OVERTURNING FORCES.
157
should get under the wall than that it should creep under any stratum of a
well-constructed masonry or puddle dam, and under those cireumstances the
presence of the water will increase the stability by diminishing the lateral
thrust of the filling. If, however, as is perhaps more frequently the case
the wall is founded on a porous stratum, the full hydrostatic pressure will
act on the base of the wall, and reduce its stability in practical cases by
about one-half.”* These mutable conditions can manifestly only be met by
providing a considerable margin of strength.
Stresses in Retaining Walls.—The forces at work in the case of an
ordinary retaining wall are three in numbei.
(1) There is the overturning influence of a wedge-shaped mass of earth,
DC E (fig. 77), behind the wall, which tends to slide down some plane of
rupture, C E, in the absence of proper support.
(2) To this must be added the effect of any surcharge upon the surface
of the ground constituting the wedge.
Fig. 77.
(3) And, lastly, there is the weight of the wall acting vertically down-
ward, and consequently offering resistance to the overturning tendency
the back of the wall be not vertical, as in fig. 78, it is obvions that the
perpendicular line, C D, niust still be con-
sidered the virtual boundary of the opposing
influences and that the weight of the earth-
work, F C D, must be included in the weight
of the wall.
It will be well to consider these forces a
Fig. 79.
little more in detail. .
Overturning Forces.-The actual extent of the wedge and its effective
pressure can only be matters of conjecture. It is common expérience that
unsupported earthwork stands at widely differing slopes, according to the
nature and condition of the particles of which it is composed. To a limited
degree, experiments have determined some of these slopes and fixed what is
termed an Ancjle oJ Repose ^ fig. 79) for the more prominent k.nds of
* Baker on “Lateral Pressure of Earthwork,” Mm. Proc. Inst. C.L., vol.
p. 180.