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• Considering the fact that there are no plans for secondary <br />mining, and that the refuse pile and colluvium are uncon- <br />solidated materials, it is improbable that a limited sub- <br />sidence can create a continuous crack extending from top <br />of the refuse pile to F-seam floor. However, to simplify <br />the wedge analysis performed, it was conservatively as- <br />sumed that a continuous crack extends from the seam to the <br />surface and that the shearing resistance is derived from <br />the F-seam floor material only. <br />i.- <br />Selection of Strength Parameters <br />As discussed previously, strength parameters of C=1350 psf <br />and ~ = 9° were obtained for the surface clay obtained from <br />boring 30 which is located in the refuse pile area. Addi- <br />tionally, studies (4,5) indicate triaxial compression <br /> <br />strength and unconfined compression strength values of 30 to <br />2,300 psi for underclay materials. Based on these test re- <br />~, • sults, the unconfined strength of 60 psi (8640 psf) is con- <br />sidered conservative, which results in a cohesion value of <br />4,320 psf. A review of lithologic logs indicated that the <br />F-seam floor materials consist of claystone, shale, siltstone <br />and sandstone. It is, therefore, unlikely that a continuous <br />clay layer will be encountered. Therefore, strength para- <br />meters of the F-seam floor materials will be much higher than <br />the above mentioned parameters for the surface clay and the <br />underclay materials. However, for the simplified analysis, <br />the following conservative strength parameters were selected. <br />• <br />f <br />~_ <br />22 <br />