Civil Engineering Reference
In-Depth Information
There are, of course, computer programs
available to carry out stability analyses of
slopes, where the circular failure charts are not
applicable. Important features included in these
programs, which allows them to be used for a
wide range of conditions, are as follows:
slope excavated in sandstone, shale and siltstone
above a proposed highway. For these conditions
the shape of the failure surfaces are influenced
by the position and thickness of the beds of the
weaker material.
8.6.5 Three-dimensional circular failure
analysis
•
Slope face can include benches and a variety
of slope angles;
•
Boundaries between the materials can be posi-
tioned to define layers of varying thickness and
inclination, or inclusions of any shape;
The program XSTABL examines the stability
of a unit width slice of the slope, which is a
two-dimensional analysis that ignores any shear
stresses on the sides of the slice (this is the
same principle that is used in the plane fail-
ure analysis described in Chapter 6). While
two-dimensional procedures have found to be a
reliable method of analysis, there may be cir-
cumstances where three-dimensional analysis is
required to define the slide surface and slope
geometry more precisely. One program that
provides a three-dimensional analysis is CLARA
(Hungr, 1987), which divides the sliding mass
into columns, rather than slices as used in the two-
dimensional mode. Figure 8.21 shows an example
of the CLARA analysis for a partially saturated
slope in which the water table is below the bottom
of the tension crack.
•
Shear strength of the materials can be defined
in terms of Mohr-Coulomb or Hoek-Brown
criterion;
•
Ground water pressures can be defined on
single or multiple water tables, or as specified
pressure distributions;
•
External loads, in any direction within the
plane of the slope cross-section, can be posi-
tioned at their correct location on the slope.
Such loads can include bridge and building
foundations and bolting forces;
•
Earthquake acceleration which is applied as a
horizontal force in order to carry out pseudo-
static stability analysis;
•
The shape and position of the slide surface
can be defined as a circular arc or straight line
segments;
8.6.6 Numerical slope stability analysis
•
A search routine finds the slide surface with
the minimum factor of safety;
This chapter has been concerned solely with the
limit equilibrium method of analysis in which
the factor of safety is defined by the ratio of
the resisting to the displacing forces on the slide
surface.
An alternative method of analysis is to exam-
ine the stresses and strains within the slope as
a means of assessing stability conditions. If the
slope is close to failure, then a zone of high strain
will develop within the slope with a shape that
will be approximately coincident with the circu-
lar slide surface. If the shear strength properties
are progressively reduced, there will be a sudden
increase in the movement along the shear zone
indicating that the slope is on the point of failure.
The approximate factor of safety of the slope can
be calculated from the ratio of the actual shear
•
Deterministic and probabilistic analysis meth-
ods that calculate the factor of safety and
probability of failure, respectively. The prob-
abilistic analysis requires that the design para-
meters be defined as distributions rather than
single values;
•
Error
messages
which
identify
negative
stresses along the slide surface; and
•
Drawing of slope showing slope geometry,
material boundaries, ground water table(s)
and slide surface(s).
One program that contains all these functions
is the program XSTABL (Sharma, 1991). An
example of the output (partial) produced by
XSTABL is shown in Figure 8.20 for a benched
