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' 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 of 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. Critical 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 found. Stability calculations were performed along three sections, ' considered critical, as shown on Plate 1 in the Appendix. Critical section selections were based on the results of the historical piezometer data evaluation. 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. No requirements for seismic loading safety factors are stated and such loading conditions are not considered t in the present analyses. MODEL AND MATERIAL STRENGTHS: ' The disposal area pile was modelled using three different material zones. Those being, refuse materials (zone 1), placed on ' foundation alluvium (zone 2), underlain by bedrock (zone 3). A fourth zone, modelled as water, represents the sedimentation pond. The model considers the pile complete to elevation 5025, according to topography provided by Powderhorn Coal, Co. The pile/alluvium ' contact was also modelled from provided topography. As noted previously, the alluvium/bedrock contact was modelled from drill log information contained in the "Foundation Investigation" report, ' with the depth of alluvium varying between 10 and 35-feet. The various piezometric surfaces modelled, are addressed in subsequent sections of this report. Material testing was not included in the scope of the present stability analysis. Therefore, the present analysis relies on information regarding material strengths cited in the "Stability ' Evaluation" design report prepared by Rocky Mountain Geotechnical. From the original "Stability Evaluation" design report, zone 1 refuse material is comprised of "black carbonaceous gravel and ' cobble material, with a considerable amount of sand and silt sized particles". The results of a remolded, triaxial test cited in the evaluation report indicates an effective friction angle of 32.12 degrees and an effective cohesion of 204.5 psf for refuse material. ' There is no reason to question these values other than to note that the triaxial sample was remolded to a "somewhat lower" dry density than was targeted. This in addition to the higher average in-place i e 1