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-2- DRAFT GEOTECHNICAL FINDINGS MT. GUNNISON N0. 1 UNDERGROUND COAL MINE PERMIT REVISION APPLICATION - UPPER WASTE ROCK PILE Coal Processing Waste and Non-Coal Processing Waste West Elk Coal Company applied for a permit revision to allow the construction of an upper waste rock pile in October of 1984. The operator had noted its intention to construct an upper waste pile in the original permit application. The permit revision application represented the submission of a detailed engineering plan for that structure, for which a general plan had earlier been submitted. West Elk Coal proposes, within the amended permit revision application, to construct a five million ton coal processing waste pile. The refuse will consist of coarse coal reject and development waste rock. The mine projects waste production at a rate of 150,000 tons for the first year and 100,000 tons per year thereafter. The completed waste pile structure will cover approximately 40 acres, and attain a maximum thickness of 160 feet. The waste pile will have 2.5 H:1V (horizontal to vertical) facial slopes with 30 foot wide benches at 50 foot vertical intervals. The overall final facial slope will be approximately 3 H:1V (horizontal to vertical slope of approximately 18.2 degrees) in order to facilitate reclamation. Samples of currently produced waste were collected and tested, in order to provide material properties for a mathematical stability analysis. Samples were also collected and tested of subgrade colluvium and bedrock for the same purpose. The stability analysis projected a minimum static slope safety factor of 1.64 for an overall facial slope of 2.5 H:1 V, steeper then the requested configuration. At the request of the Division, West Elk Coal Company also completed an analysis of the potential stability effects upon the F-Seam, from the placement of a five million ton surcharge load upon the existing workings within the mine. The plan calls for approximately the eastern half of the waste pile to be undermined. In completing this analysis, the operator assumed: 1. An ultimate configuration of the waste pile reaching elevation 6,880 feet above sea level. 2. No secondary extraction beneath the pile. 3. No barrier pillars, even though one is planned. 4. A continuous crack existing from the elevation of the F-Seam's floor to the ground surface at the top of the pile. Two separate conservative strengths were assumed for the floor clay beneath the F-Seam (angle of internal friction = 90, cohesion = 1,350 lbs per square foot: angle of internal friction = Oo, cohesion = 4,320 lbs per square foot), resulting in safety factors greater than 1.62.