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December 1986 14 863-2060 <br />r~ <br />L <br />Our experience with spoil structures constructed on a weathered <br />bedrock foundation with pronounced bedding indicates that classi- <br />cal circular failure surfaces are unlikely to develop through the <br />foundation unless the foundation has a very low strength. We be- <br />lieve that the strength of the foundation material will generally <br />preclude this type of failure in Area 9/10. Instead, the inclin- <br />ation of the spoil structure foundation and dip of the sedimen- <br />tary foundation strata will result in a potential failure surface <br />along the spoil/foundation contact. The presence of water at the <br />base of the spoil structure will tend to soften and weaken the <br />foundation material, while any pore pressures resulting from <br />water will reduce the shear strength of the foundation. <br />We believe that the failure mode most likely to develop consists <br />of a failure plane developing along the spoil/foundation contact <br />and extending up through the spoil to the surface of the struc- <br />• ture. In order to realistically model this type of failure mode, <br />we have used the non-circular failure surface capabilities of <br />CSLOPE. A variety of potential failure surfaces have been ana- <br />lyzed to insure that critical (lowest) safety factors have been <br />calculated. Circular failure surfaces, using the simplified <br />Bishop method, have also been evaluated to test the hypothesis <br />that the non-circular failure plane is more critical than a <br />circular failure surface. <br />5.2 Assumptions <br />Two assumptions were made in the Area 9/10 slope-stability <br />analyses which are conservative. It was assumed that the Wadge <br />seam and associated Mesaverde overburden will be entirely removed <br />in Area 51 downslope from Area 9/10. It was also conservatively <br />assumed that ground-water conditions in Area 9/10 will reflect <br />the maximum levels measured during the spring of 1986. <br /> <br />Golder Associates <br />