2021-06-17_PERMIT FILE - M2001046 (4)

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Mr. Joe Lamanna June 9, 2021 Page 4 factor of safety. 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. Review of the Hazard Tool indicated a maximum horizontal acceleration of 0.091g with a return period of 2,475 years for the site. The three cross section locations were selected and analyzed as described below. The locations are shown on Figure 1. ► Fresh water and Silt Ponds Cell 1: This section is on the northwest side of the Freshwater and Silt Ponds and considers the tallest mine highwall of this cell. This cell will be mined with a dredge and will not be dewatered. The stability analysis for this section assumes a mine highwall sloped at 2.5 horizontal to 1 vertical (2H:1 V). The nearest structure is a gas line located approximately 30 feet to the west.The stability analysis on this section was run with potential failure circles initiating at the gas line and terminating in the area near the base of the mine highwall. ► Cell 2: This section is on the south side of cell 2 and considers the tallest highwall in this cell. The stability analysis for this section assumed a 30 foot offset from the slurry wall and a mine slope of 2 horizontal to 1 vertical (2H:1V). The stability analysis on this section was run with potential failure circles initiating at the slurry wall and terminating in the area near the base of the mine highwall. ► Cell 3: This section is on the north side of cell 3 and is the tallest highwall in the cell. This section will be capped with approximately 2-feet of compacted overburden fill. The stability analysis for this section assumed a 30 foot offset from the slurry wall and a mine slope of 2 horizontal to 1 vertical (2H:1 V). State Highway 66 is approximately 100 feet to the north. The stability analysis on this section was run with potential failure circles initiating at the slurry wall and terminating in the area near the base of the mine highwall. MATERIAL PROPERTIES The material index and engineering strengths assumed in this slope stability report are discussed below. Overburden The strength properties for the insitu silty to clayey sand overburden were based on field testing data and on our engineering judgment;the following parameters have been used to model the overburden. Dry Unit Moist Unit Saturated Unit Cohesion C'psf i Friction Angle 0' Weight(pco Weight(pco Weight c Native 103 114 126 50 29 -Comp. Fill 108 118 128 70 30 Alluvial Sand and Gravel The sand and gravel is generally a medium to coarse-grained sand that is medium dense to dense and locally gravelly. The alluvial sand and gravel was modeled as follows: Dry Unit Moist Unit Saturated Unit Cohesion C'psf Friction Angle 0' Weight(pco Weight(pco Weight c 119 129 130 0 35 Bedrock Bedrock below the alluvium is predominately sandstone with local claystone and interlaminated to interbedded claystone and sandstone bedrock. Sandstone is typically stronger than claystone. Claystone is generally a weak bedrock. To be conservative,we modeled the bedrock as claystone. For the claystone bedrock,two potential strength conditions were considered. These strength conditions are referred to as: 1)peak strength, and 2) residual strength.