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DAMS.
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the face ribs. These planks are fitted with an outgauge or battened or otherwise calked.
Abutments.—Where
abutments are used they should be constructed so as not to contract the
width of the stream. They must be securely connected to the ends of the
dam, and, if possible, carried so far inland that high water cannot
sweep around them ; they must be sunk deep and protected from all
action of the water, and the ends adjacent to the dam should be
rounded. They are constructed of stone or cement, or are built of
timber cribs.
Masonry Dams.—Hydraulic
mining from its nature does not justify the expense of masonry dams,
unless perhaps the reservoirs are designed also for other and more
permanent uses. The subject of the construction of masonry dams has
been thoroughly investigated by engineers. The annexed profile (Fig.
3), the bounding lines of which are logarithmic curves, has been
calculated by Prof. Rankine to serve as a type for masonry dams of any
practicable height. " It presents many strong points not found in the
usual rectilinear profile, and deserves especial consideration."
The
most desirable form of profile for masonry dams is the one which
combines the greatest strength with the least amount of material. To
determine this it is necessary to know the forces to which the
proposed dam is to be subjected, whether constant or variable, and the
effects they are likely to produce. The conditions of stability (that
the dam may sustain its own weight and withstand both its own weight
and the pressure of the water) are then considered, and the profile
adopted which, combines the greatest strength and stability with
economy of material.
The
weight of the material composing the structure, and the pressure or
thrust of the water which it holds, are the only forces which may be
regarded as acting with vigor on a dam. The former is constant; the
latter depends on the height of the water behind the dam, and
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