Page 290 - Moving the Earth_ The Workbook of Excavation

PONDS AND EARTH DAMS

6.26 THE WORK

metal baffles sealed to the concrete, and the fill near the baffles should be mixed with bentonite.
Failure to take these precautions may lead to serious leakage around the core.
Fills should be placed on both sides of the wall at the same time to avoid unbalanced pressure. If
the dam is high, the fill should be carefully compacted. If it is low, this is not necessary unless final
grading is to be done immediately.
The masonry core dam is the safest and most satisfactory construction for ponds, but is too
expensive for casual use.
Removable Wood Dams. If a small pond is built on a small but fast-flowing stream subject to
flood, there is the danger that not only earth or weak masonry dams will be washed out, but that,
if the dam holds, the pond may fill completely with mud and debris in a single season, because
the slowing and widening of the stream cause it to drop a part of its burden.
A removable wood dam may be used to advantage under such conditions. If the stream is narrow,
10 feet or less, a heavy, well-founded masonry wall is put on each bank, having slots to receive
2- to 3-inch plank, as in Fig. 6.16. A masonry sill, similarly slotted, connects the piers on the
stream bottom. Planks cut to the correct size and length are slid down the pier slots, resting on the sill
and on each other, until the desired height is achieved. This structure will leak but will impede a
brisk stream enough that part of it will flow over the top board, and the desired water height may
be maintained. If the stream shrinks, the leaks may be reduced by jamming a tarpaulin in the sill
slot, upstream, and pulling it over the dam face and top, and tying weights on the downstream
side. Or tongue-and-groove planks may be used to cut leakage, with the top plank fastened down
and all the joints packed.
When a flood is expected, or pond use stopped for the season, the planks may be taken out and
stored, allowing the stream a clear passage.

FIGURE 6.16 Removable wood dam.