2019-11-12_REVISION - M2004044 (13)

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TETRA TECH Updated Tucson South Proposed Gravel Mine Slope Stability Analysis N November 8 2019 psf load is modeled to simulate the weight of the building. The setback presented is measured from the edge of the water utility easement. Section L—Challenger Pit Section L (static analysis Figures L-1; seismic analysis Figures L-2) evaluated the proposed setback for the Challenger Pit to the north of the proposed West pit boundary. A mud lens was present and modeled based on previous studies in the area'. A slurry wall is modeled for the proposed Tucson South pit and the existing Challenger Pit. The water table is modeled 13 feet below ground surface. The setbacks presented are measured from the edge of the utility easements. Offset regulations may dictate a different minimum setback. Section M — Pipeline (East Side of East Cell) Section M (static analysis Figures M-1; seismic analysis Figures M-2) evaluated the proposed setback for the Todd Creek waterline (pipeline)to the east of the proposed East pit boundary. The pipeline comes closer to the property in this area. A mud lens is not modeled based on previous studies in the areal. The proposed slurry wall is modeled. The presented setbacks are measured from the edge of the utility easement. An agreement with the Todd Creek Metropolitan Water District included in the DRMS application allows for a lesser setback than estimated in the models Section N — Highway 7 from North Cell (West) Section N (static analysis Figures N-1; seismic analysis Figures N-2) evaluated the proposed setback for Highway 7 to the south of the proposed West pit boundary. A mud lens was present and modeled based on previous studies in the areal The proposed slurry wall is modeled. A 500 psf load was modeled to simulate traffic on the Highway 7. A transmission line adjacent to Highway 7 is the critical structure which the setback presented is measured from. The model assumes a screening berm is located between the road and the mine and the slurry wall is not constructed under the berm. Section O—City of Aurora Water Line Section O (static analysis Figures 0-1; seismic analysis Figures 0-2) evaluated the proposed setback for the Aurora waterline to the south of the proposed West pit boundary. A mud lens was present and modeled based on previous studies in the areal. The proposed slurry wall is modeled. A 200 psf load was modeled to simulate the weight of a screening berm on the south side of the West Cell. The model assumes that the slurry wall is not constructed under the screening berm. The waterline is the critical structure and where the setback is measured from. 4.2 INPUT PARAMETERS Each section was modeled to resemble the existing site topography based on the site survey referenced on the Tucson South Resource Pre-Mining and Mining Plan Maps. Material thicknesses were modeled based on a geotechnical investigation perforemed at the site in 2004. In general, the sand and gravel were thinner on the west side of the site. The materials were generally assumed to be horizontal in the subsurface. A three-foot thick weathered claystone bedrock layer was modeled based on a letter from Allen Sorenson to Kate Pickford dated March 6, 20037. The sections modeled should not be used as an estimation of the aggregate resource. The input parameters for each of the materials used were those required for the 2004 analysis and are based on a memo from Allen Sorenson to Larry Oehler, dated August 29, 20048. A reclamation slope fill material was added for this revised analysis. The material properties are presented in Table 1. All materials were modeled using the Mohr/Coulomb framework. 7Sorenson, A. (2003, March 6). RE: Pit Wall Stability Analysis, Irwin Corporation, Bernhardt Resource Gravel Pie, Permit No. M-2002-120. Denver, CO. 8Sorenson, A. (2004, August 29). RE: Reclamation Cost Estimate and Pit Wall Stability Analysis, Aggregate Industries, Tucson South Resource Gravel Pit, File No. M-2004-044. TETRA TECH