Before moving on to other sorts of revetments, a clarification from last week’s post. With respect to layering of sod stretchers and headers, I left out Mahan’s diagram showing how two layers of sod cut in those orders was placed:
What this shows is the first layer (A) in an order, from left to right, of header, stretcher, header, stretcher, header. Then the second layer (B) as stretcher, header, stretcher, header, stretcher. Note how the joins are broken as required by Mahan, but most importantly notice how the headers are placed with the long sides vertical. The pattern illustrated here, matched against the photos of Fort Johnson, leads me to conclude the Confederates had modified Mahan’s instructions in regard to revetments, at least in the Charleston fortifications. Perhaps this was an adjustment made while using beach sand as opposed to soils described by Mahan.
Not wishing to get bogged down in the sod, let us turn to some of the other types of revetments. The next discussed by Mahan was the Pisa Revetment:
Pisa Revetment. Ordinary earth, if mixed with a proper proportion of clay, and the whole well kneaded with just water enough to cause the particles to adhere when squeezed in the hand, may be used for a revetment, and is termed a pisa revetment. Sometimes chopped straw is mixed up with the mass to cause it to bind better.
The pisa is laid in layers of twelve inches thick, and two feet broad, and well packed. The same precautions should be taken in forming the parapet behind it as in sod revetments. The face of the revetment may be sown with grass seed or oats, and when the stalk comes to maturity it should not be cut, but suffered to remain as a kind of thatch to protect the facing from the weather.
In short, a clay mixture to apply to the surface of the works. Oh… and let us grow some oats for the horses….
Much more work was required for the next type of revetment:
Fascine revetment. A fascine is a bundle of twigs closely bound up. There are two sizes of fascines; one size is nine inches in diameter, and about ten feet long; the other, which is generally termed a soucisson, is twelve inches in diameter and twenty feet long; it is chiefly used for the revetments of batteries.
Mahan offered this illustration of a fascine:
So let’s get to work making a fascine…
To make a fascine, straight twigs are selected, between the thickness of the little finger and thumb, the longer the better; they should be stripped of the smaller twigs. A machine, termed a fascine horse, is put up, by driving two stout poles obliquely into the ground about two feet, so as to cross each other about two feet above the ground, where they are firmly tied together; as many of these supports as may be required are put up in a straight line, about eighteen inches apart; this forms the horse, on which the twigs are laid to be bound together.
Another machine, termed a fascine choker, is formed of two stout levers about five feet long, connected near their extremities by a chain or strong cord, which should be long enough to pass once round the fascine, and be drawn tight by means of the levers.
The twigs are laid on the horse, with their large and small ends alternating; the choker is applied to bring them together; and they are bound by withes, or gads, made of tough twigs, properly prepared by untwisting the fibers over a blaze, so as to render them pliant; or else stout rope yarn may be substituted for them. The gads are placed twelve inches apart, and every third or fourth one should be made with an end about three or four feet long, having a loop at the extremity to receive a picket through it; this picket is termed an anchoring picket, its object being to secure the fascine firmly to the parapet.
Got all that? I’ll plan a follow up post in the future to discuss fascine construction in more detail. Though, I would point out we still use fascines today. You might get a quote for linear feet of fascine from a landscaping company, as the device is used today on embankments and berms. So don’t get the notion the fascine is just for war-like avocations.
Once built, the fascine need be laid on the works for the intended purpose:
To form the revetment, the first row of fascines is imbedded about half its thickness below the tread of the banquette, and is secured by means of the anchoring pickets, and also by several pickets driven through the fascine itself about twelve inches into the earth. The knots of the withes are laid inside, and the earth of the parapet is well packed behind the fascine. A second row is laid on the first, so as to give the requisite interior slope; it should break joints with the first row, and be connected with it by several pickets driven [through] them both. The other rows are laid with similar precautions; and the parapet is usually finished at top by a course of sods.
Figure 23 demonstrates this arrangement:
Focus on the parapet details here, and ignore the picket stake in the ditch for the moment. Note the depth of the anchor pickets here.
The next type of revetment also weaving twigs, but was done directly on the works:
Hurdle revetment. This revetment is made by driving poles, in the same direction as the interior slope, into the banquette, about eighteen inches below the tread, and then forming a wicker-work, by interlacing twigs between them in a similar manner to a basket work.
The poles should be nine inches apart, their diameter about one-and-a-half inches. They should be secured to the parapet by long withes and anchoring pickets. The top twigs should be bound together by withes.
Advance Figure 24 to illustrate this:
Again, ignore what is on the front side, in this case on the exterior slope, as we will reuse this illustration in a later lesson (Mahan saved ink for the printer here). Our hurdle revetment is on the interior slope to the left.
Thus far, we’ve examined four types of revetments – sod, pisa, fascine, and hurdle. These were the recommended types for field works, in descending order. Moving forward to the next installment on this line we will consider the less favored, for field works, gabions, plank, and sandbag revetments.
(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 37-39.)