die rammed solidly, which is torn off afterwards, bringing with it all the loose pieces of rock.
Where
a hard-pan bottom is used great care should be taken not to crack it.
Fanning recommends in such cases that the soil should be carefully
removed down to the impervious stratum, on which a puddle of well
rammed clay, rolled with not less than a two-ton weight, should be
placed, and a puddle wall built. He also suggests the covering of the
ground in front with a layer of gravel and clay, and at the toe of the
inside face of the dam sheet piling should be driven through the
hard-pan t« prevent any leakage under the base of the structure, which
must be water-tight and have a strong apron placed in front of it to
prevent the water from scouring the bed.
Wooden Dams.—On
light soil, where there is danger of undermining from the overflow,
wooden dams can be built in step form (i vertical to 3 or 4 horizontal)
and provided with aprons; sometimes the aprons are inclined towards the
dam, against which their lower ends abut, while at the further end
sheet piling is driven and the bed around it protected with rip-rap.
The same object is accomplished likewise by two dams erected a short
distance apart, the lower one forming a pool or water-cushion for the
discharge from the upper one.
There
are various forms of wooden dams. They are generally constructed of
round logs or hewn timber one to two feet in diameter, laid on each
other so as to form in plan a series of cribs from eight to ten feet
square, and pinned together by wooden treenails. In the better class
of crib-work the timbers are notched and bolted to each other at each
intersection with iron drift bolts, the round logs being flattened or
notched where they lie upon each other. The bottom timbers are bolted
to the bed-rock, the ties are notched and bolted to the stringers, and
the cribs are filled with rock. The face of the dam is made water-tight
bv an outer skin of plank spiked to
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