Page 286 - Moving the Earth_ The Workbook of Excavation

PONDS AND EARTH DAMS

6.22 THE WORK

FIGURE 6.14 Hydraulic fill dam.

Large Earth Dams. In large dams, a clay core should be used to stop the water, with loam or
gravel faces to support the clay, as in Fig. 6.14. In such dams, the type and amount of each material
must be calculated. The dam may be built up in carefully compacted layers from material carried
from pits in trucks or scrapers; or the fill may be mixed with water and carried to the dam in pipes
laid along its sides. When carried mechanically, the clay core is usually built up a step or two
ahead of the faces. The hydraulic method mixes all the soil types together, but as they come out
of the pipe, the coarse material is dropped first, at the edge, and successively finer particles as the
water flows inward. At the center a pond forms, and fine clay and silt particles are deposited to
build the core.

Small Dams. These methods are not well adapted to small dams. The expense of setting up
hydraulic equipment can be justified only by large-scale operations. Mechanical transportation
and spreading are handicapped by lack of width. Even small trucks, scrapers, and dozers cannot
work readily in strips less than 8 feet wide, or in comfort on less than 12 feet. The three sections
would accordingly produce a width greatly in excess of that needed. A narrower dam could be
built by the use of undersized equipment or hand labor.
A reasonably satisfactory dam may be made of mixed soil dug out of the pond or obtained
nearby. This may be piled wet by a dragline or built up in compacted layers in the same manner
as a road fill. Dusty soil should be dampened. If much sand or gravel is included, it should be
mixed with fine-grained soil, or placed on the downstream face as much as possible.
If the dam is built of dry, uncompacted material, it should be allowed several months and some
soaking rains to settle it before it is used to impound water.
If the soil is porous, the dam will leak unless sealed on the upstream side. It is not safe to wait
for sediment in the pond to accomplish this, as leakage may liquefy the soil and cause the dam
to fail.
The upstream face may be covered with a blanket of clay, heavy soil, or a bentonite mixture.
Bentonite. Bentonite is a volcanic clay which absorbs large quantities of water, changing to a
jelly that effectively seals soil against water seepage. It is the expensive variety of clay mentioned
in Chap. 3. It is used in many industrial processes and is available in bags in most cities. The pellet
size is more desirable for pond work than the powder forms, which tend to float on the water sur-
face for long periods and may be lost over the spillway.
A recommended practice is to mix one part of bentonite with four parts of sandy soil, or six or
eight parts of fine-grained loam, and place a 4-inch layer of the mixture over the areas to be water-
proofed. When more convenient, the pure material is spread over the ground and raked in. Satisfactory
results are often obtained from more economical amounts, applied either in leaner mixtures or in
thinner layers.
Either bentonite or the mixture can be shoveled into a pond over leaks, and allowed to settle
into them. This is best done when there is no overflow.