Page 267 - Moving the Earth_ The Workbook of Excavation

PONDS AND EARTH DAMS

PONDS AND EARTH DAMS 6.3

FIGURE 6.1 Digging plan for swamp pond.

been drained enough in advance to be firm, or has gravel soil, or a heavy mat of bushes, these may
not be necessary.
The bottom is kept on grade by digging just enough to let the water back over it. If there is not
enough water to cover the enlarging bottom, the grade may be checked in the same manner as in a
basement excavation.
The length of a pond of this type can be increased indefinitely without change of method. The
width, however, is limited by the reach of the dragline and the depth of the hole. The reach determines
the width of the strip in which it can dig and pile, and the height of the piles; the depth governs the
part of that width which must be reserved for piling spoil.
If a wider pond is required than the machine can dig in one round trip, as illustrated, it must
go behind the piles, drag or swing them away from the excavation, and then widen the hole.

Size and Depth. Calculation of the size and depth of the pond should involve a number of factors.
A large shallow pond gives the most for the investment, at first appearance. A deep pond is desirable
in that it can be fed by seepage from lower levels, loses a smaller percentage of its water by evap-
oration, does not lose area by silting as readily, discourages growth of bottom weeds, and is more
suitable for fishing and swimming. Against these advantages are increased cost and a possible
drowning hazard.
Deep ponds may often be obtained from shallow excavations, or without excavation, by building
of dams and dikes; but for the present we will consider results obtained by excavation only.
The pond should be dug to a clean bottom, if possible, and should yield enough spoil to build
banks 1 foot or more above the water at the edge, and sloping up away from the pond for drainage.
In a limited area that is to be reclaimed, an increase in water surface reduces the area of the banks
and the amount of fill needed for them. Fewer yards need be moved for a large shallow pond than
a small deep one of the same capacity; although a larger proportion of the yardage may have to
be moved more than once.