2023-02-08_REVISION - M2003016 (2)

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Mr. Brennan Middleton September 30,2022 Page 3 The stability of structures within 200 feet of the proposed mining limits was evaluated at the two(2)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 27,001 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 0.0896g with a return period of 2,475 years for the site. The two(2)most critical cross section locations were selected and analyzed as described below. ► South Section: This section is on the south side of the previously mined mine cell where the underdrain is—30 feet from the mine highwall. This section considers a 22-foot tall highwall at a point where the mine limit is approximately 20 feet from the slung wall.The high wall slope 3h:1v. The overburden is modelled at 6-feet thick overlying 16-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. Groundwater was modelled at a depth of 4-feet exterior to the slurry wall. Loading due to traffic along County Road 20 was simulated with a 500 pound per square foot(psf)surcharge at the road. ► Southeast Section:This section is on the southeast side of the previously mined mine cell where a concrete ditch is approximately—39 feet from the mine highwall. This section considers a 28-foot tall highwall mined at a slope of 3h:1 v. The original mine cell was mined to within approximately 12-feet of the slurry wall and approximately 1-foot of fill was placed over the top of the slurry wall. A boring at the site indicates the depth to bedrock in this area is approximately 44-feet. However,site surveys and observations indicate the cell was not mined to bedrock in this area. The overburden (including the 1-foot of fill)is modelled at 6-feet thick overlying 38-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. Groundwater was modelled at a depth of 4-feet exterior to the slurry wall. Loading due to traffic along County Road 20 was simulated with a 500 psf surcharge at the road. MATERIAL PROPERTIES The material index and engineering strengths assumed in this slope stability report are discussed below. Overburden The strength properties for the in-situ overburden were based on our experience at similar sites and engineering judgment;the following parameters have been used to model the overburden. Unit Weight Cohesion C'psf Friction Angle 0'° c 105 50 26 Alluvial Sand and Gravel The sand and gravel is generally a fine to medium-grained sand overlying a fine to coarse grained sand that is typically medium dense and locally gravelly. The alluvial sand and gravel unit was modeled as follows: Unit Weight Cohesion C'psf Friction Angle 0'° c 120 0 1 35