CHAPTER 7.
SLOPE STABILITY AND PROTECTION
Section 1.
INTRODUCTION
1. SCOPE. This chapter presents methods of analyzing stability of natural
slopes and safety of embankments. Diagrams are included for stability
analysis, and procedures for slope stabilization are discussed.
2. APPLICATIONS. Overstressing of a slope, or reduction in shear strength
of the soil may cause rapid or progressive displacements. The stability of
slopes may be evaluated by comparison of the forces resisting failure with
those tending to cause rupture along the assumed slip surface. The ratio of
these forces is the factor of safety.
3. RELATED CRITERIA. Excavations, Earth Pressures, Special Problems - See
DM-7.2, Chapters 1, 2 and 3 and DM-7.3, Chapter 3.
4. REFERENCE. For detailed treatment on subject see Reference 1, Landslide
Analyses and Control, by the Transportation Research Board.
Section 2.
TYPES OF FAILURES
1. MODES OF SLOPE FAILURE. Principal modes of failure in soil or rock are
(i) rotation on a curved slip surface approximated by a circular arc, (ii)
translation on a planar surface whose length is large compared to depth
below ground, and (iii) displacement of a wedge-shaped mass along one or
more planes of weakness. Other modes of failure include toppling of
rockslopes, falls, block slides, lateral spreading, earth and mud flow in
clayey and silty soils, and debris flows in coarse-grained soils. Tables 1
and 2 show examples of potential slope failure problems in both natural and
man-made slopes.
2. CAUSES OF SLOPE FAILURE.
Slope failures occur when the rupturing force
exceeds resisting force.
a. Natural Slopes.
Imbalance of forces may be caused by one or more of
the following factors:
(1) A change in slope profile that adds driving weight at the top or
decreases resisting force at the base. Examples include steepening of the
slope or undercutting of the toe.
(2) An increase of groundwater pressure, resulting in a decrease of
frictional resistance in cohesionless soil or swell in cohesive material.
Groundwater pressures may increase through the saturation of a slope from
rainfall or snowmelt, seepage from an artificial source, or rise of the
water table.
7.1-309
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