Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 6 SLOPE STABILITY
programs such as SLOPE/W and STABL, which offer several analysis procedures, are useful for
slope stability assessment.
6.4 PRINCIPLES OF ANALYSIS
6.4.1 Method of Analysis
The methods for analysis of slope stability broadly used in engineering practice are limit equilibrium
methods and finite element methods. The limit equilibrium method of slope stability analysis is
used to evaluate the equilibrium of a soil mass tending to move down slope under the influence of
gravity. A comparison is made between forces, moments, or stresses tending to cause instability of
the mass, and those that resist instability. Two-dimensional (2-D) sections are analyzed and plane
strain conditions are assumed. These methods assume that the shear strengths of the materials
along the potential failure surface are governed by linear (Mohr-Coulomb) or nonlinear relationships
between shear strength and the normal stress on the failure surface.
Where estimates of movements as well as factor of safety are required to achieve design
objectives, the effort required to perform finite element analysis can be justified. However, finite
element analysis requires considerably more time and effort, compared to the limit equilibrium
analysis and additional data related to stress-strain behavior of materials. Therefore, the use of
finite element analysis is not justified for the sole purpose of calculating factors of safety.
6.4.2 Stages of Stress Analysis
As mentioned in Para 3.3, shear strength of the soil varies with time. Thus, in slope stability
analysis, it is important for the designer to understand and determine at which point in time i.e.
before, during or after construction that is more critical and yield the lowest factor of safety.
Generally, the two conditions considered are:
6.4.2.1 Short-Term (or At-the-end-of-construction)
Analyses of the short-term condition of stability are normally performed in terms of total stress
(using undrained shear strength parameters), with the assumption that any pore water pressure set
up by the construction activity will not dissipate at all. However, in some construction works such as
large earth dams or embankments, the construction period is relatively long, and some dissipation
of the excess pore water pressure is likely. Under these conditions, a total stress analysis would
yield a value of factor of safety on the low side, possibly resulting in un-economic design.
For undrained shear strength of saturated soil, φ can be assumed as zero and knowledge of the
pore water pressure (i.e. the phreatic line) is not necessary since total stress can be expressed
independently of effective stress at failure. For instance, the total stress analysis must be used for
the construction of coastal bund in soft clay and it usually gives the worst critical factor of safety.
Unconsolidated Undrained (UU) Triaxial test is usually used to obtain the undrained strength
parameter of the soil. Extra care shall be given during the test when the soil samples are not fully
saturated. For soft to very soft clay such as coastal alluvium clay, in-situ strength test using in-situ
vane shear test should be used to determine the undrained shear strength. Typical values of
undrained shear strength for Malaysia coastal alluvium clay ranges from 10 to 20 kPa.
Table 6.1 gives some typical values of undrained shear strength, c which may be used for
preliminary analysis or to check laboratory test results
6-4 March 2009