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Mardlny Lawson Assoelates <br />August 8, 1996 <br />32640.19 <br />Ms. Christine Johnston <br />Mountain Coal Company <br />West Elk Mine <br />P.O. Box 591 <br />Somerset, Colorado 81434 <br />iii iiiiiiiiiiiu iii <br />999 <br />RE: SLOPE STABILITY ANALYSIS OF PIEZOMETRIC SURFACE -LOWER REFUSE PILE <br />WEST ELK MINE, SOMERSET, COLORADO <br />Dear Ms. Christine Johnston: <br />Harding Lawson Associates (HLA) is submitting the additional slope stability analysis for the Lower Refuse Pile <br />(LRP) at the West Elk Mine (WEM). The purpose of the analysis is to address the technical concerns of the <br />Colorado Division of Minerals and Geology (DMG) June 20th, 1996, letter outlining the request for incorporation <br />of a perched water table into the LRP slope stability model. HLA was requested to assume that a perched water <br />table exists in the LRP. This assumption is based on the observations of saturated soils along the east bench of <br />the LRP. It should be noted that there has not been any slope failures in this location as to date. The saturated <br />soils were observed by DMG personnel during a June 12, 1996, inspection of the LRP. <br />The slope stability analysis of the LRP incorporating a perched water table was not possible using the current <br />PCSTABLS computer program. The PCSTABLS program has certain limitations for incorporating a~erched <br />water table. To eliminate this limitation, HLA is using a slope stability program called GEO-SLOPE The <br />GEO-SLOPErt't program is a more technically advanced slope stability program than PCSTABLS. To insure <br />quality controUcontinuiry of the previous LRP factors of safety, information from original slope stability models <br />were used in the new program. A complete overview of the factors of safety are found in Table One. <br />INTRODUCTION: <br />The LRP perched water table slope stability analyses was conducted on the same profiles identified in the original <br />July, 1981, Exhibit 51 study. The three profiles encompass the critical sections of the LRP. A critical section <br />review analysis of the three profiles is discussed in the follow paragraphs. The LRP perched water table slope <br />stability analysis utilized the computer program GEO-SLOPE to determine the factors of safety against failure. <br />The factor of safety is the ratio of the forces resisting the slope movement to the forces which cause the slope to <br />fail. The resisting forces are determined by the shear strength of the imported coal refuse to resist failure along a <br />specific surface. The forces which tend to cause failure include, but are not limited to: (a) the angle of the slope; <br />(b) the location of existing roads, drainage ditches, and sediment control ponds in relation to the slope; (c) pore <br />pressure or a perched water table acting on the failure plane; and (d) the characteristics of the coal refuse, native <br />soil and bedrock found in the slope. A factor of safety of I.0 implies that the slope is marginally stable for the <br />cross-section profiles, native clay bedrock material strengths, groundwater levels, and the slope conditions. A <br />factor of safety of 1.5, and I.0 for an earthquake loading factor is the minimum level of safety requ'ved by the <br />Colorado Division of Minerals and Geology (DMG) for slopes utilizing coal refuse. <br />CRITICAL SECTIONS: <br />Three profiles were modeled for the LRP perched water table slope stability analysis (see Figure One). The three <br />profiles are located in the same critical locations as identified in the original Exhibit 51. The physical conditions <br />of each profile is as follows: <br />• Profile) One bisects the north'Cwae,s,tt~ 'ct~omer of the Lc1RP. The critical pazameter in this profile is a sediment <br />Engine~R~aAtl Pond IOC9fed -r55SO0~'~0'w95F ~~~I ~ Cd{2~~,, Utah 84tOt Telephone: (901) 363J499 Facsimile: (901) 3634079 <br />Ennronmental Services <br />