2023-09-06_PERMIT FILE - M2023032 (19)

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Mr. Phillip Courtney February 15, 2023 Page 3 structures at the Thunderbird Mine are considered critical structures. The FOS are for both static and seismic(ground accelerations from an earthquake)stability analyses are listed in the DRMS policy. For generalized strength assumptions and critical structures, an FOS of 1.5 is considered sufficient for static conditions and an FOS of 1.3 is considered suitable for seismic conditions. The stability of structures within 200 feet of the proposed mining limits was evaluated at the three (3) most critical representative sections under anticipated loading conditions as discussed below. The GALENA computer program was used for the analysis. The method for selecting the critical failure surface for each analyzed loading condition was the following. The Simplified Bishop's Method of Analysis was used to find the critical failure surface by randomly searching 20,001 trial failure circles over a broad range of the slope surface and at the structure in question to evaluate the lowest FOS. Both static stability under anticipated mining conditions and seismic stability under peak ground acceleration loads were performed. Seismic loading was obtained from the U.S.G.S. Unified Hazard Tool attached to this report. Review of the Hazard Tool indicated a maximum horizontal acceleration of 0.0847g with a return period of 2,475 years for the site. The three (3) most critical cross section locations were selected and analyzed as described below. ► Excelsior Ditch Section: This section is in the northwest corner of the Phase 3 mine cell where the Excelsior Ditch is at its closest(-80 feet)to the mine highwall. An exploratory boring drilled in this area encountered approximately seven (7)feet of overburden overlying approximately twenty (20)feet of sand and gravel on top of claystone bedrock. This section considers a 27-foot tall highwall at a point where the mine limit is 80 feet from the Excelsior Ditch. A low permeability slurry wall is modelled 25 feet from the top of the highwall. The overburden is modelled at 7-feet thick overlying 20 feet of sand and gravel. The top foot of the claystone was modelled at residual strength reflecting the weathered nature at the top of the claystone. Unweathered claystone was modelled below the top foot. Mounded groundwater was modelled at a depth of 3 feet exterior to the slurry wall. This section was analyzed with both the slurry wall lining as well as the dewatered condition in the event that mining occurs prior to slurry wall installation. In the dewatered scenario the slurry wall is not modeled. ► Overhead Transmission Lines Section: This section is in the southwest corner of the Phase 2 mine cell where the overhead transmission line is at its closest(-150 feet), the mine highwall is at its tallest(-29 feet), and the weakest soil (overburden)forms a large part of the part(-12 feet)of the highwall. An exploratory boring drilled in this area encountered these conditions. A low permeability slurry wall is modelled 25 feet from the top of the highwall. The overburden is modelled at 12-feet thick overlying 17 feet of sand and gravel. The top foot of the claystone was modelled at residual strength reflecting the weathered nature at the top of the claystone. Unweathered claystone was modelled below the top foot. Mounded groundwater was modelled at a depth of 3 feet exterior to the slurry wall. This section was analyzed with both the slurry wall lining as well as the dewatered condition in the event that mining occurs prior to slurry wall installation. In the dewatered scenario the slurry wall was not modeled. ► Baxter Road Section: This section is on the west side of the site where wet mining of the fresh water and siltation ponds may approach within approximately 75 feet of Baxter Road. At this point the cell was modeled with a high wall of 0.5h:1v with a maximum depth of 20 feet. The overburden is modeled at 12-feet thick over 8 feet of sand and gravel. Bedrock was modeled at a depth of 29 feet with the top foot modeled as residual strength reflecting the weathered nature at the top of the claystone. Unweathered claystone was modeled below the weathered interval. Groundwater was modeled at a depth of 6-feet.