2018-05-22_PERMIT FILE - M2018027 (4)

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Mr. David W. Hunt May 13, 2018 Page 5 Dry Unit Moist Unit Saturated Unit Weight(pco Weight(pco Weight(pco Cohesion C'psf Friction Angle 0'° 116 Peak= 126 Peak= 135 Peak= 100 Peak=25 Residual = 110 Residual = 133 Residual=0 Residual = 15 Soil-Bentonite Slurry Wall The proposed slurry wall will consist of a mix of the overburden clayey to silty sand, alluvial sand, and imported bentonite. The resulting mix will produce a non-Newtonian fluid with some shear strength characteristics based on a reduced friction angle of the overlying overburden. Based on engineering judgment,we modeled the slurry wall as follows: Dry Unit Moist Unit Saturated Unit Weight(pco Weight(pco Weight(pco Cohesion C'psf Friction Angle 4ti'° NA 112 115 0 0 STABILITY ANALYSES RESULTS The stability analyses assumed the mining will be per the mine plan. The plan includes dry mining in both cells as the water level in the main cell will be controlled by the slurry wall and mining in the other cell will be above the groundwater table. The perimeter mine slopes of both cells will be no steeper than 3H:1 V. Setbacks listed in Table 1 (below) indicate the setback from the structure to the mining limits. The setback distance can be increased as needed to address other restrictions. The factor of safety shown below is the minimum factor of safety of the three conditions listed above. TABLE 1 -SLOPE STABILITY RESULTS AND SETBACKS Section Location Critical Structure Static Factor of Seismic Factor of DRMS Draft FOS Structure Setback From Safety at Safety at Structure Requirement Mine Limit ft Structure 0.084 hor Static/Quake 1 Main Cell West Gas Line 30 1.95 1.45 1.5/1.3 (most critical section 2 Main Cell East Phone 70 2.27 1.58 1.5/1.3 weaker soil Line 3 Main Cell West Gas Line 115 2.63 1.79 1.5/1.3 dee est mining) 4 Main Cell Oil Burner 115 2.76 1.86 1.5/1.3 Northwest CONCLUSIONS The mining setback will be 30-feet from the high wall to the slurry wall or nearest structure. Based on the Factors of Safety listed in the table above,the mine will not be a hazard to neighboring structures provided the structure and slurry wall offsets, as well as the perimeter mine slopes, follow the mine plan. LIMITATIONS Our review is based on regional geologic mapping, present mining plans, and in part borehole data by Civil Resources and Earth Engineering Consultants. Stability analyses were performed using typical strength parameters for the various strata in the critical sections. Should the mining plans change or subsurface conditions vary from those portrayed in this