Page 304 - Moving the Earth_ The Workbook of Excavation

PONDS AND EARTH DAMS

6.40 THE WORK

FIGURE 6.27 Loading a small dredge for transportation.

of water contains about 325,000 gallons for each foot of depth (this is a measurement called an
1
1
acre-foot), or enough water for 4 ⁄2 to 6 ⁄2 hours of operation. Because of the slope of banks, each
successive foot of water will contain fewer gallons than the one above it. A pond 1 acre in area
1
and 5 feet deep might contain 2 ⁄2 to 3 acre-feet, say 1 million gallons.
An 8-inch dredge needs about 60 percent more water and moves about 60 percent more soil
than a 6-inch model.
If there is no natural inflow into a pond, it is necessary to return most of the water taken by the
dredge to be used again. The most economical way to do this is to put the disposal area upstream,
so that water will drain back. If this is not practical, a pump is placed so that it can return the water
through a hose or pipe. A heavy-duty 4-inch pump can usually handle enough water to keep a 6-inch
dredge busy.
Return water is usually dirty so that part of the dredge capacity is wasted rehandling the fines that
did not settle out. The amount of circulating soil particles is greatest when the soil is fine-textured,
and when settling ponds or areas are small.
The fine organic mud that forms a large part of the deposits on pond bottoms may stay sus-
pended in water for long periods. The experimental uses of cyclones for separating soil particles
from dredge water, and of polymers for clotting organic slime, which may reduce this problem,
are discussed in Chap. 14.
Dirt in return water increases when the suction strainer of a return pump is too low, or the water
is allowed to run on the ground and erode it. It is reduced by having a large settlement pond or area,
and/or very sluggish open flow of return water. It tends to increase as the settlement pond fills up
with use.