Civil War Field Fortifications
Strength of the Profile
The strength of a field work's profile was estimated by its relief and the thickness of its parapet.
A field work's relief was the elevation of the interior crest above the level of the bottom of the ditch. With a ditch eight feet deep, for example, and an interior crest six feet above the terre-plein a field work would have a relief of 14 feet. The relief was used to estimate a field work's quality as an obstacle to an assault. In general the relief had to be great enough to cause an attacking body of troops some difficulty in getting up the scarp of the ditch. It was assumed that an attacking body of troops, already stressed by their advance across the glacis, would be further fatigued and their orderliness completely disrupted as they attempted to climb the scarp and mount the superior slope or enter the work through the embrasures, rendering their attack easier to defeat.
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Determining the thickness of a parapet can be a tricky task since the lower portions near the terre-plein, which includes the exterior slope, are by nature thicker than the upper sections around the superior slope. Thickness was estimated an a horizontal line from the crest of the interior slope to the crest of the exterior slope. This line represented the maximum sustainable thickness of the parapet. Parapets were usually given a thickness that was twice the penetrative power of the weight of enemy shot that could reasonably be expected to be fired at the parapet. Solid shot from a 6-pounder field gun, for example, could penetrate three and one-half to four feet into packed earth so a parapet designed to absorb the fire of a 6-pounder had to be seven or eight feet thick.
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