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III IIIIIIIIIIIIIIII ~CZ Engineering and Environmental Services 1475 Pine Grove Road, Suite 109 ~'~ P.O. Box 774016 Steamboat Springs, Colorado 60477 303-879-6260 January 14, 1992 Ms. Catherine W. Begcj Colorado Mined Land Reclamation Division 1313 Sherman Street, Room 215 Denver, Colorado 80203 Re: Southfield Mine (C-81-014) Refuse Pile Stability Analysis Dear Ms. Begej: !L° l~ i._. ~~ ~ . ~ ~ JAN 1 5 1992 Mined Land Our ref: 869-0100 ACZ Inc. (ACZ) has completed a geotechnical s[abiG[y analysis of the existing coal refuse pile a[ [he above referenced mine. Our analysis was conducted in response to your September 30, 1991 letter which outlined concerns about current refuse placement practices and the assumptions utilized in the original and subsequent stability evaluations. The scope of our investigation was outlined [o you in a letter dated November 1, 1991 which included a reevaluation of the stability based on current disposal practices a[ [he mine. 1.0 Background The original stability evaluation (Geo-Testing Laboratories, Inc., 1981) was based on placement of mixed coarse and tine refuse (no designated proportion) to a maximum height of 30 feet, with 2H:1V sideslopes, and 6G percent relative compaction (ASTM D-698). The strength parameters used in the analysis included an effective friction angle o(23.6 degrees, a cohesion of 150 psf, and a phrea[ic surface 5 feet above ground level. Modeling based on these conditions resulted in a minimum calculated factor of safety of 1.52. Expanded evaluations (Rocky Mountain Geotechnical, 1982) addressed foundation conditions, in-place refuse consistency, moisture content and density, existing slope angles, and strength testing of mixed refuse samples. Sensitivity analyses of stability modeling results were evaluated for selected conditions and pile heights. For a 75 foot pile height with 2H:1V sideslopes, a mixture of at least 3 parts coarse refuse to 1 part fine refuse, 90 percent relative compaction (ASTM D-698), an effective friction angle of 35 degrees, cohesion of 0 psf, and a phreatic surface at the original ground level, the minimum calculated factor of safety for circular failure was 1.53. Curren[ refuse placement practices at [he mine vary from conditions assumed in the above analyses. The differences are described below. With [he possible exception of active working benches, actual stockpile sideslopes have been established at 3H:1V or greater compared with the 2H;1V slopes used in stability modeling. Refuse material is being placed in lilts consisting of a lift of fine material overlain by one or more lifts of coarse material as opposed [o [he original assumption of a 3:1 mixture of coarse and fine refuse.