Profile, Parapet and Ditch, Proportions and Dimensions

A profile is a cross section of the parapet and ditch taken along a line perpendicular to the general direction of the interior crest of the parapet or from the interior side of the parapet exterior side of the work. It shows, basically, a vertical slice of the work and graphically describes the elevation, thickness and general arrangement of the various elements of the work. Profiles of all major field works included two basic elements: the parapet and the ditch. The parapet served as the protecting mass of earth which sheltered the interior of the work from enemy fire and, in conjunction with the ditch, served as a commanding obstacle to enemy troops attempting to enter the work during an assault. The ditch was both an obstacle and the primary source of soil used to construct the parapet.

A well constructed parapet and ditch consisted of various elevations and purposely shaped surfaces that were arranged to improve the defensive qualities of the work, allow the garrison to sweep the ground in front of the work with its fire, and generally enhance the stability and endurance of the structure. The profile of a major work usually consisted of ten primary elements; taken in order from the interior to the exterior these elements were:

A. The terre-plein, or space enclosed by the fortification or immediately to the rear of the parapet which was usually levelled to the plain of the site.

B. The banquette slope which allowed troops manning the parapet easy access to the banquette;

C. The Tread of the banquette where troops manning the parapet stood to deliver their fire;

D. The Interior Slope, which was approximately 4.5 feet in height, which formed the interior side of the parapet and was usually revetted to retain the soil composing the parapet at an unnaturally steep slope;

E. The Superior Slope, which formed the upper surface of the parapet and was gently sloped toward the exterior of the work to allow troops to point their rifles or musket downward to sweep the ground immediate in front of the ditch;

F. The Exterior Slope, which was usually allowed to take the natural slope of loosely piled soil and which was surface that was required to absorb the impact of most enemy fire directed at the work;

G. The Berm, a flat area left between the exterior slope and ditch, which intended to decrease the pressure of the weight of the parapet on the interior slope of the ditch in order to prevent the parapet from eroding into the ditch;

H. The Counterscarp of the ditch, was the interior wall of the ditch and was usually given a slope that allowed it to take the weight of the parapet without collapsing;

I. The Bottom of the Ditch;

J. The Scarp, which formed the exterior wall of the ditch;

K. The Glacis was a raised mound of earth in front of the ditch with a long gentle slope in front which was intended to increase the elevation of the ground exterior to the work and prevent enemy troops from being able to slip under fire delivered from the parapet.

Angles where two surfaces intersected in a salient (exterior) angle were called "crests;" angles forming re-entering (interior) angle were called the "foot" of the most elevated the intersecting surfaces. The line where the interior slope intersected the superior slope, for instance, was called the "crest of the interior slope" or just "interior crest." The line where the interior slope fell on the banquette tread was called the "foot of the interior slope."

Standard Proportions

Major field works constructed during the American Civil War generally followed received methods that had been worked out through the preceding two centuries. Standard proportions were basically an expression of traditional wisdom concerning the stability of a protective embankment and the structural characteristics necessary for the embankment to serve as cover, obstacle, and an effective platform for defensive fire. Each element of a major field work parapet had a distinct function and was both proportioned and shaped to perform that function. These proportions could be altered as necessary to produce a specific effect or modified by limitations imposed by time, labor, and materials. Standard proportions were guides to producing an effective parapet, not unalterable and universally applicable laws of nature; it was the duty of engineer and other officers to creatively apply their own good sense and experience when adapting standards to specific situations.

Glacis (K)

A glacis consisted of a very gently sloped mound that extended outward from the crest of the counterscarp. A glacis was included in the general profile of a field work when the difference in height between the interior crest of the parapet combined with the degree of slope given the superior crest and the height of the crest of the counterscarp of the ditch prevent the garrison's small arms fire from reaching a point at least 2 feet above the crest of the counterscarp. The glacis raised the level of the ground immediately in front of the work and insured that an attacking body of troops could not slip into the ditch without first receiving the garrison's fire at close range. It also had the benefit of compelling an attacking body of troops to advance over an incline to reach the ditch. There were no fixed dimensions for a glacis as an element in a field fortification's profile; the general rule indicated that it had to be high enough for fire delivered from the parapet to pass no more than two feet above its crest near the crest of the counterscarp. The earth to construct a glacis was usually obtained by widening or deepening the ditch.

The Ditch (H, I, J)

Although the volume of soil necessary to constructed a parapet of a particular same could be finely calculated for the most part the width and depth of the ditch necessary to produce enough soil was estimated. The most common method was to assume the depth of the ditch and divide it by the total of the surface area of the profile of the parapet. Since the ditch was intended as an obstacle, 6 feet was considered the minimum acceptable depth and 12 feet was the minimum width. If this produced too much soil, the extra was formed into a glacis. The scarp of the ditch was usually given a base equal to 2/3 of the natural angle of the soil (that is, 2/3 of the depth of the ditch) and the counterscarp was given a base equal to 1/2 the base of the scarp. A ditch 16 feet wide at the top and 10 feet deep, in normal soil, would require a base of 6 feet for the scarp (approximately 1/3 of 10) and a base of 3 feet for the counterscarp (1/2 of 6) which would leave a width of 7 feet for the bottom of the ditch.

Berm (G)

Generally considered a defect in field works, a berm was often necessary to relieve the scarp from carrying the weight of the parapet. Its length was proportioned in relation to the tenacity of the soil. In firm soil that could take the burden of the parapet berms could be anywhere from 1 to 2 feet wide, in marshy or very light soil that tended to displace easily under a burden it could be up to 6 feet wide. Since the berm was generally level it caught rain water running off the parapet and tended to erode rather quickly; once it started deteriorating it tended to wear the crest of the scarp down and begin the more or less slow process of filling the ditch. This problem was sometimes resolved in semi-permanent fortifications by cutting away the berm and connecting the exterior slope with the scarp and letting them fall at the natural angle of the soil. It was considered a defect because it gave an attacking body of troops a foot hold where they could rest for a moment after scaling the scarp before mounting the superior slope. For this reason the berm was to be no less than six feet below the interior crest to prevent attacking troops from being able to stand on the berm and fire into the the work.

Exterior Slope (F)

The exterior slope was given the inclination that soil loosely thrown into a pile would naturally assume. This was about a 45 degree angle so the base of the exterior slope usually equalled the height of the exterior crest. This surface was expected to take the majority of enemy fire directed at the field work. Its natural angle allowed the soil composing it to be thrown into the air by the impact and shot and explosion of shells and fall back onto the slope, partially restoring the initial damage and increasing the period time required to batter down the parapet and expose the interior of the work to enemy fire. The slope also had the advantage of promoting good drainage by moving run-off quickly, but evenly, across the berm and into the ditch. Seeding or sodding was necessary for the proper maintenance of an exterior slope of a field work that was expected to stand over a prolonged period of time.

Superior Slope (E)

The superior slope was slanted downward from the interior crest to allow troops firing from the parapet to depress their muskets enough to fire no higher than three feet above and across the crest of the counterscarp. The slope could be equal to 1/4 to 1/6 of the base of the superior slope. A superior slope with a base 12 feet wide and a slope 1/4 of its base would be 3 feet lower at the crest of the exterior slope than it was at the crest of the interior slope. Slopes greater than 1/4 would be too weak to prevent enemy fire from penetrating the parapet and passing through to the interior of the field work. Slopes less than 1/6 would be too flat to allow troops to fire downward from the parapet. The superior slope determined the thickness of the parapet and was expected to absorb solid shot and shell penetrating into the mass of the parapet without collapsing too rapidly. Its stability depended on the capacity of the revetment of the interior slope to retain the mass of the parapet at a relatively sharp angle and on the capacity of the exterior slope to resist the erosive effects of the weather and to absorb the impact of solid shot and and explosion shells without breaking down and falling into the ditch. The slope also allowed rain water to run off the surface and onto the exterior.

Interior Slope (D)

The interior slope was given a horizontal base equal to 1/3 to1/4 of its vertical height so that troops could lean forward to fire over the superior slope. Due to the sharp inclination necessary to allow troops to stand close enough to the superior slope to fire effectively, the interior slope had to be supported by a revetment. A strong revetment was necessary for works that were intended to maintain their defensive qualities longer than a few days or weeks; all revetments tended to collapse under the pressure exerted by the weight of the mass of the parapet and required more or less constant maintenance and repair. The solidity of the revetment material was also important. Shot and shell penetrating into and through the mass of the parapet could be stopped by a strong revetment; this was particularly important in field works subjected to a prolonged bombardment.

Tread of the Banquette (C)

The tread of the banquette was usually 4 to 4.5 feet below the crest of the interior slope so that even the shortest men in the ranks could raise their muskets high enough to fire over the superior slope. Its width depended on the number of ranks the troops would form when manning the parapet. One rank was allowed a banquette 2 feet wide, 2 or 3 ranks would have a banquette 4 feet wide. To promote good drainage the banquette was usually given a very slight slope to the rear so that run-off could flow easily down the banquette slope and onto the terreplein. A strong defense depended in part on a banquette that was relatively free of obstructions, easy to move around on, and provided a solid and sure base for troops lining the parapet to stand on while engaged with an attacking body of troops. If made too narrow or sloped too radically, troops standing on the banquette would have a difficult time maintaining their balance and staying in position to fire and reload their rifles or muskets.

The Banquette Slope (B)

The slope of the banquette was given a base equal to twice the height of the tread of the banquette. Given an interior crest 8 feet above the level of the terre-plein, the tread of the banquette would be about 4 feet, 6 inches below the interior crest and 3 feet, 6 inches above the the level of the terre-plein. The slope of the banquette would then have a base of 7 feet. When the full base of the banquette slope cramped the interior space of a field work too much it could be cut into steps 12 inches wide and nine inches high. Easy access to the banquette tread was extremely important, especially when the garrison was unexpectedly called to man the parapet and had to rush into position in the dark or half-light of dawn.

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

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January, 2004