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01-04-1994 11:36W"I FROM SUI'IM1TT OFFICE TO 130342B54E9 P.06 • as the joints were at 50 degrees and the slope was mined z.t 60 deg- rees. Calculations for the safety factor were based upon the power laor and for probabilities of 802, 98~, and 99.9 and the confidence was computed for slope walls of 50- to 70-degrees. Since the curves for such calclations reach an azmuth4at 50 degrees, it follows that for any probability a slope that not exceed the angle of the joints is, in effect, infinitely ; but what is interesting is that if one where to apply these"d.ata to the SpecAgg pit, the wall could be mined to an agle of, say, 70 degrees and have only a prob- ability of failure of 958! Naturally this is not recommended. Since the application of the finite element technique caas used at the Kimbley Pit the literature has been full of mathematical models for computer applications and the most noteworthy are "Back Anal- x yses of Plane and Wedge Rock Slope Failures Using Micro-computer Programs" (Frizzell and Watts 1988) and the so called STABL prog- ram developed at Purdue University which Ss a carbon copy •~f the old method of slices used for many years for soil failures and design. The work of Frizzell and I~'atts cannot be applied t~~ the SpecAgg pit owing to the fact that, in the CSA! study, the slope angle has to exceed the angle of the defects. The advantaq~s of the STADL program is that it can model complex geology, such a:3 lal•er after layer of different sedimentary rocks, etc. It is understood that the nearby Cooler' Pit wall ores modelled after a sophi:atication r of the STABL program using an assumed rock mass strength. Only in the long term or in the analyses of a failure can the true rock strength be approximated. IDENTIFICATION OF FAILURE SURFACES: Failure surfaces can be characterized as surfaces along whi.th rock breaks. In the SpecAgg Pit visual observation of failure surfaces (planar surfaces with few, if any fragments of rook to sugcrest breakage through the rock rather than along the plane) oven the past 12 years have disclosed a set of fractures or joints that have a 60 dip to the east and have an almost north-south strike. Theee have a spacing from 5 to 10 feet apart, and once the rock is blasted the fractures have an almost slickensides appearance. Over the course of years it has been assumed that these fractures have little, or no bonding strength and mining practice has been that these fract- ures are never daylighted, and there have been no failures in as many years. Obviously this slope wall has a safety factor in excess of 1.51 mother prominent fracture set has a strike of about east- west and dips at an angle of about 45 degrees to the south. In mining over the years there was a period when this structure was daylighted resulting in a small failure which, upon back calculation, produce sufficient information to restrict all mining greater than 45 degrees along the north slope wall; again, after modification, there have been no more faiures on that failure surface. Naturally in this type of geologic domain there are sever.sl other surfaces that can not withstand the shock of blasting, and expose planar surfaces that are either widely spaced and/or do not extend over distances of a few feet. Further, the mine staff is constantly alert to th=_ possi- bility of faults that may appear, or changes in the planar fractures, and minimize bacY, breaks from blasts or leaving "noses" in the pit wall. To this end they have been very successful in that there has never been a massive failure and they have only experience minor ravelling or sloughing of fractured rock, usually back brea:cs. (5)