No April Fools today… we have some serious stuff to discuss as Fortification Friday resumes. Revetments is our subject, and thus far we’ve looked at those of sod, and those using pisa, fascine, and hurdle constructions. The next type to consider used gabions. Some time back, I discussed gabions as relating to the Civil War and modern applications, but that need be revisited with Mahan’s take in relation to field fortifications.
Gabion revetment. The gabion is a round basket of cylindrical form, open at each end, its height is usually two feet nine inches, and diameter two-feet.
Mahan referenced Figure 25 to illustrate the gabion:
The lower section shows the gabion in vertical profile. The upper portion looks on from above. You see the basket form with the “two feet, nine inch” stakes. From above we see concentric circles, or hoops, of a form that is made when constructing the gabion:
To form a gabion, a directing circle is made of two hoops, the difference between their radii being such, that, when placed concentrically, there shall be about one-and-three-quarter inches between them. They are kept in this position by placing small blocks of wood between them, to which they are tied with packthread. The directing circle is laid on the ground, and seven or nine pickets, about one inch in diameter and three feet long, are driven into the ground between the hoops, at equal distances apart; the directing circle is then slipped up midway from the bottom, and confined in that position. Twigs half an inch in diameter, and as long as they can be procured, are wattled between the pickets, like ordinary basket work; when finished within about one-and-a-half inches of the top, the gabion is placed with the other end up, the directing circle is taken off, and the gabion is completed within the same distance from the other extremities of the pickets. The wicker work at the two ends is secured by several withes, and the ends of the pickets being brought to a point, the gabion is ready for use.
And that is how you build a gabion. Implied as a prerequisite is the time needed to gather all those half-inch diameter twigs.
With the gabion prepared for use, where do we put it on the works? Figure 26 demonstrates a gabion in the profile:
The right end of this diagram is used to discuss obstacles later in Mahan’s text (back in the 19th century, woodcut diagrams were expensive to reproduce, don’t you know). So we need to focus on the left:
The gabion revetment is seldom used except for the trenches in the attack of permanent works, where it is desirable to place the troops speedily under cover from the enemy’s case shot and musketry. When used for field works, a fascine is first laid partly imbedded blow the tread of the banquette; the gabion, which is placed on end, rests on this, so as to give it the requisite slope; it is filled with earth, and the parapet is raised behind it, and another fascine is laid on top, and in some cases two.
I’ve added annotations for the fascine anchors required within this arrangement.
We see a gabions often in wartime photos. Those from Fort Sumter catch my attention most (as one might presume). By the later half of the war, Fort Sumter was for all practical purposes a permanent seacoast fortification turned into a field fortification. And we see gabions in use all across the interior.
Notice those gabions that appear to be partially constructed … deconstructed, or at least empty… in the foreground. We don’t see fascines on top of the gabions, when seen in profile:
Part of this is explained by the specific application. In the case of the photo above, there is a planking on which the gabions in the foreground are stacked. That planking is the top of the entrance to a gallery. If the top fascine is used, we might assume it to be buried well. But I see no evidence of such.
Moving past gabions, we still have several sorts of revetments to discuss – next up are plank and sandbag revetments…. oh the excitement builds….
(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 39-40.)