SLOPE STABILITY ANALYSES FOR EARTHQUAKES - Geotechnical Earthquake Engineering - page 262

TABLE 9.3 Assumptions Concerning Interslice Forces for Different Method of Slices

Type of method of slices Assumption concerning interslice forces Reference

Ordinary method of slices Resultant of interslice forces is parallel to Fellenius (1936)

average inclination of slice

Bishop simplified method Resultant of interslice forces is horizontal Bishop (1955)

(no interslice shear forces)

Janbu simplified method Resultant of interslice forces is horizontal Janbu (1968)

(a correction factor is used to account

for interslice shear forces)

Janbu generalized method Location of interslice normal force is de- Janbu (1957)

fined by an assumed line of thrust

Spencer method Resultant of interslice forces is of constant

slope throughout the sliding mass Spencer (1967, 1968)

Morgenstern-Price method Direction of resultant interslice forces is Morgenstern and

determined by using a selected function Price (1965)

Sources: Lambe and Whitman (1969) and Geo-Slope (1991).

Duncan (1996) states that the nearly universal availability of computers and much
improved understanding of the mechanics of slope stability analyses have brought about
considerable change in the computational aspects of slope stability analysis. Analyses can
be done much more thoroughly and, from the point of view of mechanics, more accurately
than was possible previously. However, problems can develop because of a lack of under-
standing of soil mechanics, soil strength, and the computer programs themselves, as well
as the inability to analyze the results in order to avoid mistakes and misuse (Duncan 1996).
Section 9.2.7 presents an example problem dealing with the use of the pseudostatic
slope stability analysis based on the method of slices.

9.2.5 Landslide Analysis

As mentioned in Sec. 9.1.1, the pseudostatic method can be used for landslides that have a
distinct rupture surface, and the shear strength along the rupture surface is equal to the
drained residual shear strength r . The residual shear strength r is defined as the remain-
ing (or residual) shear strength of cohesive soil after a considerable amount of shear defor-
mation has occurred. In essence, r represents the minimum shear resistance of a cohesive
soil along a fully developed failure surface. The drained residual shear strength is primar-
ily used to evaluate slope stability when there is a preexisting shear surface. An example of
a preexisting shear surface is shown in Fig. 9.7, which is the Niguel Summit landslide slip
surface that was exposed during its stabilization. In addition to landslides, other conditions
that can be modeled using the drained residual shear strength include slopes in overcon-
solidated fissured clays, slopes in fissured shales, and other types of preexisting shear sur-
faces, such as sheared bedding planes, joints, and faults (Bjerrum 1967, Skempton and
Hutchinson 1969, Skempton 1985, Hawkins and Privett 1985, Ehlig 1992).
Skempton (1964) states that the residual shear strength r is independent of the origi-
nal shear strength, water content, and liquidity index; and it depends only on the size, shape,
and mineralogical composition of the constituent particles. The drained residual friction
angle r of cohesive soil could be determined by using the direct shear apparatus. For
example, a clay specimen could be placed in the direct shear box and then sheared back and