TABLE OF CONTENTS In this chapter, we discuss instability mechanisms in relation to slopes and foundations. It is possible, in an underground excavation, for the excavation walls and floor to fail in a similar way to surface slopes and foundations, but these topics are covered in the next chapter.
In the history of rock mechanics and rock engineering, more attention has been paid to slope instability considerations than any other topic, and this topic remains one of the most important today. In Fig. 17.1, there are two slope failure mechanisms. Figure 17.1(a) illustrates slope instability when the rock is behaving as an equivalent continuum; Figure 17.1(b) illustrates slope instability when the rock is behaving as a discontinuum. One of our first considerations must be to identify the basic mechanisms of slope instability. The sketches in Fig. 17.1 also highlight the CHILE versus DIANE nature of the rock which was discussed in Chapter 10.
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 discontinuities. 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 instability is of the continuous nature, the majority...
