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{_iu i+ ;' Geottb,h, Stain;I;ty R�.-p ort <br /> August 2cl 20 19 <br /> P�-7e2o` r3 <br /> General observations of the high-walls indicate slopes were mined nearly vertical, up to 80 ft in <br /> height, with a series of benches and haul roads traversing the site prior to 2010. On the west <br /> side of the quarry approximately 230 feet of the east-facing mine wall has been exposed and <br /> generally less than 50 feet of the mine wall has been exposed on the east side (west-facing). <br /> The current mining practice utilizes a 40-ft tall bench height with a 15-ft to 20-ft bench width. <br /> 1.2 Evaluation of Slope Stability <br /> 2) Geotechnical evaluation of the stability of the existing high-wall configuration, with special <br /> consideration given to identify existing areas of potential weakness and/or instability, and for <br /> recommendations to correct these issues if observed. <br /> Based on observations, the stability of the layer of basalt at the shale contact appears good. <br /> Where "windows" of the altered Pierre shale have been exposed in the upper high walls of the <br /> quarry, evidence of slaking (disintegrating and crumbling when exposed to moisture) is evident <br /> as expected due to weathering. The area of exposed shale is minimal and limited to the upper <br /> benches. The current practice of leaving a basalt veneer over the shale appears successful for <br /> preventing unfavorable weathering of the shale. <br /> The basalt across the site does present a few local failures along dip-slope joint orientations, <br /> however no large-scale slope failures were observed at the mine site. <br /> 1.3 Conformance with Existing Permit <br /> 3) Determination if the existing high-wall conforms to the requirements of existing permit. <br /> In a 2014 analysis, Brierley evaluated the allowable slope configuration based on the minimum <br /> factor of safety (FOS) of 1.3 for mine slopes. The results of this analysis indicated vertical bench <br /> heights up to 80 ft high have an allowable FOS on all exposed aspects of the quarry walls. <br /> Additionally, a vertical height up to 200 ft has an allowable FOS on west-facing slopes (east- <br /> side of quarry). South-facing slopes (north-side of quarry) up to 400 ft high have an allowable <br /> FOS with an overall slope dip angle of 79 degrees. East-facing slopes (west-side of quarry) up <br /> to 400 ft high have an allowable FOS with an overall slope dip angle of 73 degrees. <br /> Based on the reviewed 2010 topographic survey and 2019 observations, the existing high-wall <br /> benches and overall geometry of Ralston Quarry is above the minimum allowable FOS of 1.3 for <br /> mine slopes. <br /> 1.4 Conclusion <br /> Brierley has not observed any large-scale slope instabilities during field observations. A few dip- <br /> slope failures were observed within the basalt; however, our evaluation indicates these surficial <br /> failures do not pose a significant slope stability hazard and can likely be mitigated during the <br /> ongoing mining process. <br /> The current practice of leaving a basalt veneer over the shale on the west side of the site <br /> appears successful for preventing slaking and unfavorable weathering of the shale. Planned <br /> reclamation will further stabilize the weathering of the shale. <br /> BRIERLEY <br /> ASSOCIATES <br /> Creating Space Underground <br />