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• Another type of failure potentially affecting the highwall would <br />result if a concentrated flow of water was discharged at the top <br />of the highwall. This condition would create a debris flow type <br />i of failure locally in that area. This type of failure could be <br />anticipated wherever a concentrated flow of water is dicharged <br />onto the existing highwall. However, the grading plan proposed <br />for final reclamation will significantly reduce the potential for <br />this type of occurrence. <br />An appraisal of the structural data has indicated that for the <br />~ majority of the highwall where competent rock is exposed, <br />stability will not be controlled by intact rock strength, but <br />~ rather by the presence of structural discontinuities in the rock. <br />Potential failure modes with respect to competent rock in the <br />existing highwall consists principally of wedge failures and <br />toppling failures. More specific information on the character .of <br />wedge failure within the highwall are found in the <br />Stability/Sensitivity and Conclusion sections of this report. <br />The toppling failure mechanism is not produced solely by the <br />nature of the structural discontinuities in the highwall, but by <br />the ravelling of exposed shale from beneath competent sandstone <br />beds releasing large blocks of sandstone bounded by joints. <br />The structural data has also indicated a potential for planar <br />type failures in slopes exceeding approximately 80 degrees. <br />However, the steepest existing slope observed in the highwall <br />7 area is on the order of 80 degrees, with the majority of the <br />slopes being considerably flatter. Therefore, we feel there is <br />1 9 <br />