C4 Rock slope stabilization

Abstract

The assessment of rock slope stability is dependent on whether stability is controlled by: ### C4.1.1 Failure controlled by the intact rock strength For continuous rock masses, where discontinuities play no role in controlling stability, a given slope might be cut vertically for rock with high intact strength (Fig. C4.1) or treated as a soil if weak and cut to a shallow angle. Rock strength is a function of the mineralogy, the interaction (physical/chemical) between the grains and the processes which have affected the rock after its formation (e.g. Anon 1977). Fig. C4.1. Sandstone cut vertically where discontinuities play no role in stability. Hawkins (1998) compares various methods in place (e.g. Deere & Miller 1966, IAEG 1981, ISRM 1981, BSI 1999) for classifying rock by unconfined compressive strength (UCS) in terms of MPa. Field methods for estimating UCS are described, for example, in BSI EN (2003). ### C4.1.2 Failure controlled by discontinuities The most frequently occurring rock slope failures are due to either displacement along dominant persistent discontinuities or multiple closely-spaced discontinuities (as illustrated in Fig. C4.2). The strength of the intact rock is usually ignored when rock slope stability is discontinuity-controlled. Fig. C4.2. Discontinuity-controlled rock slope stability ( a ) persistent; ( b ) multiple, closely-spaced. ### C4.2.1 Discontinuity-controlled stability (assessment method 1) Usually, rock failures occur predominantly along a single discontinuity or a combination of two or three. The geometrical relationship between these discontinuities and the angle and orientation of the hillside or cut slope is usually the most important factor in determining rock slope stability. Discontinuity-controlled rock slope failure mechanisms are described and illustrated in Part A and include planar, wedge and toppling failures (Fig. C4.3 and Hoek & Bray 1981; Wyllie & Mah 2004). Along mountain roads, true wedge failures and topples are usually far less frequent than …