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31-04-1994 11:36P~M FP.OM SUhih11TT OFFICE • TO 13034285469 P.05 • Perhaps the most useful techniques that have been an extension of the Bureau-Kennecott report has been the Canmet report 76-].4 that has a report by McMahon "Back Calculation of Slope Walls", and by Shuk "Estimates of the Upper Bounds to Rock Slopes" and by Stark "Wall Stability in the South Roberts .j.t - An Example of tY.e use of Previous Slopes". It is coincides "1 that a part of the idcMahon study was made in the walls of the rby Clear Creek Canyon, the same geologic domain as the Spec- Pit. His important contribution:. was the discovery that the hieght to length (H/L) ratio for low stress slopes is straight line on a log-loy plot or the H/L ratio had the exponential relationship H=kLb where; !c constant b = slope of the log-log line. This relationship suggests that the empirical function suggested by Jaeger may explain the straight line relationship and that the average coefficient of friction u = tan = k6 n-1 which also explains why a slope of near infinite hieght wou.Ld fail at values of lower friction. Based upon these empirical findings (with no help from the :Lab- oratory except to provide mathematical modals) we find that mass- ive igneous rocks (as SpecAgg) with few - or sngular - joi:its, the values of a and b from [•1c1•Sahons report are 139 and .28, resp- ectively. So using the values that we know for the SpecAgg Exit namely, low water pressure and p = 165#f t2, the corrected peirameters are P5 = 13 and the compressive strength is 16500 psi, and oLir meas- ured strength is in excess of 20000 psi, so the calculated 1•educt- ion factor '0.18; what this translates into is that, by bath: cal- culation, the SpecAgg pit is 1.63 times safer than the same slope in layered sandstone and 8.G8 times safer than the same slo~~e in jointed or altered crystalline rocks! Since eve can not quantitatively compute a safety factor cvithout very broad assumptions, it is suggested in the Canmet reports that the safe slopes Observed (such as SpedAgg) we can accept that the upper bounds for a slope hieght of 800 feet would be a slope length of only 250 feet! Extrapolating this Curve to failure re- quires a H/L of 1000:250 feet or an increase in H, or the weight by a factor of 1:1.2; since the SpecAgg ratio is 800:400 it follows that the safety factor is well above 1.21 Another example of the calculation of the safety factor is by stat- isical.analyses, and approach popular with several irvestigators. A study by the Colorado School of Mines is selected as an example. in this study they measured the friction of the joints and f•~und the cohesion at 23 psi and the friction angle of 31 degrees for samples from the Castledome Pit. In this study the slope caall was steeper than the joints anfl as such the joints were "c:aylighted" (4)