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Geosynteccl consultants Page 6 of 12 Client: Arcosa LWB Project: Permit Amendment Project No.: DE0374 Task: Exhibit S—Structures Evaluation Written by: Jonathan Gillen Date: 7/8/2019 Reviewed by: Youngmin Cho Date: 7/9/2019 Approved by: Poppy Staub Date: 7/15/2019 Highway 93 ROW.These results assume a friction angle equal to 32 degrees and a cohesion equal to 1,253. These strength values are lower than other strength values found in literature, as described above. Using the non-circular critical surface with the FS equal to 2.95, the values for friction angle and cohesion are independently varied and the change in FS is observed,as described below. 2. Assuming the Pierre Shale cohesion value remains constant: a. A friction angle equal to 6.5 degrees for the Pierre Shale corresponds to a 1.5 FS for the static non-circular analysis. Refer to Chart 1 in Attachment D. 3. Assuming the Pierre Shale friction angle value remains equal to 32 degrees: a. A cohesion value equal to approximately 0 lb/ft2 for the Pierre Shale corresponds to a 1.7 FS for the static non-circular analysis. Refer to Chart 3 in Attachment D. DISCUSSION OF RESULTS Global factors of safety for the existing static and seismic conditions are greater than the required minimum FS required in the DRMS Policies, and therefore global stability is not expected to be an issue. Localized failures (i.e., near-surface sloughing within 2 feet of the ground surface) may be possible during a seismic event within the Slocum Alluvium layer; however global failure is not anticipated. The inherent uncertainty associated with using non-Site-specific material properties was evaluated using the statistical analysis. The results of the statistical analysis show that, if cohesion values remain constant, the friction angle for the formation would need to be less than 6.5 degrees to achieve a FS lower than the target minimum. It is unlikely that, even if strength values were somewhat lower than the values assumed in this analysis, the calculated FS would be lower than the target minimum. Because the strength of the Pierre Shale formation is derived from cohesive and friction forces (i.e., cohesion and friction angle values) it is not anticipated that the strength of this formation will be substantially different than the strengths found in literature. Therefore, the static and seismic stability of Highway 93 is adequate for the anticipated range of strength values associated with the Pierre Shale formation. 5 The non-circular critical surface is used for the sensitivity analysis because it is the critical surface with the lowest factor of safety that encompasses the mine slope.As mentioned before,the circular critical surface with the lowest FS falls entirely within the Highway 93 limits and does not include any portion of the mine slope.By using a critical surface that incorporates the mine slope,the strength parameters effect on the stability(i.e.,FS)of the mine slope can be evaluated. Structures Evaluation Arcosa LWB Plan—Permit Amendment 2019-07-16