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~€~ <br /> <br />June 28, 2006 <br />Ms. Roy Karo <br />Reclamation Manager <br />Seneca Coal Company <br />Re: Seneca II-W Mine Stability and SEDCAD 4 Analyses <br />Dear Rop. <br />This letter is to' summarize the tads performed by MWH to develop a remediadon plan to <br />increase the Factor of Safety (FS) against continued slope failure at the II-W Mine. The tasks <br />completed by MWH include: an infinite slope back analysis of the failure slope, a Roclab analysis <br />for the undedying rock layer, aSLOPE/W rotational failure analysis, and a SEDCAD 4 analysis. <br /> <br />A back analysis was completed using the slope geometry provided to MWH by Seneca Coal <br />Company in post-mine and pre-mine topography AutoCAD drawing files. The location of [he <br />slide in these dzawtngs was located using survey pouts of the sliding body, also provided to MWH <br />by Seneca Coal Cotnpany. <br />• Overall slope height of 200 Eeet; <br />• The upper section of the shde has a slope approximately 1.5:1 (I-I:V); <br />• The lower section of the slide has a slope approximately 2:1 (H:V); <br />• Groundwater was present at the time of slope failure. <br />An infinite slope back analysis was performed to determine a FS slightly less than 1.00. Using the <br />given slope geometry and slide location, soil properties inputs were varied until the desired FS was <br />cakulated. <br />Cohesion, c= 200psf <br />Friction Angle, $= 37° <br />The soil properties obtained in the back analysis were then used in another infinite slope analysis <br />while decreasing the slope angle above the slide. The slope angle was decreased until an acceptable <br />FS of 1.30 or greater was calculated <br />The following factors of safety were calculated: <br />• Infinite Slope Analysis of a Regraded Slope of 3H:1V, FS=1.72 <br />An analysis of the intact rock beneath the slide surface was also performed using Rodab softwaze. <br />This analysis was performed to estimate the intact sheaz strength for the undeflying rock slopes. <br />These estimated shear strengths were input into SLOPE/W to analyze the potential a circulaz <br />failure in the underlying rock. <br />P.O: BOX 174018 ?e1: 9708796100 Oeli'rxring lnn°verive ..°r°(ECr, and Sun:nuns tNor7dwida <br />1475 Pine Grove toad, Suite 109 f-ax: 570 879 5048 <br />Steamboat Springs. Cniorndu <br />80477 <br />