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EXECUTIVE SUMMARY <br />Buckhorn Geotech has been retained to conduct a stability evaluation of the existing coal waste <br />(gob) stockpile at Bowie Mine No. 2, near Paonia, Colorado. Our evaluation has included <br />drilling investigations, installation and monitoring of piezometers and inclinometers, laboratory <br />testing, and limit equilibrium stability analyses. <br />A total of eleven boreholes have been drilled at the site. The purpose of the boreholes was to <br />characterize the coal waste and native foundation materials, sample the coal waste, and install <br />piezometers and inclinometers within the boreholes. Laboratory testing conducted on the coal <br />waste includes index property testing (i.e., gradation, Atterberg limits, specific gravity, and <br />proctor), unconsolidated - undrained (UU) triaxial shear, and consolidated - undrained (CU) triaxial <br />shear with pore pressure measurement. Undrained and drained shear strength parameters were <br />estimated for short-term and long -term stability analyses based on the UU and CU triaxial tests. <br />Five vibrating wire piezometers were installed in boreholes in June 2005 to monitor pore <br />pressures developing within the coal waste. The piezometers have generally been monitored on <br />a monthly basis. Measured pore pressures range from 2 to 8 psi. Two piezometers indicate <br />slight increasing pore pressure trend while one piezometer indicates a slightly decreasing trend. <br />Monitoring of three inclinometers does not indicate definite or significant subsurface movement <br />to date. We recommend on -going monitoring of piezometers and inclinometers. <br />Limit equilibrium stability evaluations were conducted for short-term ( undrained) and long -term <br />(drained) conditions. A minimum factor of safety of 1.3 was desired for the short-term analyses <br />and 1.5 for long -term analyses. For a 4H:1 V (horizontal:vertical) slope, our stability analyses <br />indicate a short-term factor of safety with respect to stability of 1.3 to 1.4 and long -term <br />(drained) factor of safety of 1.5 or greater. At this time, we recommend a maximum slope of <br />4H:1 V above the lowest existing bench for the coal waste stockpile. This recommendation is <br />contingent on continued monitoring of piezometers and inclinometers; careful and proper fill <br />placement; inspection, in -situ density testing and laboratory testing of gob material, as <br />appropriate; and proper management of surface water drainage on and around the stockpile. <br />If Bowie would like to consider slopes steeper than 4H:1 V (based on demonstrated pore pressure <br />dissipation) for the future of this waste stockpile, we recommend installation of additional <br />piezometers. The additional piezometers are recommended due to the variable gob consistency <br />and variation observed in the pore pressures to date. <br />Bowie No 2 Gob Pile Stability Dec2005 <br />Project #05- 126 -GEO <br />Page 3 of 18 <br />