Ditch, Defenses of

Although the ditch of a major field work was intended to interpose an obstacle between an attacking body of troops and the defenders lining the parapet it had the potential for seriously hindering a garrison's defensive effort if attacking troops were able to enter the ditch and find shelter from the defenders' fire. This was a particularly serious problem with unflanked field works such as redoubts and irregular indented lines. One of the most important intermediate objectives of any well arranged attack was to pass through exterior obstacles and get as many troops as possible into the ditch where they could, unless the ditch was defended, rest and re-organize before making an attempt on the parapet. All of the accessory obstacles that could be positioned in a ditch, such as palisadings and fraises, could not stop an attack if they were not well covered by defensive fire. Once an attacking body of troops had restored its ensemble in the ditch, scaled the scarp and mounted the berm, the defenders sheltered behind the parapet, still under a harassing fire delivered by the attacker's firing parties, were at both a physical and psychological disadvantage. Once on the berm, attacking troops could rush over the parapet in a single bound to drive the defenders from the banquette. The defenders, unable to raise their heads above the interior crest without extreme risk, could not be sure when the rush would come and in what strength; however well prepared to defend their work with the bayonet the suddenness of the attackers' rush usually carried great impact. Suddenly denuded of their cover and looking up at a determined enemy, the chances for a successful defense of their field work were against the harassed defenders.

The ditch of a major field work could become a decisive liability to the conduct of a good defense and presented a work's defenders with as many disadvantages as advantages if it was not well defended. There were several different ways for a garrison to defend the bottom of the ditch, none of which involved direct fire from the parapet down into the ditch immediately to its front. A major field work's profile prevented defenders on the banquette from depressing their muskets down far enough to fire directly into the section of ditch to their front; the thickness and height of the parapet along with the limits of an effective plongee of the superior slope combined to prevent a work's defenders from seeing into the ditch from the banquette; an assaulting body of troops gather in a work's ditch could mill about in almost perfect security just a few feet from their enemy on the banquette. While this condition could not be controlled or very seriously altered, the ditch of a major field could still be defended from the parapet above it by secondary means.

Defenses Exterior To The Ditch

This required some degree of preparation and collection of materials prior to an attack. Heavy stones and logs could be collected in the work and stacked at intervals near the foot of the banquette; when attacking troops had gathered in the ditch these objects would be rolled over the superior slope and into the ditch to either crush or seriously harass the enemy as they tried to restore sufficient order and ensemble to scale the scarp and mount the berm. A heavy log would carry enough momentum after rolling over the superior slope and down the exterior slope to knock enemy troops off the berm or, at least scatter troops huddled in the ditch. Grenades, usually 6 or 12-pounder shells and spherical case shot fused with quick-match, rolled over the parapet were also quite effective for making enemy troops pay more attention to their physical safety than to their officers trying to urge them up the scarp. Heavier shells that could not be picked up and thrown back over the parapet were also effective for preventing enemy troops from regaining their balance and composure in the close confines of the ditch. These measures were simple, but effective, expedients that quite often did not occur to the commanders of isolated and unflanked garrison redoubts and field works.

More effective means of defending the ditch and bringing within reach of close musket fire were available, if time, material, and labor were plentiful and the work was considered important enough to warrant extra expense and improved protection. Manipulation of a field work's trace to produce a series of faces and flanks capable of collateral defense was the easiest and least expensive way to provide a garrison the means to defend the ditch. A flanked disposition using a bastioned trace allowed all of a work's salients and re-entering angles to be covered by either direct or oblique fire from the work's parapet. Fire from the flanks of the bastions covered both the faces of the bastions and could be directed down into the ditch in front of opposite flanks. Attacking troops trapped in a bastioned front's ditch would be exposed to the garrison's fire delivered from the parapet at every point of the ditch. But reaching the bottom of the ditch with fire depended on a field work's relief (height of the interior crest above the bottom of the ditch) and the plongee of the superior slope. As a general rule fire delivered from the banquette would strike the bottom of the ditch at a distance equal to six times the relief, which meant that bastioned fronts were generally not suitable for small field works.

The length of the parapet of a bastioned front as it angled around the advanced faces and retired flanks and curtain of the front required more time and materials to construct than could usually be expended on isolated or unimportant works plus the length of parapet required a larger garrison than other traces. Although the bastion trace was the most effective trace for close defense, it was usually saved for large and important field works. Other traditional traces, such as star forts and cremaillere lines, that produced a series of salient and re-entrant angles allowed a work's garrison to cover sections of the ditch with either direct or oblique fire, but left dead ground at the re-entrant angles of the ditch that could not be reached by fire from the parapet.

Defenses Within The Ditch

The most effective means for covering the bottom of the ditch with close musket fire were also the most expensive and least used in field fortifications. Caponnieres and galleries sunken into the scarp and counterscarp provided the defenders with covered firing positions within the ditch itself. Although these were not particularly complex structures, they did add extra time, material, and labor to the cost of construction and were generally reserved for semi-permanent field works that could be developed over a prolonged period of time. In many cases the dimensions of a field work's ditch limited the size and therefore the amount of fire that could be delivered from these defensive structures within the ditch; this alone made these structures something less than a practical solution to the problem of ditch defense. These structures were usually connected to the interior of the field work by subterranean galleries that could be used by attacking troops to get inside the work if they were able to break down the wooden walls of the ditch defenses.

Caponnieres

A caponniere may be defined as any structure positioned on the bottom of the ditch specifically designed to provide covered defense for the bottom of the ditch. During the American Civil War the term was also used to describe any covered way used to connect two field works or lines of works that included parapets facing in opposite directions or any abrupt deviation in the general line and direction of a work's parapet designed to allow the garrison to cover the ditch with fire. Engineering manuals of the period usually describe double caponnieres used in field fortifications as blockhouse like structures positioned either at the salient angles at the center of the faces of unflanked field works that could cover the ditch with fire from opposite sides of the structure. Single caponnieres were described as stockade works placed at the extremities of ditches of works open at the gorge (such as redans and lunettes) that could project a single column of fire in one direction along a straight section of a field work's ditch.

Double caponnieres could either be constructed with their interior floor on the bottom of the ditch or with the floor sunken 3 to 4 feet below the surface of the ditch. Walls were constructed using vertical wall posts at least 12 inches in diameter hewn along the sides to fit tightly together when planted. Loop-holes were cut between post 2 to 3 feet apart and at least 18" above the bottom of the ditch. The structure was roofed using 12 inch diameter timbers laid transversely as tie beams across cap-sills along the tops of the walls. A second layer of halved timbers was laid on the tie beams and the whole covered with 2 or 3 feet of earth to make the structure bomb-proof. In generally double caponnieres were laid out with straight walls that covered the full width of the ditch and two short faces that joined to form a salient. The ditch in front of this type of caponniere was widen in front of the caponniere salient to maintain 12 feet of separation between the caponniere and the crest of the counterscarp to prevent attacking troops from jumping the gap and using the roof of the caponniere as a bridge to mount the berm. The structure was sunken into the scarp wall and a subterranean gallery constructed using mining frames was excavated from the rear of the caponniere to the interior of the work. As with other types of blockhouses the exterior walls could be covered by small ditches with the deblai thrown against the exterior walls to form a slight embankment to prevent enemy troops from finding shelter at the base of the walls.

Single caponnieres at the extremities of ditches were in every respect constructed as loop-holed stockades. Timbers composing the wall would be at least six feet above the bottom of the ditch and bound together with both an upper and buried riband. The end timbers were sunken into the scarp and counterscarp walls to prevent enemy troops from easily scaling either wall to break into the caponniere. Loop-holes were cut through the wall at intervals from 2 to 3 feet. The front was covered by a slight ditch with the deblai thrown against the lower portion of the exterior wall. Since this type of caponniere was open at the top it would not be connected to the interior of the work by a gallery, rather, defenders were positioned in the caponniere prior to an attack and pretty much left to fend for themselves during the attack. The open top would be covered by fire from the work's parapet, though this would not generally be sufficient to keep a determined enemy troops from entering the caponniere at the same time as they entered other sections of the ditch.

Scarp and Counterscarp Galleries

Scarp and counterscarp galleries were basically enclosed rooms sunken into the ditch walls that allowed a work's garrison to defend the bottom of the ditch from a covered position within the ditch. Counterscarp galleries, sometimes called reverse galleries, were usually positioned at salient angles; scarp galleries were positioned in the scarp wall at re-entering angles. Both could either be pierced with loop-holes for muskets or enlarged and provided with embrasures for flank casemate howitzers. These structures were relatively complex and consumed too much time and labor to be included in most field works, but they were used with greater frequency than double or single blockhouse caponnieres.

Both scarp and counterscarp galleries were constructed after the same fashion. The ditch wall was excavated inward to a width of slightly more than 4 feet and to a depth level with the bottom of the ditch. Framing posts were then planted along the interior side of the excavation at intervals of about 3 feet; sheeting planks about 1 1/2 inches thick were then slipped down behind the posts to form the inner wall of the gallery. The outer wall facing the ditch was constructed using scantling or timbers hewn to make a tight fit planted vertically along a line marking the continuation of the foot of the scarp or counterscarp. Loop-holes were cut between the vertical posts at intervals of about 3 feet. The roof was composed of cap sills joined along the front and rear walls with girders notched onto the caps at 3 foot intervals. This roof framework was then covered over with plank sheeting and finished off with about three feet of earth thrown on and rammed down to make the structure bomb-proof.

Girders could be made to extend 3 feet beyond the rear wall (under the glacis) and only about 5 feet of the rear wall covered with sheeting to produce an open space that would make it easier for troops in the gallery to handle their muskets' rammers. In this case shores attached to the rear wall posts were carried up to the roofing girders to support the extra length of the roof. The extra length would also make it more difficult for sappers to dig down from the glacis and pry the roof planking up in an attempt break into the gallery from above. Access to counterscarp galleries was provided by a narrow loop-holed door in the wall facing the ditch that could be secured with a heavy bar. Troops assigned to the gallery would necessarily have to be in position prior to an attack and would be left to defend their work until the ditch had been cleared of attacking troops. Access to scarp galleries could be provided through a subterranean gallery under the parapet or through a door opening onto the ditch. In either case the ditch wall was covered by a ditch within the ditch with the deblai thrown against the wall to form an embankment.

[This page originally appeared as a Basic Information Page on the old Civil War Field Fortifications Website.]

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