Page 289 - Moving the Earth_ The Workbook of Excavation

PONDS AND EARTH DAMS

PONDS AND EARTH DAMS 6.25

then a heavy fill of coarse rock on the downstream side. An attempt should be made to puddle or
blanket the pond side, and the top should be filled to grade. If it settles badly without slumping,
the top should be built up, preferably with compacted fill. Sandbags, if obtainable, make an excellent
temporary stop.
Sometimes a dam can be saved by partly draining the pond through a trench dug in firm ground
nearby. Undisturbed soils can often carry a heavy flow of clean water without severe gullying,
particularly if reinforced with roots, boulders, or brush mats.

Repair. When a gullied dam is fixed, the sides of the break should be smoothed and sloped suf-
ficiently that the fill can be tamped against all parts of them, but it should not be cut into a straight
ditch. The bottom should be dried up if possible. Fill should be dumped on the edge and pushed
or shoveled down gradually, while workers at the bottom spread it in thin layers, tamping or
tramping it thoroughly. If the break is large enough to allow machinery to work in it, it can do
most of the spreading and compacting, but the bond with the walls must be done by hand. Dusting
bentonite against the sides while filling should prevent seepage along them.
If it is not practical to dry up the bottom, fill should be dumped and kneaded until the water is
absorbed into a stiff mud on which a layered fill may be built.
Burrowing Animals. Earth dams may be damaged by animals burrowing part or all the way
through them. Muskrats make holes which run underwater to well under the bank, where they rise
above the water. Such tunnels will cause leaks only when they give water access to some line of
weakness that did not go through to the pond, or which had been silted shut. Muskrat damage can
be largely avoided by using a low dam not containing enough dry ground for home building, or a
wide one without porous veins.
Crayfish will at times dig burrows all the way through a dam, creating a water channel large
enough to enlarge by erosion, unless a fortunate cave-in should block it. This damage is most apt
to occur in soft peat soil, and it may sometimes be cured by injections of cement grout.
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Burrowing animals may be discouraged by including ⁄ 4 -inch mesh wire in the underwater part
of the upstream slope. This affords fairly good protection for a number of years. It is usually laid
on the dam, and 6 inches to 1 foot of fill is spread on it.
Masonry Dams. Masonry dams may be used instead of earth fills. They are most suited to com-
paratively narrow sites with firm bedrock near the surface of bottom and sides. Reinforced concrete
is the strongest construction, but fieldstone masonry is more attractive and may be less expensive
in inaccessible spots.
Earth and decayed rock should be cleaned off the dam site, and the bedrock shaped or gouged
in such a way that the dam will not be able to slide on it in any direction. Holes 2 or more feet in
depth should be drilled in the rock, and reinforcing steel grouted into them so that it will project
into the concrete or other masonry.
If the dam is to be more than a few feet high, it is advisable to have an engineer or a geologist
check the ground, as fractured rock can make a leaky and unstable foundation.
The dam should have a bottom thickness of at least 2 to 3 feet for every 3 feet of height.
Masonry Cores. A masonry core dam consists of a thinner wall, preferably reinforced concrete,
with earth piled on both sides. The masonry does not extend much above the waterline, and is
ordinarily buried under earth. The core seals off seepage, and the sides support and protect it. It
must resist the difference in pressure between the wet and dry earth on its two sides. Thickness is
about one-fourth of height.
The core should be founded on a firm, impermeable material, preferably rock. The original
surface is ditched for footings. The sides are carried into the banks until they meet rock, or until
they are far enough from the water to make seepage unlikely. Rock should be roughened to hold
the masonry against shifting.
The core is built and allowed to cure before placing the earth fill. The upstream face should be
painted with waterproofing. If its ends are not keyed into rock, they should be fitted with vertical