Over the last few weeks, we’ve moved beyond just planning an earthwork into the practical aspects of actually constructing those works to the specifications, derived from the plan. Assuming your digging project worked out as expected, the fortification would now have bare earth frowning upon any potential attacker. While achieving the defensive needs, bare earth is hard to maintain. Readers are likely aware of the natural forces that act here. To reduce the wear and thus reduce the maintenance needs, Mahan prescribed revetments:
A revetment consists of a facing of stone, wood, sods, or any other material to sustain an embankment, when it receives a slope steeper than the natural slope.
In field works revetments are used only for the interior slop of the parapet and for the scarp; for the first sods, pisa, fascines, hurdles, gabions, and plank, are chiefly used; and for the last, timber.
Keep in mind here the specification of “natural slope” being at or shallower than a 45° grade, the natural slope at which loose dirt will pile. Loosely speaking, we might assert that any slope might need revetments if use extends beyond a short time. But let us for now focus on the notion of temporary fortifications and, thus, temporary arrangements. Anything steeper than that natural angle is going to start falling down over night. So we put up a revetment to avoid a morning chore.
Notice also the list of material we might use. It is built, somewhat, in descending order of preference. So let us take a look at sods:
Revetments of Sods. Sod work forms a strong and durable revetment. The sods should be cut from a well-clothed sward, with the grass of a fine short blade, and thickly matted roots. If the grass is long, it should be mowed before the sod is cut.
Sods are of two sizes, one termed stretchers, are twelve inches square, and four-and-a-half inches thick; the others, termed headers, are eighteen inches long, twelve inches broad, and four-and-a-half inches thick.
And we have a diagram depicting these stretchers and headers:
The sod revetment is commenced as soon as the parapet is raised to the level of the head of the banquette. A course of sods is then laid, either horizontal or a little inclined from the banquette; the course consists of two stretchers and one header alternating, the end of the header laid to the front. The grass side is laid downward; and the sods should protrude a little beyond the line of the interior slope, for the purpose of trimming the course even at the top, before laying another, and to make the interior slope regular. The course is firmly settled, by tapping each sod as it is laid with a spade or a wooden mallet; and the earth of the parapet is packed closely behind the course.
So a matrix of sorts with these blocks of sod. Notice the arrangement in Figure 21, below:
A second course is laid on the first, so as to cover the joints, or, as it is termed, to break joints with it; using otherwise the same precautions as with the first. The top course is laid with the grass up; and in some cases pegs are driven through the sods of the two courses to connect the whole more firmly, which is, however, by no means necessary to form a strong sodding.
As with all things, there are some fine points to consider … which the experienced instructor was willing to impart to his students:
When cut from a wet soil, the sods should not be lain until they are partially dried, otherwise they will shrink, and the revetment will crack in drying. In hot weather the revetment should be watered frequently, until the grass puts forth. The sods are cut rather larger than required for use; and are trimmed to a proper size from a model sod.
While I am not much of an expert on landscaping, and cannot provide some analysis of the sod and soil properties, I do dabble a bit with old photos. Particularly old photos of old, sometimes no longer existing, forts that were in some hot climates. Recall this photo from our tour of Fort Johnson, circa 1865?
We see lots of sod “bricks” on the face of Fort Johnson. Particularly on the exterior slope (and do keep in mind this was a “permanent” fortification of sorts, so the rule above need not directly apply to the use of sod only on the interior). In some sections, it appears stretchers and headers were used:
In other faces, there seems no pattern:
Perhaps we see here where the grass has grown so well as to “break” over the joins of the sod courses.
On other views of the fort, it appears only twelve-inch square stretchers were used:
If we go over to the other side of the harbor, Battery Marshall’s sod appeared much worse for the exposure to the ocean breezes:
Just does not look like those are 4½ inches thick. Mahan might have called these out as an example of what not to do.
Next week, we will look at some other forms of revetments.
(Citation from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 36-37.)