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EXECUTIVE SUMMARY <br />The McClane Canyon Mine is a subsurface coal mine located in Ga~rrField County, Colorado, <br />approximately 19 miles north of Loma, Colorado. Buckhorn Geotech has conducted a stability <br />evaluation of a proposed new gob wastepile for the mine. The proposed gob pile configuration <br />includes 3H:1V slopes and an ultimate height of 40 feet encompassing an approximately one <br />acre footprint. <br />A geotechnical site investigation was conducted to characterize the subsurface conditions at the <br />proposed footprint for the gob pile. Five boreholes were drilled at the gob footprint to depths <br />of 30 to 51 feet. A sixth borehole was drilled approximately 650 feet southwest of the gob pile. <br />A monitoring well was installed in this borehole. Foundation soils at the gob pile generally <br />consisted of moist, silty sand with trace to some clay and trace to Kittle gravel and cobbles. <br />A laboratory testing program was conducted to determine index and strength properties of the <br />gob and foundation soils. A small stockpile of gob was available for testing of bulk and <br />remolded samples. No gob material has been placed to date for tu~ndisturbed sampling or in-situ <br />testing. <br />Conventional two-dimensional static limiting equilibrium stability analyses were conducted to <br />evaluate the proposed gob pile geometry. Both total and effective stress analyses were <br />conducted and both circular and block-type failure surfaces were evaluated for two cross- <br />sections. The proposed gob pile configuration resulted with a critical factor of safety of 2.4 for <br />total stress analyses and 1.7 for effective stress analyses. The senisitivity to variability in <br />. material unit weight and strength parameters was evaluated. Recommendations are provided <br />to ensure an adequate stability margin for the gob pile during and following construction. <br />1.0 INTRODUCTION <br />The McClane Canyon Mine is a subsurface coal mine owned and operated by McClane Canyon <br />Mining, LLC. The mine is located approximately 18.5 miles north of Loma, Colorado off of State <br />Highway 139 in the East Salt Creek drainage. The mine produces approximately 300,000 tons <br />of coal per year. The mine wishes to permit a new gob wastepile at the location indicated on <br />the attached aerial photo. We understand that waste material wild consist of low quality coal <br />and various waste materials (e.g., from cave-ins, etc.) <br />The proposed gob pile footprint is approximately one acre. The gob pile is proposed to have an <br />ultimate height of approximately 40 feet with 3H:1V sideslopes. Topsoil and coverfill piles <br />currently at the proposed gob pile footprint will be relocated prior to construction. <br />Buckhorn Geotech, Inc. (BGI) was retained by McClane Canyon Mining, LLC to evaluate stability <br />of the proposed new gob wastepile r^~gob pile's site. A geotechnical site investigation was <br />conducted in September 2007 to characterize the subsurface conditions at the proposed <br />wastepile footprint. Laboratory testing was subsequently conducted on soil samples obtained <br />from the native foundation soils as well as from a small gob stockpile. The geotechnical <br />investigation and laboratory testing support limiting equilibrium stability evaluations for the <br />a <br />McClane Canyon Mine stability report <br />Project #07-334-GEO <br />Page 3 of 15 <br />