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1 <br />' Marding Lawton As~oelatss <br />November 18, 1996 ~,- <br />Ms. Christine Johnston <br />' Mountain Coal Company <br />West Elk Mine <br />P.O. Box 591 <br />Somerset, Colorado 81434 <br />RE: SLOPE STABILITY ANALYSIS, EASTERN SLOPE (PROFILE 3) <br />LOWER REFUSE PILE, WEST ELK MINE <br />Dear Ms. Johnston: <br />The analysis of Profile 3, on the eastern slope of the Lower Refuse Pile (LRP), as shown in Figure 1.0, <br />has been carefully evaluated by Harding Lawson Associates (HLA) in response to the letter written by <br />Mr. David Berry of the DMG to Mr. Norm Every dated November 12, 1996. In particular, the perched <br />water table assumed from the previous (HLA Letter Report, August 8, 1996) investigation was further <br />defined using current hydrology modeling techniques. Results of HLA's current study indicate that for <br />' expected conditions, the eastern slope has a minimum factor of safety (FOS) exceeding 1.5 as required <br />by the Regulations of the CMLRB for Coal Mining regarding stability of a constructed coal refuse fill. <br />The findings of HLA's study are presented herein and corresponding data are attached. <br />INTRODUCTION <br />HLA performed a slope stability analysis for the revised lower refuse pile (LRP) configuration in July, <br />1996. Triaxial shear tests were performed on crushed samples of coal refuse currently being placed on <br />the LRP and the results were summarized in the August 8, 1996 HLA Letter Report. Refuse currently <br />' being placed on the LRP consist of 7.6% fines with a cohesion of 1100 psf and an internal friction <br />angle of 35 degrees. Worst case conditions result when the refuse consists of 10% fines. At 10% <br />fines, cohesion is 0 psf and the angle of internal friction is 39 degrees. <br />' During the July, 1996 analysis, a perched ] 0-feet thick water table was assumed at the elevation where <br />DMG personnel noted a seep along the eastern base of the LRP. The 10-feet thickness was an assumed <br />worst case situation. For this current study, HLA has further studied a perched water table through <br />' investigative modeling to determine a maximum height estimate. Modeling for the perched water table <br />was achieved using local climatic data and hydraulic conductivity information for the LRP refuse <br />material. <br />' LRP PERCHED WATER TABLE -HELP MODEL ESTIMATE <br />' The perched water table is a function of the amount of precipitation, runoff, evapotranspiration, and <br />infiltration into the pile, based upon parameters defining climate, vegetative cover, soil cap, surface grade, <br />and subsurface permeability. An estimate of the height of the perched water table was obtained using the <br />U.S. Army Corps of Engineers (COE) `'Hydrologic Evaluation of Landfill Performance (HELP) Model, <br />"Version 3, September, 1994. <br /> <br />Eng~neer~ng and 165 South 300 West, Swte 103. Salt W ke City. Utah 84101 Telephone: (601) 3634455 Facsimile' (601) 3634476 <br />Environmental Services <br />1 <br />