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C-1981-010 PR-06 <br />Engineering Adequacy Review <br />28 April 2009 <br />Page 2 of 3 <br />The toe of the HG Fill does rest on a slight downslope, but the stability analysis has <br />demonstrated that a Factor of Safety greater than 1.5 can be achieved without the use of a <br />keyway or toe buttress. However, AAI has identified steeper portions of natural ground <br />on the northern sideslope of Horse Gulch, and states that benching is required for these <br />areas (AAI HG Fill Stability Analysis Figure 28). Based on the auger drilling data <br />(Tables 1 and 2), these slopes may be composed of thick, weak dark-brown clay. The <br />application does not address the proposed bench configurations and dimensions. <br />Please provide a description of the-benching activities proposed for the steeper side <br />slopes that exist within the Horse Gulch Fill area. <br />2. 4.09.1(11) - Inspection - The AAI Report on the HG Fill discusses inspection of <br />construction of that fill and the underdrain, but inspections are not mentioned in either the <br />K-Pit Buttress stability analysis or within the permit test. <br />Please provide specific information within the pages of the Permit that thoroughly <br />addresses the requirements of Section 4.09.1(11) regarding inspection of construction <br />of the Horse Gulch and K-Pit Buttress Fills. <br />3. 4.09.1(14) requires that the foundation and abutments of the fill shall be stable under all <br />conditions of construction and operation. Maps have been provided which depict the <br />mining plan and post-mining topography, and the method of construction of each fill is <br />described, e.g. dragline and/or truck dump. Very little information has been provided <br />regarding the anticipated phasing (including length of time required) for the dragline <br />portion of HG Fill to be constructed, or construction of K-Pit Buttress, or construction of <br />the remainder of the HG Fill. <br />Please provide additional detail about the timeline and phasing of the construction of <br />the HG Fill, in particular, including a discussion of the stability of the f Il under all <br />conditions of construction and operation. <br />4. 4.09.2(2)(d) requires that in constructing the underdrains, no more than 10 percent of the <br />rock may be less than 12 inches in size and no single rock may be larger than 25 percent <br />of the width of the drain. Figure 24 and the Appendix A Horse Gulch Uriderdrain detail <br />of the AAI report indicate that no more than 10 percent of the rock will be less than 10 <br />inches in size. <br />Please adjust the size from 10 inches to 12 inches for the underdrain material <br />requirement.