etc., to make the turn.

Approaches are usually 4 feet deep, 9 to 12 feet wide at bottom, with slopes in front and rear as steep as the earth will stand, and have a rough parapet not less than 4½ feet high, separated from the trench by a berm of 18 inches, or more if necessary (Pl. VIII, Fig. 82). When drainage requires it, as it very frequently does, the bottom of the trench is sloped from front to rear about 6 inches; a ditch cut along the reverse slope, discharging into the drainage ditches of the vicinity, or into drainage pits excavated in rear of and outside the approaches. These may be lined with a gabion to prevent their sides falling in.

TRACING AND CONSTRUCTING PARALLELS AND APPROACHES.

=14.= =Tracing Parallels.=--Parallels are located by engineer officers after careful reconnoissance of the ground. Guiding points and lines are marked so as to be readily found in the dusk, but so that they cannot be seen by the defence. When completely screened from view important points are marked by posting sappers at them.

When no other practicable method can be used, the directions are determined by the use of a pocket compass fastened to a straight measuring-rod.

Tracing is begun as soon as the approaching darkness will conceal the parties from the defence, while close objects are still visible. The length of parallel traced by each officer should not exceed that occupied by one unit, usually a battalion, as a working party. (A battalion of 500 men will occupy 800 yards).

The tracing party consists of one officer, one N. C. officer, and one sapper to each 50 yards of parallel, with one or two extra men.

The officer is provided with a pocket compass and measuring-rod. The N. C. officer has a tracing-lantern and a mallet with muffled head. Each sapper carries a roll of tracing-tape, a tracing-picket, and a six-foot measuring-rod. The officer stations the first sapper at the initial point, and, taking one end of his tracing-tape, moves along the line of the parallel, accompanied by the rest of the sappers; the first sapper places his picket between his feet, and the N. C. officer drives it into the ground far enough to make it secure. The sapper drops the ball of tape on the ground and lets it run out through his hands until nearly out, when he checks it, and when it is all out places the loop over his picket and lies down to await the arrival of the working party. The N. C. officer, as soon as he has driven the first picket, follows on to the second, etc.

The officer, having run out the tape of the first sapper, halts the second, takes the end of his tape, and proceeds as before until the parallel is traced, and a sapper is left at each 50 yards of its length. Each sapper is told the designation, by number and section, of the point he occupies.

=15.= =Tracing Approaches.=--The approaches are traced in the same manner as the parallels, but at each turning point of the zigzags a picket is driven, around which the tape is carried. After tracing the branch in front, the tape is cut at five yards in rear of the picket, and the end carried out to the rear in prolongation of the branch in front to indicate “the return,” which is then prolonged to the proper length (from 10 to 20 yards) by a short piece of tracing-tape.

POSTING THE WORKING PARTIES.

=16.= The working parties are commanded by their own officers, under the guidance of the engineers. They carry their arms and ammunition. Each battalion (or other unit) is marched in column to the depots, where the tools are laid out in lines, so that each man can take up his pick and shovel when drawn up in front of them. But when time does not allow of this arrangement, they are piled, the picks in one pile, the shovels in another, and the men pass the piles in single file to the right of the picks and the left of the shovels, each man receiving a pick and shovel as he passes the pile. If gabions are to be carried, they are distributed in a similar way. When two are carried, the shovel is secured inside one and the pick inside the other, and the gabions are then carried by the picket inserted for that purpose. When one only is carried, the pick is usually secured inside it, the gabion carried on the shoulder, and the shovel in the hand.

The working party, in column, provided with its tools, etc., is then led by the engineer officer to the parallel, and forms on right (or left) into line along it in single rank at five-foot intervals, beginning at the initial point of each section.

The N. C. officer of engineers assists in this formation, and each sapper points out to the men of the working party the five-foot intervals marked upon the tracing-tape of his 50 yards, verifies their positions along it, and subsequently superintends their work. Each man when properly placed drives his pick into the ground at the left of his task, places his shovel beside it, and lies down until the command “Commence work” is given.

When gabions are used the working party is posted in a manner entirely similar, except that the column is of necessity formed in single rank when marching to the initial point. The men form on right (or left) into line and place their gabions in front of the tape and touching each other; each man then takes out his tools, places them behind the gabions, lies down, and awaits the command to commence work. The sapper sees that the gabions of his 50 yards are properly aligned and touch each other throughout.

Both the sappers and working parties are divided into reliefs, usually of eight hours. The sappers of the tracing parties superintend the work of the first relief of the working party, but are relieved long enough before them (½ hour to 1 hour) for the second relief to become acquainted with the details of its sections before the second relief of the working party arrives. A similar arrangement is made for the third relief.

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.

SPLINTER-PROOF COVER.

=20.= =Splinter-proofs= for the guards of the trenches, for field-hospitals, latrines, etc., should be provided as soon as possible after the parallels are finished. They may be placed in the returns of the approaches, or in rear of the parallels, and be connected with them by trenches. Pl. II, Figs. 14-16, show their construction when in rear of the parallel and revetted with logs, fascines, or sawn lumber. The trench is 9 feet wide by 4½ feet deep. Its front edge should be 25 or 30 feet in rear of the reverse slope of the parallel. This width of trench will allow 2 to 4 men per yard of its length to work advantageously. They should finish it in 8 hours.

In digging the trench the earth is thrown out on both sides, leaving a berm of about 1½ feet on each side to allow the woodwork to be properly placed. When this is completed the earth in rear is thrown forward to complete the cover, as shown in the plate.

Steps for egress and openings for light and ventilation may be placed at intervals along the rear, and, when desirable, bunks may be built, as shown in the figures.

When the splinter-proofs are built in the returns of the approaches, the overhead cover may extend entirely across, steps and openings being provided as in the previous case; or posts and longitudinal beams may be set in the trench to hold up the rear end of the cross-beams, leaving the rear of the splinter-proof open. Portions of this may be closed, if desired, by leaning short posts or fascines against the longitudinal beam and banking earth against them.

The splinter-proofs may generally be drained into the parallels or approaches. When this cannot be done drainage-pits must be used. Limited portions of the splinter-proofs may be protected against leakage through the earth cover by first filling over the covering beams with earth, packing it to a smooth surface with a gentle slope, placing upon it raw hides, roofing felt or other waterproof material, and then completing the cover by adding the necessary thickness of earth, giving finally to its top surface a slope to carry off the rain.

BOMB-PROOFS.

=21.= When, in the close attack of the work, the besiegers are subject to vertical fire from small mortars, better overhead cover must be obtained by bomb-proofs, constructed by deepening the trench, using stronger beams, and a greater thickness of earth. Twelve-inch timbers laid touching each other, with spans of 5 feet and 5 feet of earth cover, have been considered sufficient; but with the improvement of high-angle fire and the use of high-explosive shells greater protection will be needed in the future. Experimental data for fixing the amount is not now available; an approximate thickness of earth cover may be computed as indicated below.

The mining effect of dynamite in common earth is something less than twice that of an equal weight of gunpowder. (See Military Mining, Arts. 13 and 14.)

An explosive enclosed in a strong case, however, expends a part of the energy due to explosion in rupturing this case. The stronger the explosive the less will be the percentage of the total energy required for breaking the case, and the greater the percentage remaining for performing other work. For this reason equal weights of high explosives and of gunpowder enclosed in strong shells will not produce the same relative effects in forming craters, &c., that they would if contained in paper cases. The effect of the high explosive is relatively much greater when contained in a strong shell. Experiments made at Fort Hamilton, 1890-91, with 8-inch shells loaded with explosive gelatine, showed this explosive to have between 4 and 5 times the effect of gunpowder, while in paper cases the relative effects were as 1.7 to 1.0. (See Report of Board of Ordnance and Fortification, Ex. Doc. No. 12, 52d Congress, 1st Session, January 5th, 1892.)

Since the mining effects of the charges contained in shells are, however, less than when packed in thin cases, the thickness of cover determined by the use of the usual mining formulas should err on the side of safety.

Knowing the charge contained in shells to be fired against a bomb-proof, and their probable penetration, the formulas given in Arts. 7 to 12, Military Mining, may be used for finding equivalent common mines and radii of rupture for dynamite and explosive gelatine by substituting in them 1/17 for 1/10.

The values given in Art. 11 will probably be sufficiently accurate for the radii of rupture. The cover given in the direction of the fire must be greater than the sum of the penetration and the radius of rupture.

When the penetration is equal to or greater than twice the L. L. R. of an equivalent common mine a camouflet will probably be formed, whose radius of rupture, from the formulas, will be equal in all directions and may be assumed as ⅕ of the L. L. R. of the equivalent common mine.

In this connection, see par. 61, p. 58.

SAPPING.

=22.= When the trenches have been carried so near the work that the simple or flying trench cannot be used without undue loss, recourse must be had to the _sap_.

A =sap= is a narrow trench (subsequently widened), which is continually prolonged in the desired direction by digging away the earth at its head and throwing it to the front and exposed flank as a cover for the working party.

When the sap is subject to an oblique front-fire, exposing one flank only, the parapet is constructed on that flank and at the head. This is known as a _single_ or _full sap_. When both flanks and the head of the sap are exposed to fire two full saps are driven parallel and very near to each other, each with its parapet on the outer flank. The tongue of earth left between them is removed to widen the narrow trenches, thus making a single trench with a parapet on each side. This is called a “_double sap_.”

The trench is sometimes deepened and given a splinter-proof roof or cover. This is known as a “_blinded sap_.” A sap gaining ground to the right and front is called a “_right-handed sap_;” its parapet is on its left flank. A “_left-handed sap_” has its parapet on its right flank.

To expedite the work in sapping several reliefs should be employed, and task-work should be adopted to induce the men to work rapidly.

In all sapping operations the use of the simple trench and flying sap will be resumed when circumstances admit without involving too great losses.

=23. The full sap= (Pl. II, Figs. 17-21) requires a detachment, or “=brigade=,” of 1 non-commissioned officer and 8 men, provided with the following tools, viz.:

For No. 1, a miner’s pick, a miner’s shovel, a measuring-rod,
4' 6", marked at 3', and a
sand-bag fork.

No. 2, a measuring-rod of 1' 6" and a shovel.

No. 3, a pick, a shovel, and a measuring-rod 5'
long, marked at 4' 6".

No. 4, a shovel and a scraper with a handle 9'
long.

For the rest of the detachment, a 6' measuring-rod, knee-caps for 4 men, 2 shovels and 1 pick (in reserve), and, when necessary, a crowbar, axe, and bill-hook. From 100 to 150 sand-bags are supplied to each detachment.

=24. Organization and Duties of the Detachment.=--The sappers are numbered 1, 2, 3, and 4 in each rank; the front rank extends the sap 1 yard and is then relieved by the rear rank, and so alternately. The sappers change places at each relief; those who serve as Nos. 1 and 2 during their first task becoming Nos. 3 and 4 during the second, and so on throughout their tour.

If a detachment is reduced below 8 in number by casualties it nevertheless keeps 4 men at work driving the sap, and reduces its reserve until new men are supplied.

=25. Driving the Sap.=--The sap is driven as follows, viz.: Nos. 1 and 2 dig a ditch 4' 6" deep, 1' 6" wide at bottom, and 3' or more at top; the berm side has a slope of 3/1, and the reverse is vertical, or as nearly so as the earth will stand. They leave no berm, as they need all the cover they can get. Nos. 3 and 4 widen this trench 2 feet and form a berm of 1' 6" by digging away the foot of the parapet and throwing the earth upon its top and exterior slope. The head of their work is kept at 9' in rear of the head of the sap.

The side parapet made by Nos. 1 and 2 is about 2' 6" high and bullet-proof (about 2' 6" to 3' thick) at 18 inches above the ground. The head parapet is made up of about 60 sand-bags, from ½ to 2⅔ filled. It joins the side parapet and extends across the head of the sap. It is about 2' 6" high.

As the sap is driven forward the head parapet is advanced by throwing the rear sand-bags over to the front by hand or by the use of the sand-bag fork. In excavating the trench No. 1 kneels down, undermines, and digs down enough earth to advance his trench about 9 inches. He is replaced by No. 2, who shovels this earth upon the side parapet toward the head of the sap. No. 1 then resumes his place and throws the sand-bags at the head of the trench over the parapet until he has uncovered about a foot in advance. He uses a sand-bag fork when necessary. The trench is advanced 9 inches more in the same manner. No. 2, besides throwing out the earth dug by No. 1, trims up the slopes and gives the trench its proper width and depth.

Nos. 1 and 2 change places when they have advanced the head of the sap 1' 6", and are relieved as before stated when 3 feet is gained.

Nos. 3 and 4 work together upon their task. In shovelling the earth upon the parapet they throw it somewhat toward the front and regulate its height with the scraper. The rate of advance is usually from 2 to 4 feet per hour.

_Widening the Sap._--The sap is widened by working parties, usually of infantry, who work at about, but not less than, 25 feet in rear of the head of the sap. In an approach their task is equal in volume to that of the sappers, and can be finished in one relief. In a parallel when steps are to be provided, a second relief makes the steps, drainage ditches, drainage pits, etc.

_A change of direction_ in a full sap (Pl. II, Fig. 20) is made by No. 1 turning in the new direction and working through the old side parapet; No 2 throwing the earth over the old head parapet. The sand-bags of the old are gradually removed and used for a new head parapet, 20 or 30 additional sand-bags being ready for use if needed before the others can be safely removed. Nos. 3 and 4 follow on as before.

A _return_ can be driven back when desired by another detachment of sappers. No head parapet will be needed, but the side parapet will be kept a little in advance of the head of the sap.

=26.= =Breaking Out a Sap from a Parallel.=--The head parapet of a sap is about 2' 6" high. The parapet of a parallel is about 4' 6" high. A sap of the usual form driven through the parapet of a parallel will expose the latter to fire through the opening formed. To reduce the danger from this exposure, the sap is broken out at night, and to cover the opening in the shortest time two or three men may creep over the parapet of the parallel and cover themselves by rapidly digging a hole, from which they may work back and join the sappers, who are working outward. The sap being driven obliquely to the front, the trench widened, and the parapet made full size (Pl. II, Fig. 21), the opening will be covered; or a few men may in some cases construct in front of the parallel a short section of flying sap, under cover of which the full sap may be broken out (Pl. III, Fig. 28).

Preliminary preparations for breaking out should be made during the day, but should be so conducted as not to indicate the selected point to the enemy.

=27.= =Circular places of arms= (Pl. IX, Fig. 83) may be formed in front of a parallel by breaking out two single saps from points 80 to 100 yards apart and so directing them as to meet at 25 or 30 yards in front of the parallel. They may be used by the guards of the trenches or as depots for trench material, etc.

=28.= =Shallow Sap.=--When the presence of water in the soil or of rock near the surface prevents driving the full sap 4' 6" deep, a shallow or modified sap (Pl. II, Figs. 22 and 23) may be used, provided a trench 3' deep can be driven forward. In this case Nos. 1 and 2 must both work kneeling, and Nos. 3 and 4 must throw the earth well to the front and keep the parapet as high as possible, leaving the construction of the berm to the widening party, who will give to the trench the necessary width, and will then obtain earth for strengthening the parapet by deepening the trench when practicable and widening it when necessary, making, however, no irregularities which will injure it as a communication and no depressions which will collect water. This sap advances about as rapidly as the full sap.

=29.= =Overground Approaches.=--When the water or rock comes to the surface of the ground, approaches can, under favorable circumstances, be driven for short distances by carrying forward earth in sand-bags, forming with them head and side parapets, and moving forward by continually building up the latter and advancing the former as before described.

The expenditure of time, labor, and sand-bags is so great in driving approaches in this way that the minimum height of parapet (possibly 5') should be made with sand-bags. This may be subsequently heightened and strengthened with earth brought forward in barrows or hand-carts and thrown upon the top and exterior slope.

=30.= =Double Sap.=--(Pl. III, Figs. 24, 25, and 26).--The double sap consists of two parallel single saps driven side by side, the cutting lines of the berms, usually 10 feet apart, making the bottom of the completed trench 7 feet wide. It is used when the zigzags, to avoid enfilade, make such a slow advance as to be no longer profitable, i.e., when the amount gained in advance does not exceed ⅓ the length driven. The double sap is directed toward the work, and is exposed to an enfilading and also to a slant fire from both directions. It must therefore have a parapet in front and on both flanks.

=31.= =Execution of the Double Sap.=--The double sap is driven by two detachments, each organized and equipped exactly as for a single sap, except that a greater number of sand-bags should be supplied when practicable. The sappers work as in driving a single sap, with the following modifications only: The Nos. 1 prolong their head parapets until they meet, and in advancing their heads of sap leave undisturbed the 4 feet of head parapet intervening between their trenches, but, by continually throwing sand-bags or earth obliquely to the front, keep the head parapet continuous and nearly straight.

This leaves between the trenches made by the Nos. 1 and 2 of the two detachments a tongue of earth 4 feet thick, surmounted with a parapet about 2' or 2' 6" high, which serves as a parados and protects these sappers from reverse fire.

Nos. 3 and 4 of both detachments, in completing their tasks, remove this tongue and pass forward the sand-bags forming its parapet for use by Nos. 1 and 2; otherwise the tasks are as in driving a single sap. When sufficient sand-bags are not available the middle part of the head parapet must be made of loose earth, giving much less protection to Nos. 1 and 2.

This sap, from its method of construction, is completed by the sappers without the assistance of the infantry working parties. Its rate of advance is about the same as that of the single sap.

=32.= =Changing Direction of a Double Sap.=--When a change of direction is to be made No. 1 of the first detachment marks on the berm the width of the top of the trench (10'), Nos. 1 and 2 of the wheeling flank come around the tongue and the leading sappers of the two detachments start their sap-heads in the new direction. Nos. 3 and 4 of both detachments remove the tongue of earth and complete the parapets of the original trench and then follow up their Nos. 1 and 2 as before.

=33.= =Breaking Out a Double Sap from a Parallel.=--A double sap is broken out by methods entirely similar to those already described for the single sap (Pl. III, Figs. 27 and 28). The figures explain themselves.

=34.= =Traversed Sap.=--A sap may be traversed to protect it against enfilade by frequent changes of direction, generally rectangular (Pl. III, Figs. 29, 30, and 31), or by making hollow traverses by blinding the sap at points separated by limited distances (Pl. III, Figs. 32-35).

In traversing a double sap by change of direction, a single sap is broken out to the right or left (or one in each direction) and pushed forward to the desired length. From the flank of this the double sap is again broken out and driven to the front until another traverse is required. When the saps are driven to both right and left a double sap is driven to the front from the extremity of each, and at the next change of direction the single saps are driven towards each other until they meet, and the double sap is driven in the prolongation of its original direction. This forms what is called a _cube traverse_, and gives additional room in the communications. The single sap is used in making traverses, since by throwing all the earth on one side better cover is given. When the sap is so far advanced that it becomes subject to a reverse fire the double sap will have to be used in making the traverses.

_Length of Traverses._--Traverses should extend at least 12 feet beyond the trench in their rear, which will give them a length of from 25 to 30 feet on the berm. The salient angles of the sides of the trenches should be rounded as much as practicable to allow the easy passage of guns, and those of the parapets should, when necessary to screen the trench, be held as nearly vertical as practicable by sand-bags. Ramps leading to the surface of the ground may be made in rear of the traverses when needed. For guns they are 8' wide, with slope not greater than 1/4. The work in making traverses being considerable, they should be spaced as far apart as practicable.

_Spacing Traverses._--In driving the sap to the front the low head parapet of the sap will defilade a less length in the rear than would the finished parapet of the traverse, which is from 2 to 3 feet higher. The sap is, nevertheless, pushed forward to as great a length as will be defiladed by the traverse when finished, the sappers meanwhile passing the partly protected portion by stooping or creeping when necessary.

=35.= =Traverse by Blinded Sap.=--In traversing a sap by blinding a part of its length (Pl. III, Figs. 32-35) the sap is first deepened 2 feet over this part; mine-cases, frames similar to mining-frames, or regular blindage-frames (see Military Mining, Arts. 53-55) are then put in position, the side slopes are held up by sheeting, when necessary, and the top is covered with sheeting, fascines, rails, or other material; earth is then thrown upon the top to bring it up to the desired height for a traverse, which will usually give at least 3 feet of earth covering. When a considerable thickness of earth is to be used the frames must be made correspondingly strong. For a clear opening, 6 feet at bottom, 8 feet at top, and 6' 6" high the English engineers recommend frames at 3-foot intervals, with 6-inch square posts, 2-inch thick sills, and 9 inch × 6 inch caps 12 feet long. The end frames should be braced against outward thrust by 6" × 6" struts.

These traverses are usually made at least 20 feet in length. They can be used only when good drainage can be secured.

=36.= =Crowning the Covered Way= (Pl. IV., Fig. 36).--The traversed sap is used for “_crowning the covered way_,” which consists in constructing a battery or infantry parapet along the crest of the covered way, from which a fire can be brought upon the ditch and the scarp-wall of the work. To accomplish this the sap is run parallel to the crest, with its nearer cutting line 18 or 20 feet from it. For an infantry trench the traverses may be of the dimensions already given.

To cover a battery they should be about 33' long. It will generally be necessary to use the double sap altogether in their construction, but usually the earth excavated by Nos. 3 and 4 of both brigades will be thrown on the parapet next the work, the parapet on the reverse side formed by Nos. 1 and 2 affording sufficient cover for constructing the sap and traverses. The parapet is prepared for infantry fire as described for the parallels. The emplacements for guns, the service magazines, etc., etc., are prepared and the embrasures are cut, or the parapet prepared for the overbank carriages at the last moment, under cover of the small-arm and machine-gun fire from the parallels and places of arms, and the artillery fire from batteries, which do not endanger the working parties.

=37.= =Trench Cavalier.=--In order to obtain a greater plunge upon the covered way and ditch, short lengths of double sap are sometimes run at right angles to the direction of the crests of the covered way at about 30 yards outside its salient. The parapet on the side of the sap towards the work is thrown forward and built up to the desired height with gabions and sand-bags, and provided with steps and sand-bag loopholes, giving a short length of parapet with considerable command, firing directly along the covered way at short range.

This construction is called a _Trench cavalier_. It will be seldom, if ever, used in the future.

=38.= =Former Methods of Sapping.=--Before the general introduction of machine and rapid-fire guns and of small arms of extreme accuracy and penetration saps were constructed by No. 1 sapper driving a trench 18" × 18", which was enlarged successively by Nos. 2, 3, and 4. Cover for the sappers was obtained by the use of a sap-roller (a large gabion, 7' 6" long and 4' in diameter, stuffed with fascines and rods) as a movable head parapet, and the construction of the side parapet was expedited by the use of gabions, sap-fagots, etc. This method cannot be used against an enemy well equipped with modern weapons. It is referred to only as a suggestion that a readily improvised modification of it might be used to capture, with the least possible loss, a party of rioters, criminals, or other badly-armed men occupying an isolated house or other cover.

PASSAGE OF THE DITCH.

=39.= When breaches which are practicable are made in both counter-scarp and scarp of a dry ditch an assault may sometimes be made successfully; but when the scarp-wall is not breached, or when, for other reasons, an assault from the crowning of the covered way is not considered advisable, the ditch must be crossed and the breach, when made, must be crowned by regular approaches. This is accomplished by the use of a _sap_, _single_ or _double_, depending upon whether it is exposed to fire upon one or both flanks. Owing to the plunging fire of the defence it may be necessary to make the sap deeper than 4' 6", or in some cases to _blind_ it for a part or the whole of its length. It is generally impracticable to drive the sap down the slope of a breach in the counter-scarp; therefore a _blinded descent_ (Military Mining, pars. 53 and 54) is used. It is so directed that when the counter-scarp is reached the floor of the gallery will be at the required depth below the bottom of the ditch; i.e., at the depth fixed for the bottom of the sap. When the small-arm fire of the defence is so severe as to necessitate blinding the sap from the counter-scarp across the ditch, it will usually be imperative to provide a _shield_, under cover of which the sappers may start the blinded sap. This may be made of boards covered with bullet-proof iron-plates, and of such width and length that it may be carried through the gallery, thrust out into the ditch, and then turned, placed in position, and blocked up at such angle and to such height as may be wished, by men who move on their hands and knees and support the shield on their backs. Under cover of this shield the head parapet of the blinded sap may be thrown up and the sap then driven in the usual way.

For method of breaching by mines, see Military Mining, pars. 91 and 92.

After breaching the scarp, if an assault is to be made, the counter-scarp, for a length equal to or greater than the length of the breach, should be blown down, to give the assaulting party access to the breach. If the breach is to be crowned, and approaches are to be driven against interior retrenchments, a gallery of descent should be driven to the counter-scarp at one side of the breach before the assault is made. From this a trench should be driven to the crowning of the breach (usually by flying sap), by means of which communication is maintained between the crowning of the breach and the exterior.

=40.= =A wet ditch without current= may be crossed by building a causeway, upon one or each side of which a parapet is constructed (Pl. IV., Figs. 37, 38, and 41).

The floor of the gallery of descent should strike the ditch at about one foot above the level of the water. The counter-scarp wall having been broken through, a shield similar to that described in the preceding paragraph may be used to cover the sappers working at the outlet of the gallery in the first stages of the succeeding work. The causeway is built by throwing into the ditch short fascines or brushwood mats, having bound up with them stones enough to sink them, broken-stone gravel or other available material, until the causeway is 8 or 10 feet long, about one foot above the level of the water and wide enough for the roadway and parapets. The head and side parapets are then constructed with sand-bags, which are passed out under the shield and piled at its head and sides. The causeway is continued by throwing material over the head parapet, and the approach is driven forward somewhat like a sap. So soon as the head and side parapets are made the shield may be raised up and supported upon two or three cross-balks resting upon the side parapets. It may then be progressively moved forward and the approach in its rear be blinded by sappers working under its protection. Unless the plunge of the fire upon the approach is equal to or greater than 45° this shield will, however, when vertical, cover a greater length of trench than when in a horizontal position. It may, therefore, if desired, be turned into a vertical position and be supported by a frame built for that purpose, which can be moved forward as the approach advances (Pl. IV., Fig. 41).

When the head of the approach is subjected to the fire of rifle-bullets only, it may be practicable to dispense with the head parapet, replacing it with a bullet-proof screen covering the head of the approach and with wings extending back and overlapping the side parapets (Pl. IV., Fig. 40); the shield floating on a raft of light logs, or other material which cannot be sunk by rifle fire. This shield would, of course, be erected and launched under cover of the one already referred to. To save sand-bags, etc., the interior slope of the side parapets may be revetted with gabions about 4' 6" long, resting on two short fascines and crowned with three others, giving a height of about 6' 3". Upon these, when necessary, cross-beams are laid and the blinding of the approach is finished with sand-bags thrown on top.

=41.= =A wet ditch with current=, or one in which the water level may be varied by the defence, presents greater difficulties. The method of crossing which seems most promising is by a causeway made of materials which will allow the water to pass freely through it. For this purpose it is usually recommended to use casks with their heads knocked out, or strong gabions lashed to balks, so as to form continuous tubes, which are loaded with stones and sunk with their axes parallel to the counter-scarp by sappers working under cover of a shield. When the top of the causeway is about a foot above the high-water mark it is levelled off with fascines and the approach driven forward as previously described. When available, iron or terra-cotta pipes of large diameter may, perhaps, be advantageously substituted for casks or gabions. Cormontaigne, at Philipsburg, in 1734, successfully used floating bridges of fascines with parapets of gabions and fascines covered with raw hides. Two bridges were made. They were 128 feet long, 48 feet wide, and 6 feet thick. The water was about 15 feet deep. To construct and hold in place, in a strong or varying current, a floating bridge of sufficient width and depth to support, without sinking or capsizing, the parapets necessary for protection against modern small arms and machine guns is a task presenting such great difficulties that it will hardly be undertaken, except as a last resort, and then with a very uncertain issue.

When, however, the fire of the work is nearly or entirely silenced, a floating bridge of pontoons, casks, spars, or other materials, with a slightly masked roadway, may furnish a sufficiently good crossing and may be constructed with little difficulty and loss.[1]