pierces the base or any horizontal joint within certain defined limits.
If the line of the resultant intersects any horizontal plane of the
dam outside of these limits, stability is not assured.
The following conditions are indispensable for the stability of dams:
ist. The courses of masonry must be incapable of slipping one over the other, and the wall incapable of sliding on its base.
2d. Neither the material employed nor the foundation must be required to bear too great a pressure.
stones must not be laid in horizontal courses extending from front to
rear, and binders should be freely used. The stability of all dams (or
walls sustaining pressure) requires that there should be no continuous
reservoirs of moderate depth earthen dams are frequently used.
Experience sanctions for these dimensions not less than ten feet on
top, and a height of over sixty feet is considered risky by many
engineers. Trautwine suggests that in properly constructed earthen
dams " the top width should be equal to two feet plus twice the square
root of the height in feet." The inner slope should be 2-1/2 (base) to
1 (height), and the outer slope 1-1/2 to 1. Flat inner slopes
are most desirable, as they increase the stability of the structure and
likewise prevent displacement of the pitching. In some instances the
toes of the slopes abut against retaining walls in cement. The inner
slopes should be carefully faced up to the top with dry rubble-stone
pitching at least one and one-half feet deep.
Pillarcitos reservoir, San Mateo County, has an earthen dam six hundred
and forty feet long, twenty-six feet wide on top, and ninety-five feet
high. The San Andreas dam is six hundred and forty feet long,
twenty-five feet wide on top, and ninety-five feet high. The former has
a slope of 2-3/4 (base) to 1 (height) on the inner, and 2-1/2 to 1 on the outer side. In the latter the inner