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Greosyntecc> consultants Page 4 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 described above were selected based on the use of"preliminary" physical-mechanical properties in accordance with the DBMS Policies (DRMS 2018). As described above, the Mohr-Coulomb strength criteria for materials is employed for soil and rock in this analysis. Material strength properties required for the methodology described above include unit weight, friction angle, and cohesion. Table 2 lists each stratum and the corresponding material properties modeled for this analysis. Material strength property references for Slocum Alluvium were not found during the desktop references review and Site-specific data was not available, nor obtained. Based on the geologic characterization provided above, the Slocum Alluvium is a well-graded (i.e., poorly sorted) sand material. As such, the friction angle value was conservatively assumed based on experience and a range of published values for similar materials. The friction angle selected for the alluvium is 29 degrees, which generally corresponds to a loose sand 3. Based on a lack of data,the cohesion value for the alluvium was also conservatively assumed to be 0 lb/ft2 although it is likely that the actual material has some cohesion due to the presence of silt in the formation. Two references for Pierre Shale formation strength parameters were found. The first reference Geotechnical Engineering Circular No. 5—Evaluation of Soil and Rock Properties,issued by the Federal Highway Administration(FHWA)and dated April 20024 provided friction angle((p')value of 32 degrees, and a cohesion value of 1,253 lb/ft2. The second reference Inventory of Shale Formations in the US Including Geologic, Hvdrological, and Mechanical Characteristics, issued by the United States Department of Energy(USDOE) and dated November 2013 provided friction angle value of 21 degrees and a cohesion value of 13,824 lb/ft'-. The USDOE information corroborates the FHWA values and suggests the FHWA strength values are more conservative. These stability and sensitivity analyses utilize the FHWA strength values for Pierre Shale, and utilize the USDOE unit weight for the shale. SENSITIVITY ANALYSIS A sensitivity analysis was performed to observe the effect of variations in the Pierre Shale strength property values on the calculated FS for static conditions. The sensitivity analysis included the following steps: 1. Evaluate static stability of Highway 93 for the selected slope configuration using Pierre Shale friction angle equal to 32 degrees and cohesion equal to 1,253 lb/ft2(i.e.,the USDOE friction angle). Observe the static factor of safety for circular and block failure modes. 9 Das,B.M.(2006). Evaluation of Soil and Rock Properties.Geotechnical Engineering Circular No. 5,prepared by Sabatini et al.,for the FHWA in April 2002. Structures Evaluation Arcosa LWB Plan—Permit Amendment 2019-07-16