Engineering Rock Mechanics

288 Surkrce excavation instabiliiy mechanisms

In Fig. 17.1(a), the failure surface has been created through the rock mass
(behaving as a continuum), whereas, in Fig. 17.1(b), the failure surface is
dictated more directly by the presence of specific pre-existing disconti-
nuities. It is also possible to have intermediate cases where the failure
occurs partly along the discontinuities and partly through bridges of intact
rock, but we are concentrating here on the essential differences between
continuous and discontinuous behaviour. Although most soil slope insta-
bility is of the continuous nature, the majority of rock slope instability is
caused by individual discontinuities. This is because the strength of the
intact rock can be high, with the result that the pre-existing discontinuities
are the weakest link.
As an amplification of the sketches in Fig. 17.1, the four diagrams in
Fig. 17.2 and associated photographs in Fig. 17.3 illustrate the mechanisms
which are traditionally regarded as the four basic instability mechanisms
for rock slopes. The geometry of the slip in Fig. 17.2(a) is a function of the
geometry of the slope and the strength of the material forming the slope,
but the slope instabilities in Figs 17.2(b)-(d) show how the boundaries of
the instability are governed by the discontinuities, giving essentially planar
faces to the sliding and toppling blocks. Each of these mechanisms is
discussed separately in Sections 17.1.1-17.1.4.

Figure 17.2 The four basic mechanisms of rock slope instability: (a) circular

slip; (b) plane sliding; (c) wedge sliding; and (d) toppling ((b), (c) and (d) from

Matheson, 1983).