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'.~ ' The laboratory derived shear strengths are shown in Table 4. The samples were tested moist and dry since the ultimate ph will be dewatered and therefore the ' material will be in a moist state prior to excavation. This is reflected In the slope design by incorporating moist sample strengths with a water table located below the level of excavation. ;' 6.3.1 Rock Mass Strengths ' The laboratory tests are on small diameter cores (HO) and are not representative of rock mass strengths. An overall rock mass assessment must be done prior to estimating rock mass strength. Bieniawskl (1974) introduced a geomechanics classification of rock masses for application to ' tunnelling. In this classification system, the rock mass is assigned rating points for five factors and the resulting total is termed the Rock Mass Rating or RMR. These values were subsequently ~ correlated by Hoek and Brown (1980) to a rock mass strength as determined from back analysis i' and large scale testing. Robertson (1987) subsequently found that the relationship was only applicable to rock slope strength estimates for stronger materials (RMR > 40) (Appendix I). He proposed a modified rating system (SRMR) in which the parameters critical to weaker materials is more effectively reflected. This SRMR was correlated with shear strength by hack analysis of failed slopes at the Island Capper and Getchell pits as summarized on Table 5. ' TABLE 4 ' LABORATORY DERIVED SHEAR STRENGTH -WEAK MATERIALS 1 Material Friction Angle Cohesion Description (degrees) (psi) Santa Fe 44 0 SRMR=1;~,moist 32 6 SRMR=t;V,dry 21 1 SRMR=t0,dry` 35 1.5 remouldecl,moist Pink Gneiss 42 0 SRMR=tS,moist,dry 33 2 remoulded, moist GreenCtay 21 10 SRMR=10,moist 32 0 SRMR=10,dry ` sample crumbled under applied normal load, retested in remoulded state ,.1 LJ