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April 21, 2020 Page 12 In addition to the numerical modeling, AAI performed a foundation bearing -capacity analysis to test if the old spoil foundation could withstand the load of the new spoil pile. The new spoil pile was assumed to exert a constant strip load over a 1,200-ft-wide space, which is again conservative given the loading under the actual spoil pile will increase at a 27' slope to the top of the pile. A constant load equivalent to 140 ft of uniformly thick new spoils, at a unit weight of 110 pcf, amounted to a strip load of 15,400 psf. This load when applied to the old spoils of semi - infinite thickness assigned with cohesion of 475 psf and friction angle of 34°, yielded an SF of 5 against foundation bearing -capacity failure. As a comparison, building foundations are typically designed to have an SF value of 4. Overall, the results of the study indicate that global instability of the new spoil pile over the projected life of the structure is unlikely, as is the likelihood of foundation failure into the old I -Seam spoils. Conclusions and Recommendations AAI has completed a geotechnical study to assess the global stability of the proposed spoil piles southwest of the N-Strike Pit, which will be placed on a dipping topography over the next 2- 3 years. A summary of the findings of the study and relevant recommendations are as follows: AAI has developed a depth -dependent shear strength matrix for the N-Strike Pit spoils based on laboratory characterization of in -situ spoils and field -verified approaches utilized by other researchers to analyze the global stability of the proposed spoil pile. Numerical modeling results indicate that the proposed spoil pile is likely to have an SF value of 1.6 and is likely to be stable over the anticipated storage period (4-6 years). Trapper has indicated that the spoil pile will be constructed in small lifts (25 ft), allowing for better compaction of the spoil particles, which will also minimize the segregation of constituent particles. Even though the numerical model representative of the spoil pile was simulated assuming 20-ft benches, the conclusions of the study are still valid given that the overall shape/surface slopes of the proposed pile will not be significantly different from the analyzed geometry. Although this study has assumed a water table 25 ft below the new spoil pile bottom, effects of transient ingress of surface water into the spoil pile have not been studied. It is recommended that adequate drainages must be created and maintained along the southern perimeter of the spoil pile to direct water flow away from the pile. Additionally, underdrains may be created at the foundation level to provide drainage paths away from the new and old spoil interface. End -dumping over large heights creates drainage channels automatically at the bottom of spoil piles, as coarser particles segregate to the bottom. Given how the N-Strike Pit spoil pile will be constructed, such pathways aren't expected to form at the bottom of the pile. Therefore, regularly spaced V-shaped drainages, with the merge point aligned approximately along the centerline of the proposed pile and pointing upslope, may be constructed to provide underdrainage during the storage period. Selective placement of coarser spoil particles (gravels, cobbles, boulders) may be used. Such drains may be placed at 500-ft spacing, north to south. See Figure 1 for approximate location and orientation of such underdrains. Agapito Associates, Inc.