2018-03-26_PERMIT FILE - M2017049 (6)

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Hitch Rack Ranch Pit Wall Geotechnical Assessment—Supplemental CDRMS Response NORWEST 65018 March 2 ,218 COR PORAT ION Page constructed from the major rock types based on the block model geology. This analysis conservatively considers saturated groundwater conditions(disturbance factor=0). The alignment of the selected cross- section is shown in Appendix A. Currently,the available drillhole data is limited to the vicinity of the Phase 1 pit slopes. The ultimate wall heights for the proposed quarry range from approximately 300ft to 500ft. A preliminary overall pit wall angle of 45*has been considered and consequently modelled to confirm that factor of safety requirements as highlighted in Table are satisfied. Table 1 Design Criteria,Overall Slope Static FOS?1.5 Limit Equilibrium Pseudo-Static FOS>_1.1 7 *FOS=Factor of Safety. Limit equilibrium analyses shows the proposed 45 degree pit slopes for Phase 1 pit have FOS values greater than 2 for both static and pseudo-static conditions even assuming fully saturated wall conditions (conservative groundwater assumption). Appendix 1 contains printouts of the stability analysis results. Note that both circular failure and block specified failure surface were examined to determine the sensitivity of the results to the shape of the failure surface. In all cases examined, the FOS exceed the design criteria. Pit walls constructed at 45°from horizontal are adequate to satisfy the above design criteria for the Phase 1 cross-section under the conditions considered. The use of a 45 degree inter-ramp wall configuration can also be used as a design guideline for the ultimate pit slopes in order to determine the disturbance area for reclamation bonding. Detailed design of the Phase 2 and subsequent quarry walls can be completed as additional geotechnical drilling and rock mass characterization is completed in the proposed quarry area. Pit walls in excess of 250-300 feet may have slightly shallower overall slopes as ramps or a wider inter-stack catch bench configurations are incorporated into the wall design. 4. CONCLUSIONS AND RECOMMENDATIONS 4.1. Overall Pit Slope Design A cross-section through the highest area of the Phase 1 pit wall was analyzed using slope stability software to check the factors of safety for both static and pseudo-static conditions. An overall pit wall angle of 45* was considered under saturated conditions and results show that this configuration satisfies the design criteria. For preliminary design purposes, it is recommended that overall quarry pit slopes do not exceed 45*from horizontal. The current drillhole data supports the development of the Phase 1 pit slopes.