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Western Associated Coal <br />t•fay 26 , 1 982 <br />Page 2 <br />Interbedded sandstones, siltstor.e and shale <br />(assumed shear strength parameters) <br />c = 4000 psf <br />.8" = 33° <br />RESULTS OF ANALYSIS <br />Stability conditions were analyzed utilizing a computer-assisted <br />limit equilibrium method of slices. This particular program <br />utilizes Spencer's method for analysis ~ahich is capable of <br />analyzing stability conditions not only in circular modes of <br />failure, but also for randomly specified shear surfaces. Types <br />and conditions of failure modes utilized in this analysis are <br />presented on the enclosed figure and were as follows. <br />Case I - Access road excavated entirely into colluvial soil <br />(bedrock surface parallel to slope surface and intersecting <br />inside edge of access road) which results in a total thickness of <br />colluvium perpendicular to the slope of approximately ?0 feet, <br />and a piezometric level of approximately 4 feet above the top of <br />rock surface. <br />Case II - Partially colluvial excavation ar.d partially rock <br />excavation (total thickness of colluvium equal to 12 feet) and a <br />piezometric level 4 feet above top of rock surface. <br />Case III - Complete rock excvation with total colluvial thickness <br />of approximately 5 feet (upper slope only) and a piezometric <br />level from 0 to 4 feet above the top of rock surface. <br />Each case was evaluated utilizing both residual strength ~nd peak <br />shear strength behavior. <br />The overall results of the analysis may be described as follows. <br />The residual strength conditions in the colluvium modeled the <br />worst possible stability condiions and the worst possible shear <br />strength performance in the colluvium. The peak strength <br />conditions modeled the most optimistic stability conditions and <br />most optimistic shear strength performance in the colluvium. <br />Three types of failure modes were evaluated. The first was a <br />deep circular failure mode which is expected to occur for <br />excavated sections founded entirely in rock. The secondwas a <br />shallow circular failure mode which is expected to be the <br />critical failure surface for sections founded entirely in <br />colluvium. The third was a shallow wedge type failure mode which <br />models intermediate conditions, i.e., conditions with partially <br />colluvium and partially rock excavation. <br />