Page 285 - Moving the Earth_ The Workbook of Excavation

PONDS AND EARTH DAMS

PONDS AND EARTH DAMS 6.21

Front Shovel. A front shovel can break up the bottom, provided that drainage is adequate and
dependable. The machine can operate on pontoons and ramp itself down to the required depth, and
dig as wide a cut as it can reach, or is required, dumping the spoil in a ridge behind it. It may come
out of the pond site elsewhere, or turn and emerge near the entrance point, as in Fig. 6.13. Material
broken up in this way can be easily dug by a dragline, but may be so soft as to be unsafe even on plat-
forms, until it is well drained.
It rarely happens that a layer under a swamp is sufficiently firm to carry trucks, but in this case
regular basement-digging techniques can be used.
Bulldozer digging in softer mud is done by the methods described below for pond cleaning.

Water Level. The highest water level in a pond depends on the height of the overflow point, whether
it is a streambed or an artificial spillway.
The best way to decide upon the new water level is to lay out a grid and take elevations through-
out the area. The boundaries of a pond at any level can readily be sketched in, and the amount of cut
required for desired depth, and the spoil to be disposed of in the dam and the banks can be roughly
calculated.
A less laborious method which is usually satisfactory is to select some spot that would make
a good shore and use a transit or hand level to find the corresponding shoreline at other points.
This is done by reading a rod set on the selected point and moving the rod up and down any slope
in question until the reading is the same, at which place it will be on the same level as the original
point.
Readings can be taken on the dam site and on high, low, and normal points in the pond basis,
and distances measured with a tape or by stadia.

DAMS

Digging may be done according to patterns outlined previously, with one or more cuts made
across the bottom of the pond and piled for the dam. Or the digging may be done parallel with the
dam and all the spoil used in its construction.
A dam should fulfill three requirements. It must be high enough in relation to the spillway such
that water will never flow over unprotected parts; it should be stable enough not to break, slump,
or move under any conditions; and it should not leak.
Usually earth dams for small ponds are given a freeboard, or height above water, of 2 feet. If
the spillway is wide, wave action very weak, and the material thoroughly consolidated, 1 foot may
be enough; but under reverse conditions, 3 or more feet may be required. For further protection,
an earth dam should be covered with rocks, a strong sod, or bushes and trees.
No dam should be built to hold a depth of over 6 feet of water, or any considerable volume of
water which would flood an inhabited area if released, without competent engineering advice. In
many localities, plans must be filed and permits obtained before building any dam.
Earth Dams. For stability, an earth dam should rest on a base of firm soil or rock without strati-
fication, dipping away from the pond. It should be well bonded to its base by removing vegetation
and plowing or ditching parallel to the axis of the dam.
The dam should be at least 6 feet thick at water level, and slopes should not exceed 1 on 2 on
the downstream face, nor 1 on 3 upstream. If the top is to be used as a roadway, it should be at least
10 feet wide.
The soil used should be stable enough to hold itself up, to resist both the push and the softening
effect of the water, and to carry any traffic or other loads on the top of the dam. It should also be
fine-grained and compact enough to give maximum resistance to movement of water through it.
Stability in the presence of water is best obtained by the use of broken rock or clean gravel,
but these materials allow easy passage of water. Clay, and soils rich in clay, is best for sealing off
water, but may be inclined to slump and flow when saturated.