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~ICZ ACZ Ine. Englneerlnq and Environmental Servlcsa 1475 Plne Grove Roatl. Swte 709 P.O. Box 774018 Steamboat Springs. Colorado 80477 303-679.6260 January 14, 1992 Our ref: 869-0100 Ms. Catherine W. Begej Colorado honed Land Reclamation Division 1313 Sherman Street, Room 215 Denver, Colorado 80203 Re: Southfield Mine (C-81.014) Rcfuse Pile Stability Analysis Dear Ms. Begej: ACZ Inc. (ACZ) has completed a geotechaical stability analysis of the epsting coal rcfux pile at the above rekrenced 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 to you in a letter dated November 1, 1991 which included a reevaluation of the s[abiliry based on torrent disposal prauices at the mine. 1.0 Background The original stabiliry evalualioa (Geo-Testing Laboratories, Int., 1981) was based on placement of mixed coarse and fine refuse (no designated proportion) to a maximum height of 30 feet, with 2H:IV sidulopes, and 66 percent relative compaction (ASTM D1i98). The strength pazame[ers used in the analysis included an effeuive [riction angle of 23.6 degrees, a cohesion of 150 psf, and a phreatic surface 5 fee[ above ground level. Modeling baxd on these conditions resulted in a minimum calculated fauor of safety of 152. Expanded evaluatioos (RocJcy Mountain Geoterhaical, 1982) addressed foundation conditions, in-place refuse cotuistea~y, moisture content and density, existing slope angles, and strength testing of mixed refuse samples. Sensitivity analyses of stability modcling ruults were evaluated for selected conditions and pile heights. For a 75 foot pile height with 2H:1 V sideslopes, a mixrure of at Teas[ 3 parts coarse refuse to 1 part fine refuse, 90 percent relative compauion (ASTM D-698), an effeuive friuion angle of 35 degrees, cohesion of 0 psf, and a phreatic surface at the original ground level, the minimum calculated fauor of safety for dreulaz failure was 153. Current refuse placement prauices at the mine vary from conditions assumed is the above aaalyxs. The differences are desvbed below. • Wi[h [he possible exception of auive working benches, auual stockpile sideslopes have been • established at 3H:1V or greater compared with the 2H:1V slopes used N stability modeling. • Refuse material u being placed in lifts tonsuring of a lift of fine material overlain by one or more lifts of toarx material as oppoxd to the original assumption of a 3:1 mitaure of coarx and fine refuse.