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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