Page 288 - Moving the Earth_ The Workbook of Excavation

PONDS AND EARTH DAMS

6.24 THE WORK

may be quite thin if of dense concrete treated with waterproofing on the pond side. Since a leaking
dam is liable to have extensive soft spots in its interior, such a ditch may be dug safely only if the
pond has been drained for several months, or very heavy bracing is used.
Driving a single line of sheet piling, or tongue-and-groove sheeting down the centerline of the
dam, with grouting on the upstream side, is often effective.
The leaks may be stopped by laying a clay blanket on the pond side. The pond should be
drained, if possible, to allow inspection. The leakage may be through the upstream side of the dam
or in the pond bottom nearby. If the spots cannot be found, clay or fine-grained soil should be laid
6 inches to 2 feet thick on the whole face of the dam, and on the bottom, back about twice the
height of the dam, and should be thoroughly tamped. The slope should be gentle, 1 on 4 to 1 on
6, and the clay should be covered with gravel or cobbles where subjected to action of waves. If
the leaks are found, dig them out about 2 feet deep, then tamp in clay or heavy earth patches.
Bentonite mixture may be used instead of clay.
Impervious patches should never be applied on the downstream side, where the water is leaking
out, if the leaks are low on the face. The water will generally work into or around the patch, soften
it, and force it out. If the patch holds, the water held in the dam may liquefy parts of it, causing
slumping and possibly complete failure.
The best treatment for the downstream slope of a leaking dam is to face it with gravel, with an
underdrain below the bottom of the dam opening into the outlet brook, as in Fig. 6.15. The first
coating of gravel should be bank run to allow the passage of water, while holding back any soil
particles carried with it. Over the bank gravel should be clean coarse gravel or crushed stone, to
correct any tendency toward sliding when saturated. If the area is to be planted with grass, stone
should be covered with straw, hay, or cut weeds before placing topsoil.
This gravel blanket does not reduce loss of water, but it does stop damage to the dam and elim-
inates surface wet spots.
Seepage at the foot of the dam may be kept underground by tile and gravel, or stone drains, of
the same type used in draining farmland.
If the dam is of pervious material, the methods suggested later in this chapter for stopping
seepage into porous soil may be of use.
Overtopping. If the water is allowed to flow over the top of an ordinary earth dam, it may cut a
gully to the bottom of it, draining the pond, wrecking the dam, and perhaps causing flood damage
below. Freshly built dams are much more subject to damage from overtopping than old estab-
lished ones that have set and are covered with vegetation.
Overtopping is due to the dam’s settling or slumping below a safe height, or an inadequate or
too high spillway allowing the pond level to rise too much.
If a dam starts to slump, the water should be drained if possible and the dam allowed to dry,
then the dam should be rebuilt with more or better material. If it is not possible to drain the pond,
and pumping or siphoning is not practical, the dam should be reinforced by putting first gravel,

FIGURE 6.15 Seepage apron, small earth dam.