stated. This arrangement will lessen the exposure of the lower face and toe of the dam.
In
time of great flood the crest will be submerged to a greater or less
degree, depending on the width of the structure and the volume of water
discharged by the stream. This would be of little consequence, as the
work should be especially designed to permit of the flood waters
passing over it, the stability of the dam being assured by the size
and weight of the stones exposed to the water.
The
stability of a structure of this character is dependent upon
conditions differing from those which apply to a structure composed of
stones united by a bond, such as an ordinary retaining wall. In the
latter case, if the bond is sufficient to make the wall practically a
monolith, its stability will be complete if it be given weight enough
to prevent it from sliding on its base, and such proportions that it
can have no motion of rotation about its toe.
The
force tending to move or overthrow the bonded dam is equal to the
weight of a prism of water whose base is the area immersed, and whose
height is the vertical distance of the centre of gravity from the
water-level. The point of application of this thrust is situated at
one-third of the height of the water measured from the base. The
direction of the thrust is normal to the surface.
The
problem is an exact one. The thrust is known in its magnitude, its
point of application, and its direction, and the problem of
proportioning a wall of masonry to resist this thrust admits of
complete solution.*
But the detritus barriers are composed of pierres-per-dues, or
what is commonly known as " rip-rap." There can be little bond in such
a structure. Careful placing of stones may, it is true, impart
something like a bond, but
* See Rankin, Krantz.
