Slope instability 289
Figure 17.3 Photographs illustrating the four basic mechanisms of rock slope
instability shown in Fig. 17.2: (a) circular slip; (b) plane sliding; (c) wedge sliding;
and (d) toppling.
- 7 Curvilinear dip
The term ’curvilinear slip’ is used to describe the group of instabilities
shown in Fig. 17.4. This term should be regarded as synonymous with the
more usual one ‘circular slip’, which is normally understood to also include
non-circular slips. Only in exceptional circumstances will instabilities
occurring in a continuum have truly circular slip surfaces; they will usually
be curvilinear. Hence, we will analyse general curvilinear slips, and present
truly circular forms as a special case.
The text in the following sub-section is based on lecture notes produced
by Dr J.W. Bray (formerly of Imperial College and co-author with
Professor E. Hoek of the seminal book Rock Slope Engineering), to whom we
should like to express our gratitude here.
In Fig. 17.4 there are five diagrams of geological circumstances under
which curvilinear slips may develop. Experience indicates that with these
materials, the slip surface is curved and usually terminates at a tension
crack at the upper ground surface. The shape and location of the slip
surface depends on the strength characteristics of the ground mass, which
in turn depend on the structure-as indicated in Fig. 17.4.
In analysing the potential for slip, one has to consider (a) the location of
the slip surface and (b) determination of the factor of safety for a given slip
