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11 <br />' Gated in most unfavorable topographic and geotechnical con- <br /> ',, ditions and it should be eliminated if the number of mine <br /> ;~ entries is reduced. Entries 2, 3, and 4 are located in a more <br />' favorable to o rah assumin that the <br />p g p y g portal cuts remain at <br /> ~'i approximately the same locations as indicated. Future changes <br /> ~~~ in the bench elevation and the cut locations could of course <br /> change these conclusions. <br />1 ~ , <br />~ <br /> ; Entries 1, 3, and 5 were selected for slope stability analy- <br />' ;' ses; the method of analyses is decribed in the Appendix A. Plate <br /> I. No. 7 shows a back analysis (a back analysis is the stability <br /> j; analysis which assumes the natural slope without any excavation; <br /> ;! it either assumes the safety factor equal to one or it calcu- <br /> lates the safety factor for known strength properties) of the <br />' ~~ slope at mine entry 5 prior to any excavation. The back analysis <br /> ;, shows that if we assume the soil strength parameters as they were <br />' <br />1 <br />obtained by the soil testing and "saturated" conditions with <br />,' <br /> ground water coinciding with the topographical surface, the <br /> <br />' minimum factor of safety is on the order of 1.0 (0.985 exactly). <br /> This confirms the earlier discussed condition of marginally <br />' stable slopes under saturated conditions. It also means that no <br />d <br />i <br />h <br />ld <br />d <br />h <br /> unsupporte <br />excavat <br />ons s <br />ou <br />be performe <br />on suc <br />a steep <br /> slope. <br />1 ! <br /> Results of further analyses of the same portal are shown on <br />' Plate 8. A first analysis was run assuming that a cut was ex- <br /> cavated (saturated conditions, Case 5-2). As it could be ex- <br /> ~ pected,the minimum factor of safety is slightly lower than in <br /> the back analysis (0.932); it is evident again that the exca- <br /> ' vation is not feasible. Two further cases (5-3, 5-4) assume the <br />d <br />l <br />d <br />10 <br />d 2 <br />l <br />h <br />f <br /> owere <br />groun <br />water <br />to <br />an <br />ow t <br />e surface. Mini- <br />0 <br />eet be <br /> mum factors of safety increase to F.S. ~ 1.321 and 1.535. For <br /> fully drained conditions, the factor of safety increases to <br /> 1.758. b7e feel that for the permanent solution, safety factors <br /> i higher than 1.5 should be obtained. <br />uo+no.o caasur.n Mc <br />