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I 1 _3_ <br />iii) Rotational F--_edom ~Fioure I~iii)) <br />' This Porn of failure may be illustrated by the failure <br />i surface that would fora through a gravel pile. The individual <br />' rock fragments, which would interlock if they were prevented <br />from rotating, have rotational freedom resulting from the loose <br />packing of the gravel particles. With a high rationality <br />freedom (such as exhibited by even sized marbles in a pile) very <br />low shear strengths result. Rotational freedom increases if the <br />intact rock fragments are equidimensional, rounded and within a <br />matrix oP either voids or a very low shear strength (soft) <br />' material. An example of a rock mass with very high rotational <br />freedom would be spherically weathered dolerite boulders in a <br />matrix of soft (in-situ weather dolerite) clay. Again i~= is <br />difficult to cut, prepare and test laboratory sized specimens <br />' for strength testing. <br />Mast often, weak rock masses occur as a result of a <br />' combination of each of (i) to (iii) above. Typically a rock <br />material may have weathered to produce a soft rock (R2) cor very <br />soft rack (R1) rack material between closely spaced fractures <br />' along which more extensive weathering has occurred to prceduce <br />zones of S3 to S5 strength material. The failure surface is a <br />complex combination of failure through soft intact rack, along <br />weak joints and through very soft (soil like) weathered zones <br />' which squeeze and deform to allow harder rock blocks to rotate. <br />Figure 2 is a typical example. <br />' C7.~lSSIFICATION OF WF~K ROC{ IdASScS <br />Bieniawski, 1974, introduced a geomechanics classification <br />of rock masses for application to tunnelling. This <br />t classification system, shown in Table 2, has been widely adopted <br />as the CSIR rock mass classification system. In this <br />classification system the rack mass is assigned rating points <br />for five factors and the resulting total is termed the Rock Mass <br />'~ Rating or RMR. Hoek and Brown, 1980, correlated the RMR •~alues <br />with rock mass strength as determined from back analysis •~f <br />stress condition surrounding underground failed and unfai:Led <br />openings. Strength estimation using the Hoek and Brown <br />correlation is referred to as the MS system. <br />' Robertson, Olsen and Pierce, 1987, applied the RMR and MS <br />rating and strength estimation systems to the rock mass in the <br />pit walls of the Island Copper mine in British Columbia. They <br />' performed back analyses of pit slope failures and found tY..at for <br />weak rack masses, this method provides a very poor and <br />inaccurate strength estimate. They proposed a modified rating <br />system, which has evolved into the SRIC Geomechanics <br />Classification of Rock Masses as described in Table 3. Since <br />1 this rating system was developed specifically for rock mass <br />strength estimation for rock slope stability analyses, the <br />' resulting rating is referred to as the Slope Rock Mass Rating or <br />SRMR. <br />7 <br />1 <br />