(3) Progressive decrease in shear strength of the soil or rock mass
caused by weathering, leaching, mineralogical changes, opening and softening
of fissures, or continuing gradual shear strain (creep).
(4) Vibrations induced by earthquakes, blasting, or pile-driving.
Induced dynamic forces cause densification of loose sand, silt, or loess
below the groundwater table or collapse of sensitive clays, causing
increased pore pressures. Cyclic stresses induced by earthquakes may cause
liquefaction of loose, uniform, saturated sand layers (see DM-7.3, Chapter
1).
b. Embankment (Fill) Slopes. Failure of fill slopes may be caused by
one or more of the following factors:
(1) Overstressing of the foundation soil. This may occur in
cohesive soils, during or immediately after embankment construction.
Usually, the short-term stability of embankments on soft cohesive soils is
more critical than the long-term stability, because the foundation soil will
gain strength as the pore water pressure dissipates. It may, however, be
necessary to check the stability for a number of pore pressure conditions.
Usually, the critical failure surface is tangent to the firm layers below
the soft subsoils.
(2) Drawdown and Piping. In earth dams, rapid drawdown of the
reservoir causes increased effective weight of the embankment soil thus
reducing stability. Another potential cause of failure in embankment slopes
is subsurface erosion or piping (see Chapter 6 for guidance on prevention of
piping).
(3) Dynamic Forces. Vibrations may be induced by earthquakes,
blasting, pile driving, etc.
c. Excavation (Cut) Slopes. Failure may result from one or more of the
factors described in (a). An additional factor that should be considered
for cuts in stiff clays is the release of horizontal stresses during
excavation which may cause the formation of fissures. If water enters the
fissures, the strength of the clay will decrease progressively. Therefore,
the long-term stability of slopes excavated in cohesive soils is normally
more critical than the short-term stability. When excavations are open over
a long period and water is accessible, there is potential for swelling and
loss of strength with time.
3.
EFFECT OF SOIL OR ROCK TYPE.
a. Failure Surface. In homogeneous cohesive soils, the critical
failure surface usually is deep whereas shallow surface sloughing and
sliding is more typical in homogeneous cohesionless soils. In
nonhomogeneous soil foundations the shape and location of the failure
depends on the strength and stratification of the various soil types.
b. Rock. Slope failures are common in stratified sedimentary rocks, in
weathered shales, and in rocks containing platy minerals such as talc, mica,
and the serpentine minerals. Failure planes in rock occur along zones of
weakness or discontinuities (fissures, joints, faults) and bedding planes
(strata). The orientation and strength of the discontinuities are the most
7.1-313
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