Geologic Hazards - Geotechnical Engineering Investigation - page 711

Slope Height
Slope height is increased by filling at the top, erosion below the toe, or tectonic activity. It
is decreased by excavation and erosion at the top, or by placing a berm at the bottom. The
driving forces are affected in failure forms where the limited slope condition applies (see
Figure 9.8).
Material Structure ( Figure 9 .9b)
Significance
Material structure influences the failure form and the location and shape of the potential
failure surface, and can be considered in two broad categories: uniform and nonuniform.
Uniform Materials
Uniform materials consist of a single type of soil or rock, essentially intact and free of dis-
continuities. From the aspect of slope stability, they are restricted to certain soil formations.
Rotational failure is normal; the depth of the failure surface depends on the location of the
phreatic surface and on the variation of strength with depth. Progressive failures are com-
mon, and falls and flows possible; flows are common in fine-grained granular soils.
Nonuniform Materials
Formations containing strata of various materials, and discontinuities represented by bed-
ding, joints, shears, faults, foliations, and slickensides are considered nonuniform. The
controlling factor for stability is the orientation and strength of the discontinuities, which
represent surfaces of weakness in the slope.
Planar slides occur along the contacts of dipping beds of sedimentary rock and along
joints, fault and other shear zones, slickensides, and foliations. Where a relatively thin
deposit of soil overlies a sloping rock surface, progressive failure is likely and may
develop into a debris avalanche. Along relatively flat-lying strata of weak material, failure
can develop progressively in the form of lateral spreading, and can develop into a flow.
Rotational slides occur in horizontally bedded soil formations, and in certain rock forma-
tions such as clay shales and horizontally bedded sedimentary rocks.
Falls occur from lack of tensile strength across joints in overhanging or vertical rock
masses. Changes in the orientation of the discontinuities with respect to the slope face
occur normally as a result of excavation, but can also be caused by tectonic activity. Joint
intensity can be affected by construction blasting.
Material Strength (Figure 9.9b)
Significance
Material strength provides the resisting forces along a surface of sliding. It is often neither
the value determined by testing, nor the constant value assumed in analysis.
Variations along the Failure Surface
Slopes normally fail at a range of strengths, varying from peak to residual, distributed
along the failure surface as a function of the strains. Slopes that have undergone failure in
the past will have strengths at or near residual, depending upon the time for restitution
available since failure.
Changes with Time
Chemical weathering is significant in residual soils and along discontinuities in rock masses
in humid climates, and provides another possible explanation for the sudden failure of