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<br />~~~ ACZ Inc. <br />Engineering end Environmental Services <br />1475 Pine Grove Road, Suite 109 <br />P.o. Boz 77ao1a <br />Steamboat Springs, Colorado 80477 <br />303-879-6260 <br />November 1, 1991 <br />Ms. Catherine W. Begej <br />Colorado Mined Land Reclamation Division <br />1313 Sherman Street, Room 215 <br />Denver, Colorado 80203 <br />~II ~I~~I~II~~~~~'ll <br />s9s <br />~~~~~~~~ <br />NOV (1 4 1991 <br />Mined Land <br />Reclamation Divisior+. <br />Re: Southfield Mine (C-81-014) Review of Refuse Pile Stability Evaluation <br />Dear Ms. Begej: <br />This letter responds to the concerns expressed in your September 30, 1991 letter <br />relative to consistency between refuse placement practices and the assumptions <br />utilized in the original and subsequent stability evaluations. <br />The original stability evaluation (Geo-Testing Laboratories, Inc., 1981) was based <br />on placement of mixed coarse and fine refuse (no designated proportion) to a <br />maximum height of 30 feet, with 2H:iV sideslopes, and 66 percent relative <br />compaction (ASTM D-698). The strength parameters used in the analysis included <br />an effective friction angle of 23.6 degrees, a cohesion of 150 psf, and a phreatic <br />surface 5 feet above ground level. Modeling based on these conditions resulted in <br />a minimum calculated factor of safety of 1.52. <br />Expanded evaluations (Rocky Mountain Geotechnical, 1982) addressed foundation <br />conditions, in-place refuse consistency, moisture content and density, existing slope <br />angles, and strength testing of mixed refuse samples. Sensitivity analysis of stability <br />modeling results for selected conditions and pile heights resulted in determination <br />that ultimate refuse pile height could be increased to 75 feet while maintaining an <br />adequate factor of safety. Fora 75 foot pile height with 2H:1 V sideslopes, a mixture <br />of at least 3 parts coarse refuse to 1 part fine refuse, 90 percent compaction (ASTM <br />D-698), an effective friction angle of 35 degrees, cohesion of 0 psf, and a phreatic <br />surface at ground level, the minimum calculated factor of safety for circular failure <br />was 1.53. <br />Additional testing (GeoWest, 1988) focused on measurement of in-place densities <br />and included strength testing for fine refuse materials. <br />Current refuse placement practices vary from assumed conditions utilized in <br />developing the stability analyses in the following respects: <br />