2022-10-27_PERMIT FILE - M2022001 (3)

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Mr. Drew Damaniano December 30, 2021 Page 3 XSTABL was used for the analysis. The method for selecting the critical failure surface for each analyzed loading condition is the following. The Modified Bishop's Method of Analysis is used to find the critical failure surface by randomly searching with 20 termination points and 20 initiation points(400 failure circles)with 7-foot line segments over a broad range of the slope surface and at the structure in question. This procedure is repeated over different initiation and termination locations until the most critical factor of safety failure surface is identified. The range is narrowed and 20 initiation points and 20 termination points(400 failure circles)with 7-foot line segments for the final run of 400 circles to determine the lowest factor of safety. Therefore, prior to submitting the final stability run,at least 800 failure surfaces were analyzed to determine the lowest 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 attached to this report. Review of the Hazard Tool indicated a maximum horizontal acceleration of 0.082g with a return period of 2,475 years for the site. The three cross section locations were selected and analyzed as described below. ► Section 1: This section is in the northwest corner of the site in an area that will be wet mined prior to being backfilled with silt. This section considers a 50-foot tall highwall at a point where the mine limit is 40 feet from a gas line. The overburden is modelled at 5-feet thick and a 2-foot thick mud lens is modeled within the sand and gravel. This is the most critical section to be wet mined as it marks the point where a tall highwall (deepest bedrock)and closest structure coincide. ► Section 2: This section is the most typical section at the site. This section considers a 50-foot tall highwall at a point where the mine limit is 30 feet from the slurry wall. The nearest structure is a gas line located 47.6 feet beyond the slung wall (total distance 77.6 feet from the highwall). The Western Midstream gas line running through Cell 6 may potentially be rerouted around Cell 6 to 20 feet beyond the slurry wall (total distance 50 feet from the highwall. The overburden is modelled at 5-feet thick and a 5-foot-thick mud lens is modeled within the sand and gravel. The analysis was run with failure circles starting at 40-feet from the highwall. Because the FOS meets DRMS standards,structures can be as close as 40-feet from the highwall. The nearest structure is actually 77.6-feet from the highwall. ► Section 3: This section is on the southeast part of the site and is the section with the greatest highwall. This section considers an 85-foot tall highwall at a point where the mine limit is 73-feet from the slurry wall. The nearest structure is a gas line located 20 feet beyond the slung wall (total distance 93-feet from the highwall). The overburden is modelled at 5-feet thick and a 15-foot-thick mud lens is modeled within the sand and gravel. The analysis was run with failure circles starting at the slurry wall. Because the FOS at the slurry wall meets DRMS standards and all structures are further from the highwall than the slung wall,all structures in this area will be stable. MATERIAL PROPERTIES The material index and engineering strengths assumed in this slope stability report are discussed below. Overburden and Mud Lens The strength properties for the in situ silty to clayey sand overburden and mud lens 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 Friction Angle 0'° —Weight c Weight c Weight c 103 114 126 50 1 29