important factors influencing the stability of rock slopes. Discontinuities
can develop or strength can change as a result of the following
environmental factors:
(1) Chemical weathering.
(2) Freezing and thawing of water/ice in joints.
(3) Tectonic movements.
(4) Increase of water pressures within discontinuities.
(5) Alternate wetting and drying (especially expansive shales).
(6) Increase of tensile stresses due to differential erosion.
Further guidance pertinent to rock slopes can be found in DM-7.2,
Chapter 1.
Section 3.
METHODS OF ANALYSIS
1. TYPES OF ANALYSIS. For slopes in relatively homogeneous soil, the
failure surface is approximated by a circular arc, along which the resisting
and rupturing forces can be analyzed. Various techniques of slope stability
analysis may be classified into three broad categories.
a. Limit Equilibrium Method. Most limit equilibrium methods used in
geotechnical practice assume the validity of Coulomb's failure criterion
along an assumed failure surface. A free body of the slope is considered to
be acted upon by known or assumed forces. Shear stresses induced on the
assumed failure surface by the body and external forces are compared with
the available shear strength of the material. This method does not account
for the load deformation characteristics of the materials in question. Most
of the methods of stability analysis currently in use fall in this category.
The method of slices, which is a rotational failure analysis, is
most commonly used in limit equilibrium solutions. The minimum factor of
safety is computed by trying several circles. The difference between
various approaches stems from (a) the assumptions that make the problem
determinate, and (b) the equilibrium conditions that are satisfied. The
soil mass within the assumed slip surface is divided into several slices,
and the forces acting on each slice are considered. The effect of an
earthquake may be considered by applying appropriate horizontal force on the
slices. Figure 1 (Reference 2, Soil Mechanics, by Lambe and Whitman)
illustrates this method of analysis applied to a slope of homogeneous sandy
soil subjected to the forces of water seeping laterally toward a drain at
the toe.
b. Limit Analysis. This method considers yield criteria and the
stress-strain relationship. It is based on lower bound and upper bound
theorems for bodies of elastic - perfectly plastic materials. See Reference
3, Stability of Earth Slopes, by Fang, for further guidance.
7.1-314
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