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^III IIIIIIIIIIIII III 999 Harding lawaon llsaoelatsa November 30, 1995 HLA #32640.14 Mrs. Kathy G. Welt, Environmental Supervisor Mountain Coal Company West Elk Mine P.O. Box 59l Somerset, Colorado 81434 RE: FACTOR OF SAFETY SCTMMATION FOR THE PARKING LOT EXTENSION AT THE WEST ELK MINE Dear ~~Y The following letter summarizes the factors of safety for the parking lot extension located to the south of the present parking tot at a general elevation of 6320 feet above mean sea level. The parking lot extension and adjacent cut slope is 50 feet wide by 360 feet long, and extwds to the south of the present parking lot. The total surface area of the parking lot ext®sion including the cut slope area is approximately 17,500 square feet. The following pazagraphs describe the slope stability analysis required for permit submittal. A slope stability analysis was performed on two profiles extending up the slope on either end of the proposed parking lot extension. The first set of calculations determine the fattors of safety prior to exravation of the parking lot extension. The second set of calculations determine the factors of safety after the parking lot has been completed. The slope stability analysis utilized the PCSTABLS program to determine the factors of safety against failure. The factor of safety is the ratio of the forces resisting the slope movement to the forces which cause the slope to fail. The resisting forces are determined by the shear strrngth of the native soils to resist failure along a specific surface. The forces which tend to cause failure include; but aze not limited to: (a) the angle of the slope; (b) the location of existing access roads and drainage ditches in relation to the slope; (c) pore pressure or water table atting on the failure plane; and (d) the charatteristics of the native soil and bedrock found is the slope. A factor of safety of l.0 implies that the slope is mazginally stable for the cross-section profiles, native clay/topsoil material strengths, groundwater, and the slope conditions. As required by the State of Colorado Mined Land Reclamation Board, 1991, a minimum factor safety of l.3 will be requved for the cuVfill slopes of the parking lot extension. It was concluded that two unrelated slope failures could occur at this site based on observations in the field, borehole information, and previous experience in this area. Looking to the south, or uphill from the present parking lot, the first failure would occur in the east half of the slope in the area designated as Qlsl (refs to figure 1). The second soil failure would ocnrr in the western half of the slope in a second area designated as Qlsl. Qlsl is defined as a surficial slump or weep involving the over burden soils and thin seams of bedrock. It is characterized by hummocky or boulder strewn topography. Qlsl soil compositions ranges from clays to gavel derived as slope wash and residual soils. A description of the criteria uses in the P~~" 'BLS prnera~ *o evaluate the facors of safety for the proposed mar .! storage •a ~s as follows: O' . hoot ~eometrv: The resent slope profiles w Ke ~veloped rsom a 1992 aerial -opogaphy map supplied by Mountain Coal Company. Groundwater: Groundwa .- was assumed at approximately IS feet below surface, and has been previoLLSly identified as two springs in the slope above the parking lot extension at the general Engmeenng ana t76 East $mih Temple. $wca 1570. Salt take Clry. Utan Ban! Te~eonone: 18011 36&~55 ieleccoy: X90:. 753s.:;5 EnvllOrlTenid $erviCeS Ib,nva nn .q H.mn•~c.r~...uun,.•v •,~~..,,,,,,,~i~