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-6- 1 IIsing the reduced value SRMR profile, the rock mass can be divided into zones of stronger and weaker rock mass strength, as 1 illustrated in Figure 3. Where the rating is 40 or greater it is anticipated that slope stability will be determined by the orientation and strengt.'~ along discontinuities. Where the rating is less than 30 failure may occur through the rock mass at any orientation. For such weak rack zones the rock mass strength is estimated from a rating/strength correlation as is illustrated by the results on Table 4. Until considerably more 1 case histories of such correlations have been performed it is premature to accept the values in Table 4 as typical of all weak rock masses. STABILIZATION MEASURES Where weak rock zones are anticipated the slope angle can ` be reduced through the region of weak rock. Appropriate slope angles can be determined using limit equilibrium analyses which allow far general failure surfaces bounded by joint suri~aces through strong rock regions and arc failures through the weak rock zones. A suitable and useful analytical method is i:he y=o method described by Robertson, 1982. If slope flattening is not done, failure may occur, the subsequent control of which may ' require step-outs or widened catch betas which results in even greater slope flattening. Such unplanned slope flattening can result in substantial are losses from the mineable reser<~es. Where weak rock zones are encountered unexpectedly, it is preferable to install step-outs immediately, if possible prior to initiation of failure. Weak rock zones can be identified in ' regular inspections of the rock exposed in new benches. Sten- outs have been utilized vert successfully at Island Copper Mine, where, after failure, step-outs have been successful wits designed factors of safety as low as 1.15. ~, Unfortunately, slope failure often initiates when the soft rock zones are still unexposed under the tae o= the slope, or as they are exposed but before there is an opportunity to take corrective action. There is no substitute for adequate, gre- mining exploration and identification of the weak rock zones. Such exploration can be integrated with the mineral exploration program, since the required core logging is readily perfo:raed by t.'~e exploration geologist and oriented core is not required. REFERENCES Bieniawski, R., 1974, "Geomechanics Classification of Rack ' Masses and Its Application to Tunnelling", Proc. 3rd Initial. Cong. Rock Mechanics. Hoek, E. and Brown, 1980, Underground Excavations in ,Rock, The Institute of Mining and Metallurgy, London.