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the acceptable slope angle for this project, unless it is demonstrated that pore pressures are <br />dissipating. <br />7.0 CONCLUSIONS AND RECOMMENDATIONS <br />Short -term and long -term slope stability evaluations were conducted for the existing coal waste <br />stockpile at the Bowie Mine No. 2 near Paonia, Colorado. The evaluations were based on <br />information gathered from drilling investigations, laboratory testing of gob material, on -going <br />piezometer and inclinometer monitoring, and our professional experience. <br />The current batter of the existing coal waste stockpile is a maximum of 5H:1 V above the lower <br />50 vertical feet (which is at a slope of 2.5H:1 V). Based on our stability analyses, we recommend <br />a maximum overall slope for the stockpile of 4H: I V. For a 4H:1 V 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. The 4H:1 V slope may begin immediately above the <br />lower bench. Fifteen -foot wide terraces at 50 vertical foot spacing with drainage to the lateral <br />diversion ditches are recommended. <br />This recommendation is contingent on continued monitoring of piezometers and inclinometers; <br />careful and proper fill placement; inspection, in -situ density testing and laboratory testing of gob <br />material, as appropriate; and proper management of surface water drainage on and around the <br />stockpile. The damaged piezometer VWP -01 will need to be replaced as this was the only <br />piezometer at the eastern portion of the stockpile. We recommend that piezometers continue to <br />be monitored on a monthly basis. If a stable trend is observed, the monitoring frequency can be <br />decreased. Inclinometers should be monitored on a quarterly basis (i.e., every 3 months). <br />If Bowie would like to consider slopes steeper than 4H:1 V in the future, we recommend <br />installation of additional piezometers. This recommendation for additional piezometers is due to <br />the variable gob consistency and variation observed in the pore pressures to date. <br />Monitoring of piezometers will provide an indication of the increase or decrease of pore <br />pressures with time and in response to fill placement. It must be emphasized that pore pressures <br />are a crucial component of the stability of the gob stockpile. Due to the fines content of the gob <br />material, pore pressure dissipation is anticipated to be a slow process. Therefore, it is critical <br />that gob materials are not placed wet of the optimum moisture content. Gob should be sealed (by <br />compaction) to minimize infiltration from precipitation and should be graded to shed water per <br />the drainage plan. Upgradient surface water should be diverted around the gob pile and away <br />from gob and foundation soils. <br />If necessary, pore pressure dissipation can be enhanced by installation of drainage within the fill. <br />This may consist of wick drains, vertical sand or gravel drains, drilled horizontal drains, or <br />geosynthetic drainage materials. <br />8.0 REFERENCES <br />Bowie No 2 Gob Pile Stability Dec2005 <br />Project #05- 126 -GEO <br />Page 17 of 18 <br />