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Mr. Steve Hinkemeyer <br /> September 16, 2020 <br /> Page 9 <br /> conservative given the actual shape of loading under the spoil pile will be pyramidal in shape. A <br /> constant load, equivalent to 80 ft, of uniformly thick new spoils, at a unit weight of 110 pounds <br /> per cubic foot (pcf), amounted to a strip load of 8,800 psf. This load, when applied to the <br /> foundation soil of semi-infinite thickness (cohesion of 418 psf and friction angle of 25°) yielded <br /> an SF of 3 against foundation bearing-capacity failure. The actual SF against foundation failure is <br /> likely to be higher in light of the conservatively high load assumption and presence of relatively <br /> more competent mudstone at a depth of 30 ft below the upper soil mass.As a comparison,building <br /> foundations are typically designed to have an SF value of 4. <br /> Overall, the results of the study indicate that global instability within the new spoil pile <br /> over the projected life of the structure is unlikely, as is the likelihood of foundation failure into the <br /> subsurface soils. <br /> CONCLUSIONS AND RECOMMENDATIONS <br /> AAI has completed a geotechnical study to assess the global stability of the proposed spoil <br /> piles between the proposed I-Pit East and J-Pit cuts. The spoil piles will store spoils over 6-8 years <br /> and will be placed on a westward-dipping ground surface. A summary of the findings of the study <br /> and relevant recommendations are as follows: <br /> • Analysis results obtained from 2D numerical models developed along four distinct vertical <br /> sections through the proposed spoil pile indicate that the pile is likely to have SF values <br /> much greater than >1.5, which is the criterion for long-term stability. Therefore, the <br /> proposed spoil pile is likely to be stable over the anticipated storage period (6-8 years). <br /> • Trapper has indicated that the spoil pile will be constructed in small lifts (20-40 ft), <br /> allowing for better compaction of the spoil particles. <br /> • Although this study has assumed a water table 20 ft below the new spoil pile, effects of <br /> transient ingress of surface water into the spoil pile have not been studied. It is <br /> recommended that adequate drainages be created and maintained along the updip perimeter <br /> of the spoil pile to direct water flow away from the pile. <br /> • Additionally,underdrains may be created at the foundation level to provide drainage paths <br /> away from wetting the thicker spoil zones downslope. End-dumping over large heights <br /> automatically creates drainage channels at the bottom of spoil piles, as coarser particles <br /> segregate to the bottom. Given how this specific spoil pile will be constructed, such <br /> pathways are not expected to spontaneously form at the bottom of the pile. Therefore, <br /> selective placement of coarser spoil particles (gravels, cobbles, boulders) may be used to <br /> construct underdrains to facilitate drainage away from the thicker zones within the spoil <br /> matrix. Figure 1 shows the approximate location and orientation of such underdrains. <br /> • Minor tension cracks are anticipated to develop along the bench crests during and after <br /> construction. Such cracks should be regraded to cut off water ingress into the tension <br /> cracks,thus limiting their progressive downward extension.Dumping should be suspended <br /> if tension cracks greater than 6 inches wide and 2 ft deep develop. <br /> Agapito Associates, Inc. <br />