Civil Engineering Reference
In-Depth Information
Circular failure
8.1 Introduction
Although this topic is concerned primarily with
the stability of rock slopes containing well-
defined sets of discontinuities, it is also necessary
to design cuts in weak materials such as highly
weathered or closely fractured rock, and rock
fills. In such materials, failure occurs along a sur-
face that approaches a circular shape (Figure 8.1),
and this chapter is devoted to a discussion on the
stability analysis of these materials.
In a review of the historical development of
slope stability theories, Golder (1972) traced the
subject back almost 300 years. Much of the
development of circular failure analysis meth-
ods was carried out in the 1950s and 1960s,
and these techniques have since been used to
prepare computer programs that have the ver-
satility to accommodate a wide range of geo-
logic, geometric, ground water and external
loading conditions. This chapter discusses the
principles of the theoretical work, and demon-
strates their application in design charts and in
the results of computer analyses. During the
past half century, a vast body of literature on
the subject of circular failure has accumulated,
and no attempt will be made to summarize the
material in this chapter. Standard soil mechan-
ics text books such as those by Taylor (1937),
Terzaghi (1943) and Lambe and Whitman
(1969), and papers by Skempton (1948), Bishop
(1955), Janbu (1954), Morgenstern and Price
(1965), Nonveiller (1965), Peck (1967), Spencer
(1967, 1969) and Duncan (1996) all contain
excellent discussions on the stability of soil slopes.
The approach adopted in this chapter is to
present a series of slope stability charts for circu-
lar failure. These charts enable the user to carry
out a rapid check on the factor of safety of a
slope, or upon the sensitivity of the factor of safety
to changes in ground water conditions, slope
angle and material strength properties. These
charts should only be used for the analysis of
circular failure in slope materials that are homo-
genous and where the conditions apply that were
assumed in deriving the charts (see Section 8.4).
More comprehensive methods of analysis are
presented in Section 8.6. These methods can be
used, for example, where the material properties
vary within the slope, or where part of the slide
surface is at a soil/rock interface and the shape of
the slide surface differs significantly from a simple
circular arc.
This chapter primarily addresses the stability
of slopes in two dimensions, and assumes that
the slope can be modeled as a unit slice through
an infinitely long slope, under plane-strain condi-
tions. Section 8.6.5 discusses three-dimensional
circular failure analysis, and Section 10.3.1 dis-
cusses the influence of the radius of curvature of
the slope on stability.
8.2 Conditions for circular failure and
methods of analysis
In the previous chapters, it has been assumed
that the failure of rock slopes is controlled by
geological features such as bedding planes and
joints that divide the rock into a discontinuous
