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waste bank slope of 5H:1V was necessary for a factor of safety of at least 1.50. The drained <br />analyses indicated that upper slopes of 3H:1V to 4H:1V resulted in a minimum long-term factor <br />of safety of at least 1.50, depending on the behavior and magnitude of the pore pressures. <br />Regulations of the Co%rado Mined Land Reclamation Board for Coa/ Mining (DRMS, 2005) do <br />not differentiate between end of construction and long-term conditions when specifying the <br />minimum required factor of safety for coal mine waste banks. Therefore the total stress <br />(undrained) conditions dictated the allowable slopes accepted by the Colorado Division of <br />Minerals and Geology, and the upper slopes of the CWDA No. 2 were limited to maximum <br />5H:1V. This is documented by DMG (2006). <br />Placement of coal mine waste at the Bowie CWDA No. 2 has progressed to nearly its ultimate <br />height at the writing of this report. Since additional coal waste capacity is needed by Bowie, <br />BGI was requested to re-evaluate the stability based on instrumentation monitoring conducted <br />over the past three years. This report presents the results of this instrumentation monitoring <br />along with an updated stability evaluation of the coal mine waste bank. <br />3.0 INSTRUMENTATION MONITORING <br />Vibrating wire piezometers and inclinometers were installed in the Bowie CWDA No. 2 in 2005 <br />to monitor pore pressures and deformation of the waste bank. A summary of these installations <br />is presented below. Amore comprehensive discussion of the instrumentation installation is <br />presented by BGI (2006). BGI has conducted monitoring of the inclinometers and vibrating <br />wire piezometers on roughly a quarterly basis since their installation in 2005. Monitoring results <br />to date are discussed below. <br />3.1 Vibrating Wire Piezometers <br />A total of five vibrating wire piezometers, designated VWP-01 through VWP-05, were installed in <br />four boreholes in June 2005 to monitor pore pressures over time within the coal mine waste <br />bank. The pore pressures are an important component of the soil strength and are valuable for <br />the stability evaluation. The pores pressures may be used in effective stress stability analyses. <br />Two of the piezometers have been damaged and are no longer functioning. WP-04 was <br />damaged between May and November 2006 and VWP-01 was damaged between October 2006 <br />and February 2007. Diagnostic tests indicate that a short has developed in the piezometer <br />cables. This is likely due to construction equipment, but could be for other reasons. <br />Fortunately, the remaining working piezometers (VWP-02, VWP-03, and VW-05) are all located <br />along the maximum section of the waste bank, which is the critical section modeled in the <br />stability evaluations. Locations of the piezometer are presented in Table 1. <br />r1 <br />U <br />Bowie No 2 2008 Reevalution of Coal Mine Waste Stability.doc <br />Project #OS-126-GEO <br />Page 4 of 15 <br />