this
cannot be safely relied upon as a source of strength. Each stone, being
practically independent of its neighbors, must rely upon its own
resisting quality to maintain its place in the structure.
It
follows that where the floods are great and the exposure consequently
large the stones must be proportionately large and heavy.
The
interior of a structure of this kind, being protected from the action
of the water and held in place by superincumbent weight, may be
composed of sizes of stone which it would be unsafe to place on the
crest and exposed surfaces. The stones of the crest and on the lower
slope are most exposed, and consequently must be of the largest sizes.
The force that tends to move them is not hydrostatic pressure, but the
force and impact of great volumes of water moving with high velocity.
Such
a structure, composed of rubble stone and unable to impound water,
would be exposed to the pressure of the material which is slowly
deposited behind it. The maximum horizontal pressure from this source
alone would be reached when the plane of fracture of the earth bisects
the angle which will be formed by the earth sloping back from the foot
of the wall on its angle of repose ; therefore the weight of such a
prism can be easily calculated.
As
the dam fills up, the pressure of the material on itself, owing to its
composition, would cause it to consolidate (cement), thus continually
changing the angle of repose, until finally, when even with the crest,
there would be comparatively no horizontal thrust or pressure on the
clam, the structure simply protecting the face of the deposit from
erosion. Therefore such barriers, constructed with proper materials on
the well-known principles of dam-building, could not fail to hold back
the debris.
As
these dams are not water-tight, and are composed of large masses of
rubble stone without bond, it is difficult to see how, in the event of
a breach, the inhabitants below
