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C -1996 -083 TR -72 <br />Geotech Review — MLT <br />23- Mar -2012 <br />Page 3 of 3 <br />reflect the proposed changes. I have a number of adequacy comments regarding the proposed <br />changes to CMWDA No. 2 (Gob Pile #2). <br />BRL is proposing to excavate the ridge of material back to a slope of 1.5H: IV. It is <br />apparent, from the contour lines shown on Figure 1, that CMW will be placed only part <br />way up the cut slope, and that a portion of the slope will remain permanently exposed. <br />TR -72 does not provide a prediction of the long -term stability of this slope. As <br />referenced under my summary of TR -44, above, Buckhorn Geotech determined that a cut <br />slope of 2H: IV into the native soils (for CMWDA No. 4) would provide a factor of <br />safety of 1.50. Please ask the operator to address the question of long -term stability of <br />the permanent cut slope that appears to be proposed above the placement level of the <br />Coal Mine Waste. <br />2. With TR -72, BRL is also proposing a modification to the shape of CMWDA No. 2. <br />Previously, as shown on the current permit maps, the elevated central portion of the pile <br />was limited to a linear strip at the crest of the valley fill portion, parallel to the natural <br />drainage. The configuration currently being presented shows the flattened top of the pile <br />extending westward, outside of the valley fill area and onto the surface proposed for <br />excavation. The "back side" of the pile crest then drops away to the northwest, into West <br />Diversion Ditch 92. In my review of the various geotechnical analyses that have been <br />conducted for the Bowie No. 2 Mine, I do not find that this configuration, i.e. <br />construction of the pile to elevations higher than the catch point with the native slope, has <br />been analyzed for stability. Please ask the operator to determine the expected factor of <br />safety for this configuration, ensuring that the requirements of the performance standards <br />will be met. <br />3. Figure 1 provides numerous cross - sectional views of the Coal Mine Waste Disposal <br />Area. Given the unique configuration proposed with TR -72, an additional cross section <br />is warranted. This section should be drawn approximately perpendicular to the West <br />Diversion Ditch #2, should intersect with (or terminate at) Section D -D', and should <br />show the original ground surface, the limits of excavation proposed, any CMW that is <br />presently in- place, and the proposed final limits of CMW placement. <br />4. In the text of Volume IX, the Introduction section beginning on page 1 is somewhat <br />confusing, and partially obsolete. The tenses are contradictory, and the chronology is <br />difficult to follow. Please request that the operator re -write the second paragraph on page <br />1 to clearly communicate the history of development of Coal Mine Waste Disposal Areas <br />1, 2 and 4. The information I compiled for the introductory paragraphs of this letter <br />might be useful in that effort. <br />This concludes my preliminary adequacy review for the geotechnical aspects of TR -72 for the <br />Bowie Mine No. 2. Please do not hesitate to contact me if further explanation is required. <br />