Revetments

A revetment is a covering or facing placed upon an earth slope to enable it to stand at an inclination greater than it would naturally assume. Some revetments also increase the tenacity of slopes and diminish the injury by fire. The upper parts of revetments that may be struck by projectiles which penetrate the cover of earth must not be made of materials of large units which will splinter when struck. The upper part of the revetments is technically known as crowning.

Sandbags are made of coarse canvas or burlap. They are 33 inches long and 14 inches wide. They are filled loosely with earth or sand about 1/2 cubic foot to a bag. Having been placed in position they are pounded down with a shovel to a rectangular form when they will fill a space about 20 by 13 by 5 inches.

The sandbag revetment is constructed by laying alternate rows of headers and stretchers, breaking joints. The tied ends of the headers and seams of the stretchers are put into the parapet. Men working in pairs lay the bags and set them firmly in place with a spade or mallet.

The advantages are:

1. The portability of the empty bags. Only 62 pounds per one hundred bags.

2. They may be filled with any kind of soil.

3. They are rapidly filled and easily placed in position.

4. They are invaluable in making repairs.

5. They will not splinter.

The only disadvantage is that they are not durable. The cloth soon goes to decay and the filling material crumbles away.

Plate 29 shows the appearance of a sandbag revetment as seen from the front and from the end.

A squad of six men with two shovels and one pick should fill 150 bags in an hour. One man uses the pick, two shovel the dirt into the bag, one holds the bag open and two men tie the bags. Having the filled bags ready to hand ten men will lay 75 square feet of revetment in an hour. Four men lay the bags and flatten them out while six carry them.

Brush is used in many forms for revetting. Almost any kind will serve the purpose. For weaving, it must be live and is most pliable when not in leaf. It should not be more than 1 inch in diameter at the butt. When cut it should be assorted in sizes for the different class of revetments. Poles 2-1/2 inches in diameter are cut for the supports.

A fascine is a cylindrical bundle of brushwood tightly bound. The usual length is 18 feet, the diameter 9 inches, and the weight normally about 140 pounds. Lengths of 6 and 9 feet, which are sometimes used, are most conveniently obtained by sawing a standard fascine into two or three pieces.

Fascines are made in a cradle which consists of five trestles, the outer ones being 16 feet apart.[Pg 67]
[Pg 68] The trestle is made by driving two sticks about 6-1/2 feet long and 3 inches in diameter in the ground and lashed at the intersection as shown in Plate 29a. In making the cradle, plant the two end trestles first. Stretch a line from one to the other over the intersection. Place the others 4 feet apart and lash them so that each intersection comes fairly to the line.

To build a fascine, straight pieces of brush, 1 or 2 inches at the butt, are laid on, the butts projecting at the end 1 foot beyond the trestle. Leaves should be stripped and unruly branches cut off, or partially cut through, so that they will lie close. The larger, straighter brush should be laid on the outside, butts alternating in direction, and smaller stuff in the center. The general object is to so dispose the brush as to make the fascine of uniform size, strength, and stiffness from end to end.

When the cradle is nearly filled, the fascine is compressed or choked by the fascine choker (Plate 30), which consists of two bars, 4 feet long, joined 18 inches from the ends by a chain 4 feet long. The chain is marked at 14 inches each way from the middle by inserting a ring or special link. To use, two men standing on opposite sides pass the chain under the brush, place the short ends of the handles on top and pass the bars, short end first, across to each other. They then bear down on the long ends until the marks on the chain come together. Chokers may be improvised from sticks and rope or wire.

Binding will be done with a double turn of wire or tarred rope. It should be done in twelve places 18 inches apart, the end binders 3 inches outside the end trestles. To bind a fascine will require 66 feet of wire.

Improvised binders may be made from rods of live brush; hickory or hazel is the best. Place the butt under the foot and twist the rod to partially separate the fibers and make it flexible. A rod so prepared is called a withe. To use a withe, make a half-turn and twist at the smaller end. Pass the withe around the brush and the large end through the eye. Draw taut and double the large end back, taking two half-hitches over its own standing part.

A fascine revetment is made by placing the fascines as shown in Plate 31. The use of headers and anchors is absolutely necessary in loose soils only, but they greatly strengthen the revetment in any case. A fascine revetment must always be crowned with sod or bags.

In all brush weaving the following terms have been adopted and are convenient to use:

Randing.—Weaving a single rod in and out between pickets.

Slewing.—Weaving two or more rods together in the same way.

Pairing.—Carrying two rods together, crossing each other in and out at each picket.

Wattling.—A general term applied to the woven part of brush construction.

A hurdle is a basket work made of brushwood. If made in pieces the usual size is 2 feet 9 inches by 6 feet, though the width may be varied so that it will cover the desired height of slope.

A hurdle is made by describing on the ground an arc of a circle of 8-foot radius and on the arc driving ten pickets, 8 inches apart, covering 6 feet out to out. Brush is then woven in and out and well compacted. The concave side of a hurdle should be placed next the earth. It warps less than if made flat.

In weaving the hurdle, begin randing at the middle space at the bottom. Reaching the end, twist the rod as described for a withe but at one point only, bend it around the end picket and work back. Start a second rod before the first one is quite out, slewing the two for a short distance. Hammer the wattling down snug on the pickets with a block of wood and continue until the top is reached. It improves the hurdle to finish the edges with two selected rods paired. A pairing may be introduced in the middle, if desired, to give the hurdle extra endurance if it is to be used as a pavement or floor. If the hurdle is not to be used at once, or if it is to be transported, it must be sewed. The sewing is done with wire, twine or withes at each end and in the middle, with stitches about 6 inches long, as shown in Plate 33. About 40 feet of wire is required to sew one hurdle. No. 14 is about the right size, and a coil of 100 pounds will sew forty hurdles. Three men should make a hurdle in two hours, two wattling and the third preparing the rods.

Continuous Hurdle.—If conditions permit the revetment to be built in place, the hurdle is made continuous for considerable lengths. The pickets may be larger; they are driven further apart, 12 or 18 inches, and the brush may be heavier. The construction is more rapid. The pickets are driven with a little more slant than is intended and must be anchored to the parapet. A line of poles, with wire attached at intervals of two or three pickets, will answer. The wires should be made fast to the pickets after the wattling is done. They will interfere with the weaving if fastened sooner. Two men should make 4 yards of continuous hurdling of ordinary height in one hour.

Brush Revetment.—Pickets may be set as above described and the brush laid inside them without weaving, being held in place by bringing the earth up with it. In this case the anchors must be fastened before the brush laying begins. The wires are not much in the way in this operation.

Gabion Making.—A gabion is a cylindrical basket with open ends, made of brush woven on pickets or stakes as described for hurdles. The usual size is 2 feet outside diameter and 2 feet 9 inches height of wattling. On account of the sharp curvature somewhat better brush is required for gabions than will do for hurdles. The gabion form is made of wood, 21 inches diameter, with equidistant notches around the circumference, equal in number to the number of pickets to be used, usually eight to fourteen; less if the brush is large and stiff, more if small and pliable. The notches should be of such depth that the pickets will project to 1 inch outside the circle. The pickets should be 1-1/2 to 2 inches in diameter, 3 feet 6 inches long, sharpened, half at the small and half at the large end.

To Make a Gabion.—The form is placed on the ground. The pickets are driven vertically in the ground, large and small ends down, alternately. The form is then raised a foot and held by placing a lashing around outside the pickets, tightened with a rack stick. (See Plate 36.)

The wattling is randed or slewed from the form up. The form is then dropped down, the gabion inverted, and the wattling completed. If the brush is small, uniform, and pliable, pairing will make a better wattling than randing. If not for immediate use, the gabion must be sewed as described for hurdles, the same quantity of wire being required.

The gabion, when wattled and sewed, is completed by cutting off the tops of the pickets, 1 inch from the web, the bottom 3 inches. The latter are sharpened after cutting and driving a pairing picket through the middle of its length and a little to one side of the axis. Three men should make a gabion in an hour.

Gabions may be made without the forms, but the work is slower and not so good. The circle is struck on the ground and the pickets driven at the proper points. The weaving is done from the ground up. The entire time of one man is required to keep the pickets in their proper positions.

If brush is scarce, gabions may be made with 6 inches of wattling at each end, the middle being left open. In filling, the open parts may be lined with straw, grass, brush, or grain sacks to keep the earth from running out.

Gabion Revetment.—The use of gabions in revetment is illustrated in Plate 37. If more than two tiers are used, the separating fascines should be anchored back. Gabion revetment should be crowned with sod or sandbag.

The advantages of gabion revetment are very great. It can be put in place without extra labor, faster and with less exposure than any other. It is self-supporting and gives cover from view and partial cover from fire quicker than any other form. Several forms of gabions made of material other than brush have been used. Some of them are sheet iron, empty barrels and hoops. The disadvantages of iron are that it splinters badly, is heavy, and has not given satisfaction. If any special materials are supplied, the methods of using them will, in view of the foregoing explanation, be obvious.

Timber or Pole Revetment.—Poles too large for use in any other way may be cut to length and stood on end to form a revetment. The lower end should be in a small trench and have a waling piece in front of them. There must also be a waling piece or cap at or near the top, anchored back. Plate 38 shows this form.

Miscellaneous Revetments.—Any receptacles for earth which will make a stable, compact pile, such as boxes, baskets, cans, etc., may be used for a revetment. Canvas or burlap stretched behind pickets is being used to a great extent on the battle fronts of Europe. If the soil will make adobe, an excellent revetment may be made of them, but it will not stand wet weather.

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