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C-1981-010 PR-06 <br />2°d Engineering Adequacy Review <br />02 July 2009 <br />Page 3 of 4 <br />Please adjust the size from 10 inches to 12 inches for the underdrain material <br />requirement. <br />Response: Figure 24 and Appendix A of the report have been revised to reflect the <br />12 inch minimum rock size. <br />Item resolved - no further action is required. <br />AAI Horse Gulch Fill Stability Analysis <br />Section 3.0 - Model Results <br />For Model 3, AAI uses a highly weathered, slakable material, with fair compaction <br />(corresponding approximately to the last entry in Tables 5 and 7), in the analysis. The <br />estimated composition of the Horse Gulch Fill will be 40% shale and 60% sandstone <br />(Table 6), with high variability and segregation present throughout the fill. There may be <br />no accurate way to model for such variable composition, and AAI's stated approach is, <br />"not to model for the worst case but rather the worst reasonably expected case" (AAI, <br />Page 9). Of the various materials included in Tables 5 and 7, it appears to the Division <br />that "Shaley material fair compaction" may exist in the HG Fill in rather large proportion. <br />The estimated cohesion for this material is a mere 200 psf, and the Slope-W analysis <br />predicted a failure surface occurring in this layer rather than within the clay beneath. <br />Please provide a discussion evaluating the sensitivity (if any) of the proposed Horse <br />Gulch Fill to the possibility of as much as 40% of the volume of the fill being comprised <br />of this un-weathered shale material with fair compaction. <br />Response: Section 3.0 has been revised to include a model (and discussion of the <br />model) of the Horse Gulch Fill comprised of un-weathered shale material with fair <br />compaction. <br />Item resolved - no further action is reauired. <br />Section 4.0 - Underdrain Design and Construction <br />6. 4.3 Drain Design - A bulldozer will cut a trench along the proposed alignment for the <br />underdrain. According to the second paragraph on Page 20, dragline spoil will be <br />allowed to drape over the core to form the filter system. The drain design details in <br />Appendix A would seem to indicate that the core will be constructed of very coarse <br />boulders ahead of the advancement of the dragline fill, but the text does not specifically <br />state that to be true. <br />Please provide additional detail in the text to describe the method of underdrain core <br />construction in advance of the dragline operation.