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• <br />-14- <br />for the full height of the highwall. This data indicates that rather major <br />failure could occur for cuts steeper than 75 degrees. A factor of safety <br />of o.2 was calculated for cuts steeper than 75 degrees assuming a friction <br />angle of 3 degrees between blocks. <br />Stability of a 75 degree cut was analyzed for mass stability using two <br />methods which consider failure mechanisms different from Marklin. The <br />first method assumes a shear or failure surface which passes through indivdual <br />blocks forms by bedding and joint planes. This failure mechanism was analyzed <br />using Patton's equation and described in "Rock Slope Engineering ". The cal- <br />culated factor of safety is dependent upon the inclined angle of the sliding <br />surface as shown in Appendix B (Fig. B -5), as well as the angle of internal <br />friction of the rock. For the purpose of analysis, wE'. assumed the entire high- <br />wall had similar friction angles. A friction angle of 27 degrees was used since <br />this is the lower bound friction angle indicated for the shales based upon the <br />results of testing performed by Science Applications Inc. Published data <br />suggests no cohesion for the types of rocks exposed in the highwall, therefore, <br />none was used in our analysis. A summary of our analysis is shown on Fig. 8 <br />and indicates lower bound factor of safety of 3.4 occurs at an inclination <br />angle of about 30 degrees. <br />The second method used to evaluate mass stability is based upon the <br />Modified Ladanyi and Archambault's eqaution. This procedure assumes a similar <br />failure mechanism as Patton, but considers the height, of the slope, weathering <br />of block surfaces and the unconfined compressive strength of the rock. We <br />used an angle of 27 degrees and considered an average! unconfined strength of <br />about 2800 psi. The analysis method reduces the unconfined strength by one- <br />quarter to account for weathering. The results of this analysis indicates a <br />