Page 295 - Moving the Earth_ The Workbook of Excavation

PONDS AND EARTH DAMS

PONDS AND EARTH DAMS 6.31

A very thin grout made with 45 gallons of water to a sack of cement is good for sealing fine
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porous soil, but will escape readily through small channels. The thickest grout used, 4 ⁄ 2 gallons
of water to a sack of cement, will escape only through large openings, but does not seal fine pas-
sages effectively. Sand mixtures are not recommended for amateur use because of the tendency
to separate, but sawdust or fine shavings may be mixed with grout used from pressure containers
if the grout is otherwise washed out by water which cannot be stopped.
If grout is applied at a pressure of more than a few pounds, care should be taken that it does not
lift or break the spillway, or even split bedrock beneath. A tractor grease gun can develop pressure
of thousands of pounds per square inch, and will break up strong masonry with little effort.
All grouting equipment should be thoroughly cleaned immediately after finishing the job, or
for any shutdowns of more than a few minutes.
Detailed information on the use of grout for stopping leakage and for other purposes may be obtained
by writing to the Portland Cement Association, 5420 Old Orchard Road, Skokie, Illinois 60077.

Wood Spillway. Trouble from settling under a spillway may be avoided by putting in a temporary
structure upon completion of the pond, and removing or destroying it after complete settlement,
then building the permanent spillway. Tongue-and-groove plank made into a box is a satisfactory
construction. The dam surface on which the wood rests should be coated with bentonite, clay, or other
fine-grained soil, and puddled until semifluid. The spillway should be stirred around or vibrated
when set, and mud packed in along the sides.
A wood spillway may give satisfactory service for a great many years under favorable
conditions.
Horizontal Pipe. Concrete, tile, or corrugated steel pipe of large size may be used, either, as
described, under drains, or laid horizontally through the dam at water level, with the same pre-
cautions against seepage.

WATER SUPPLY AND LOSSES

Water Supply. The ability of a pond to remain nearly full of water through a dry season is to a
large extent the measure of its usefulness, except in semiarid sections where it is considered a success
if it retains any water at all.
A pond level is kept up by water entering it through rainfall, surface wash, springs and seepage,
and streams. It is lowered by evaporation, outflow, leaks, and seepage through sides and floor.
Once a pond is built, little can be done to add water to it except by pumping water from a well, by
windmills or engine-driven pumps, or more rarely, diverting water into it. It is therefore important
to locate and build it in such a manner as to take full advantage of sources of water.
Ponds dug in swamps may depend primarily on the water table existing before work is started.
If possible, fluctuations of this should be watched for a year or two.
A dug pond may cut into active springs or extensive seepage areas which had previously been
draining below the site, so that the pond may keep a higher level than the groundwater did. On the
other hand, the swamp water might overlie a layer of clay or hardpan, which, when cut, would allow
all the water to drain down into unsaturated porous soil, in which case it might be difficult to keep
water in the pond.
The best way to estimate the water supply is to measure the drainage area. Figure 6.21 indi-
cates approximate requirements throughout the country.

Seepage into Porous Soil. Outgoing seepage can be greatly reduced and sometimes stopped
altogether by keeping mud in suspension in the pond water for some time. The water in seeping
out of the pond takes the suspended particles with it and lodges them in the fine passages through
which it travels, thus clogging them up. This process operating naturally over a period of years
makes possible the existence of rain-fed ponds and swamps on sand dunes and gravel banks, high
above the water table.