Fortification Friday: Timber for Scarp Revetments

Thus far we’ve looked at Mahan’s advice for revetments, taking each type in detail – sod, pisa, fascine, hurdle, gabion, plank, and sandbag. Mahan offered one more type for use with field fortifications, calling it the scarp revetment.  The name implies use specifically on the scarp within the ditch.  However, it was also referred to as a timber revetment by other authorities, indicating such could be used at other places within the fortification.  But as we focus on Mahan’s instruction here, let’s proceed with the notion of revetting the scarp.

Scarp revetment. This revetment is formed of a framework of heavy timber, and is used only for important field forts. A piece, termed a cap, or cap-sill, is imbedded in a trench made along the line of the berm; other pieces, termed land-ties, are placed in the trenches perpendicular to the cap, with which they are connected by a dove-tail joint; they are about eight or ten feet asunder. Cross pieces are halved into the land-ties about two feet from their extremities, and two square piles, about five feet long, are driven into the angles between the land-ties and cross pieces; inclined pieces, which serve as supports to the cap, are mortised into its under sides at the same points as the land-ties.  These supports usually receive a slope of ten perpendicular to one base; they generally rest on a ground-sill, at the bottom of the ditch, to which they are mortised, this being held firm by square piles. The ground-sill may be omitted by driving the supports below the bottom of the ditch.

Figure 27 of Plate III was offered to illustrate the scarp revetment, in “section” and “plan”:

PlateIIIFig27

You can see how these are matched across the diagram with dotted lines between the section and plan.  For some clarity, I’ve also broken these down into individual cuts for each. Here’s the section:

PlateIIIFig27A

And here’s the plan:

PlateIIIFig27B

The annotations in the diagram follows the description (not in alphabetical order, though):

  • B is the cap-sill.
  • D is the ground-sill.
  • C are the uprights between the cap and ground.
  • A are the land-ties. Notice how those are dovetailed into the cap-sill.
  • E depicts the cross pieces attached to the land-ties.
  • F shows the short piles used to anchor the land-ties leveraging the cross pieces.
  • G are piles anchoring the ground-sill.

Note the wood is connected by way of joints – specified as dovetail or mortise joints.

With this framework constructed, Mahan called for planks over the scarp:

Behind this framework, thick plank, or heavy scantling, is placed side by side, having the same slope as the supports; or else a rabate may be made in the cap and ground-sills, and the scantling be let in between those two pieces serving as a support to the cap. This is the more difficult construction but is the better, since, should the heavy supports be cut away, the cap will still be retained in its place.

Thus the face of the scarp would be covered by a revetment of plank.  And in the best case, those planks added to the support of the framework, using a rabbet (Mahan says rabate.. a rose is a rose…) joint.  Note the uprights (C) and the planks both ran vertical in the preferred construction.  There’s a reason, as Mahan noted next:

Scarp revetments are sometimes formed by laying heavy timber in a horizontal position; but this method is bad, as it enables the enemy to gain a foot-hold by thrusting their bayonets between the joints.

So don’t be lazy and allow the enemy gain a foothold.  Take the time, get out the wood-working tools and make the right joints.

Another bit of advice Mahan offered was, “The length of the land-ties should be at least equal to two-thirds the depth of the ditch.”  Thus the length of “A” had a direct relation to the length of “C”.  Those are 9 and 12 feet, respectively, in the diagrams above. Thus extended, the land-ties would firmly anchor the weight of the revetment with enough strength to resist an enemy’s attempt to pull the structure loose.

Looking to the other side of the ditch, Mahan gave a little attention to the counterscarp:

The counterscarp is seldom reveted. A framework similar to that for the scarp might be used, and thick boards, laid horizontally, be substituted for the inclined scantling.

Observe, the horizontal orientation was fine for the counterscarp.  Who cares if the enemy gets a foothold while trying to retreat… and at the same time, those footholds might prove useful should the defender need to pursue. But above all, Mahan saw little need to waste time with fancy woodworking joints for the counterscarp.

Now this is all good talk about revetment of the scarp.  But we have to consider how it fits into the overall construction process:

When a scarp revetment is made, the excavation of the ditch must be conducted in a different manner from that already explained. In this case, after the cap-sill and land-ties are laid, the excavation is continued to the bottom of the ditch, by removing only earth enough to allow the framework to be put up. A scaffolding of plank is then raised in the ditch on which the earth that remains to be excavated is thrown, and from there to the berm.

Yes, it would be rather difficult to plant those land-ties after the parapet was piled up!  So the scarp revetment needed to be set before digging out the ditch.

To sum up the discussion of revetments, these were supplemental structures within the works.  But were considered necessary improvements to strengthen the works, adding resiliency against erosion and enemy action. We’ve seen a progression here from simple, low-labor cost solutions to more elaborate and labor-intensive options.  Each type of revetment offered different qualities that an engineer could consider within the overall plan.  And he couldn’t just go to some home improvement store to purchase materials!

(Citation from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 41-42.)

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