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SLOD°_ CONT4oL IN INT°_NSEL'i wEATYER~D aTID ~'RAC^'7R°D RoC~ was5£S <br />.lndre~d M. Robertson, President <br />Steffen, Robertson and I{irsten <br />Consulting Geotec.'uiical & Mining Eng. <br />Vancouver, Canada. <br />' ABSTRACT <br />Techniques for the estimation of the strength of ro~r_k <br />' masses has developed considerably over the past 20 years. Soft <br />or hard rack material strengths can be determined from <br />laboratory or field testing. The effects of discrete <br />' discontinuities and jointing patterns on the strengt.'~ of rock <br />masses can now be estimated using well established procedures. <br />Difficulty is still encountered in rock mass strengt.'~ <br />determination in those instances where the rock material <br />' strength is low and variable, such as for intensely weat:;er <br />rock, or where the fracturing is very intense. <br />Where zones of low strengt5 rock mass are encountered in <br />pit walls, failures can develop which differ kinematically from <br />t'~ose anticipated from an analysis of jointing patterns. The <br />' resulting failure surface may be a combination of planar joint <br />bound sur=ace in the strong rock regions and a curved arc <br />t'~rough the weak rock zone. Weak rack failures have been <br />observed in many pit slopes but par=icular'_y ir. rock masses <br />' which are intensely and variably weat'~ered together with intense <br />fracturing. Such intense variability in weathering and <br />fracturing is often observed where the ore genesis is associated <br />' with hydrothernal permeation along fracture zones with <br />associated alteration and mineralization. Examples include some <br />of the epithermal gold deposits cur-.ent'_y being developed. <br />' The design of safe slopes in such variable rock masses <br />requires exploration, logging and testing tec.'uiiques which <br />enable weak rock zones to be identified and their strengths to <br />' be accurately determined. In this paper a system of consistent <br />rock mass classification is described which enables weak ;rock <br />mass material to be described and classified from drill core. <br />' Correlation with back analysis determined strengths allow:a <br />strength estimates to be assigned to each rock mass <br />classification. A metY:od for the determination and defin_,tion <br />of the weak rock zones is described. After the zones of weak <br />' rock, and their associated strengths, have been determinec(, <br />slopes may be analysed to determine safe slope angles. The <br />potential failure surfaces are combinations of joint conts~olled <br />' planar surfaces, determined using conventional joint analyses <br />techniques, and curved surfaces through the weak .rock zones. <br />Limit equilibrium analyses methods, which permit the evaluation <br />of non-circular curved surfaces, are used for the stability <br />' analyses. <br /> <br /> <br />