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<br />MainCressonSlopeEvaluation AdrianBrown <br />7.2 Analysis of the East Watl <br />7.2.7 Hoek and Bray Method <br />Analysis of the slope stability of the highest mine wall at the Main Cresson Mine can be achieved by the <br />most conservative method for translational failure, that is circular failure (Hoek and Bray, 1981). This <br />analysis has been performed using the following parameters and assumptions, derived above: <br />Dry slope <br />Effective stress cohesion (c) = 2.32 MPa (336 psi) <br />Effective stress friction (~) = 44° <br />Maximum slope height (~ = 700 feet <br />Material density (y) =143 lb/cult <br />The dimensionless Hoek and Brown chart parameter is computed as follows (adjusting for unit <br />conversions): <br />c / [y H tan ~] _ (3361b/in2 * 144 in2/ft2) / [ 1431b/ft3 * 700 ft * tan(44°)] = 0.501 <br />The analysis process is illustrated in Figure 21. The chart parameter value is used to draw a radial on the <br />Circular Failure Chart No. 1 (for dry slopes) from Hoek and Bray (1971), p. 234. The points where this <br />• radial intercept the slope angle curves produce values of the variable tan ~ / F (where F =Factor of <br />Safety). The Factor of Safety can be found from: <br />F = tan ~ / [tan ~ / F (from chart)] <br />The result of the analysis are indicated on the figure, and the computation of the factor of safety is <br />performed as follows: <br />Slope angle tan ~ / F ~ tan ~ Factor of safety <br />80° 0.38 44° 0.966 2.5 <br />70° 0.32 44° 0.966 3.0 <br />60° 0.27 44° 0.966 3.6 <br />50° 0.24 44° 0.966 4.0 <br />C <br />Based on these values, the factor of safety against overall collapse of the slope by a predominantly <br />circular failure would appear to be high, provided that the parameters which have been determined for the <br />upper half of the slope prove to be applicable to the remainder of the slope. <br />7385D.980672 49 <br />