Civil Engineering Reference
In-Depth Information
Wedge failure
7.1 Introduction
The previous chapter was concerned with slope
failure resulting from sliding on a single planar
surface dipping into the excavation, and strik-
ing parallel or nearly parallel to the slope face.
It was stated that the plane failure analysis is
valid if the strike of the failure plane is within
±
20
◦
of the strike of the slope face. This chapter
is concerned with the failure of slopes contain-
ing discontinuities striking obliquely to the slope
face where sliding of a wedge of rock takes place
along the line of intersection of two such planes
(Figure 7.1). Wedge failures can occur over a
much wider range of geologic and geometric con-
ditions than plane failures, so the study of wedge
stability is an important component of rock slope
engineering. The analysis of wedges has been
extensively discussed in geotechnical literature,
and the manual draws heavily upon the work of
Goodman (1964), Wittke (1965), Londe (1965),
Londe
et al
. (1969, 1970), John (1970).
In this chapter, the structural geological con-
ditions that result in the formation of a wedge
formed by two intersecting planes are defined,
and the method of identifying wedges on the
stereonet is illustrated. The stereonet defines the
shape of the wedge, the orientation of the line
of intersection and the direction of sliding. This
information can be used to assess the potential for
the wedge to slide from the cut face. The proced-
ure is termed
kinematic analysis
, the purpose of
which is to identify potentially unstable wedges,
although it does not provide precise information
on their factor of safety.
Figure 7.1
Typical wedge failure involving sliding on
two persistent joints with line of intersection of joints
daylighting at toe of rock face, and an upper plane
that formed a tension crack (strong, volcanic rock on
Interstate 5, near Grants Pass, Oregon).
The chapter presents design charts that can be
used to find the factor of safety of wedges for
which friction is the only component of the shear
strength, and there are no external forces such as
water pressures or bolting acting on the wedge.
