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1 -z- ' Slopes containing weak rock zones may be stabilized by slope flattening or incorporating step-outs. The forces is used when designing the slopes, and the latter when unexpec~ed weak rock zones are encountered during mining. ' INTRODUCTION The rock mass classification methods described in this ~. paper evolved on projects undertaken by the consulting fire Steffen, Robertson and Kirsten (SRK). Back analysis of failed slopes involving rock mass materials which have been described ' by the SRK classification has enabled rock mass strengths to be ~ assigned to the various class intervals. Much of the initial work in the development oP this technique was done by personnel ' Prom the Island Copper Mine, BHP-Utah Mines, Port Hardy, British Columbia, to whom much of the credit goes for turning it into an effective technique. ' FACTORS CONTROLLING THE STRENGTH OF WEAR ROCF: MASSES Weak rock masses may occur as a result of a number of ' independent factors. These are illustrated in Figure 1. it Weak rock material ~Fiaure 1(i)1 ' ' Where the sole reason far a weak rock mass strength is the low strength of the rock material it is more properly classified as having a sail like strength. An anaronriate classification ~ ' for rock and soil, based on simple tests, from which an estimate can be made of the uniaxial compressive strength is given in Table 1 (Jennings and Robertson, 1969). Extremely weak "rocks", or more properly materials with a soil strength but a rock '. appearance, can be tested in the laboratory to deteraine the shear strength. Such material may be referred to as either ~, soils or rocks but use of the strength qualifies S1 to S5, as indicated on Table 1, serves to indicate that their strength falls into the soil classification. t iil Intense Jointing (Figure 1(ii)) Where the jointing is sufficiently intense and has a ' sufficient range of orientations, a stenned failure surface can develop througri the mass, at any orientation, with a resu:.taut low shear strength. Thus while the jointing may be non- isotropic, the shear strength may be considerably less so. The ' spacing between joints may be sufficiently large that it ~.s impractical to select a laboratory scale specimen which is representative. The difficulty of cutting and preparing a~ specimen without disturbing the joints and effecting the shear strength of the specimen is an additional impediment to such testing. 1 1 1