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• The critical failure surfaces for total stress analyses were generally deep block-type surfaces through the gob, extending to and along the contact with the native ground surface. As expected, the critical failure surfaces for effective stress analyses were shallow. For these analyses, the failure surface was forced a minimum of 5 feet into the gob to avoid insignifjcant critical failure surfaces at or near the surface of the gob. The factor of safety was not affected for analyses considering a high water table (groundwater up to 5 feet below the native ground surface). 6.0 CONCLUSIONS AND RECOMMENDATIONS A stability evaluation of the proposed Gob Pile No. 3 has been conducted for Bowie Resources No. 2 mine near Paonia, Colorado. The stability evaluation was based on a geotechnical site investigation and previous and on-going testing of gob and foundation soils. Limiting equilibrium stability analyses were conducted using total and effective stress analyses for static conditions. Various gob configurations, defjned by gob slope and ultimate wastepile height, are presented that result in a minimum critical factor of safety of 1.50. Foundation preparation, surface and subsurface water drainage, and gob placement all affect the stability of the gob pile. The following recommendations are provided to fulfill assumptions made in our evaluation and to ensure an adequate safety margin for the gob pile: 1. To provide an adequate foundation bearing surface, topsoil, loose fill, man-made debris, organic material, and all other deleterious material must be removed prior to gob placement. If soft or yielding soils are encountered, they should be removed until competent soils are encountered. 2. Foundation underdrains should be designed and constructed to keep the near surface foundation soils drained. We understand these drains are to be designed by others. Improving the lining system of the upslope Fire Mountain Canal is recommended to reduce seepage into the gob pile foundation. 3. To improve stability along the base of the gob pile (reducing potential for ablock-type failure along the contact of the native soil and gob), the native subgrade should be "benched" or "keyed" into the slope. 4. Upgradient surface water should be captured and routed away from or around the gob pile by adequately sized diversion channels. 5. The gob pile should be designed and graded to shed surface water and precipitation off all portions of the gob pile both during and after gob placement. 6. Gob must be placed at, or slightly below its optimum moisture content. Gob placed significantly above the optimum moisture content likely will not achieve adequate compaction or undrained shear strength. Placement techniques that allow for drying gob may be required at times. Once a suitable moisture content is achieved, gob should be sealed (by compaction) to minimize infiltration from precipitation and should be graded to shed water per the drainage plan. Rowie Gob Pile No. 3 stability report Project N06-48I-002 Page I S of 17