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1 t 1 STABILITY ANALYSIS: GENERAL: The present stability analyses were performed using the UTEXAS2, analytical modelling program developed by Dr. Stephen Wright at the University of Texas at Austin. The Spencer method cf analysis was used. The Spencer method provides a determinate solution by assuming that all side forces on slices act at the same angle and requires both force and moment equilibrium. Trial shear surfaces were located using the automatic circular search mode option of the program. In this mode the program searches the model surface considering several different shear surfaces until the shear surface returning the lowest factor of safety is fou:d. Because the pile consists of a strongly layered system which affects strengths as well as pore pressures, the possibility exists that the critical shear surface would not be circular but would have linear elements following zones of higher pore pressure. Therefore, the pile was analyzed using the program option for non- circular slip surfaces. Given the boundary and piezometric conditions which exist for the present analyses, the automatic non- circular search mode of the program cannot be depended on to locate the most critical shear surface. Therefore, numerous individual trials were necessary to assure that the minimum safety factor was found. Stability calculations were performed along two sections, as shown on Plate 11 in the Appendix. Section selections were based on the results of the Phase I stability analysis data. The State's Mined Land Regulations, supplied by Powderhorn Coal, Co., require that stability analyses for the subject pile satisfy a safety factor of 1.5 under static loading conditions. Igo requirements for seismic loading safety factors are stated and such loading conditions are not considered in the present analyses. MATERIAL STRENGTHS: As in the Phase I analysis, the disposal area pile cans modelled using three different material zones. Those being, refuse material (zone 1), placed on foundation alluvium (zone '_), underlain by bedrock (zone 3). A fourth zone, modelled as water, represents the sedimentation pond. The Phase I analysis raised questions regarding the strength values of the alluvium foundation material applied to the original stability design. Since that time (of the Phase I analysis) additional strength test data for the alluvial material has been supplied by Powderhorn Coal, Co. This test data comes from triaxial tests performed by Rocky Mountain Geotechnical, Inc on the alluvium material below disposal pile No. 2 which is located directly west, across Coal Canyon from disposal area No. 1. Given the close proximity of CRDA No. 2 to CRDA No. 1 and the more conservative results of triaxial tests from the CRDA No. 2 foundation, the lowest of these material strength values were applied to the present Phase II stability analysis. Strength 25 1