Page 298 - Moving the Earth_ The Workbook of Excavation

PONDS AND EARTH DAMS

6.34 THE WORK

FIGURE 6.22 Silt trap.

it will not only support platforms but will stay in a dragline bucket. This hardening process, which may
reduce its bulk as much as 80 percent, can be greatly accelerated by hand ditching into the subsoil for
more thorough drainage. The ditching, however, is a sloppy job, and will be very discouraging at
first because of mud flowing or slumping into the ditch and blocking it. The first digging should be very
shallow and can be gradually deepened as the banks drain.
If the pond is narrow and accessible enough that all parts can be reached by a dragline on the
banks, or if the mud overlies firm material that will support a dozer which can push the mud to a
dragline, it may not be absolutely necessary to let the mud dry. If it is too thin to be picked up in
the bucket, digging the ground under it several times may suffice to get enough of it out. In any
event, such undercutting will eventually lower the mud so much that it will no longer be a nuisance.
It is seldom practical to just skim even dry mud off the old bottom. At least several inches of
native soil are ordinarily dug with it, and this opportunity is often taken to deepen the pond sub-
stantially. In some cleaning methods, it is necessary to take enough subsoil to build firm piles.
The cleaning process differs from the original digging in the usually shallower cuts, the pecu-
liar nature of the mud, the fact that trees and landscaping on the banks often must not be disturbed,
and the undesirability of reducing the pond area by piling spoil inside it.
Bottom mud is generally useless for agriculture when freshly dug, but makes excellent topsoil
after curing in piles for a year or two. Mixing with sandy subsoil speeds curing and improves its
quality. It is often necessary to add lime to correct acidity.
Dragline. If a dragline can do the necessary cleaning from the banks, the problems are chiefly
avoiding or cutting trees, and providing either places to pile the spoil or means of access for trucks
to haul it away.
If the width is too great for the boom length, an unassisted dragline must work from the pond
bottom, usually on platforms. From there it may pile spoil on the banks to be leveled off later;
against the banks, to make a new shore for a smaller pond; load it in trucks on the bank, or build
one or more windrows in the pond to be trucked out later.
Trucking windrows must contain enough inorganic soil that they will become firm as they dry,
and must be high enough that capillary water will not keep them soft. This height will vary from
about 3 feet for a sandy mixture to 7 to 10 for silt or clay. Lower piles, or any piles containing a
lot of humus, may require a surfacing of better soil or gravel before they will support trucks. The
height of the roadway will be substantially lower than that of the top of the original windrow.