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 4.1 ANALYZED SECTIONS Critical manmade structures within 200 feet of proposed mining were modeled during this evaluation. A critical structure is defined by the DRMS as a manmade structure that poses a potential human safety risk, major environmental impact, and major repair cost if the slope were to fail'. A mud lens was modeled in sections evaluated where a mud lens was present at the site based on borehole information prepared for the 2004 evaluation. The cross sections and profiles were created using Figures 1 to 6. These figures show the proposed site layout and locations to critical structures, easements, and the topography in the area. Sections were created at each of the critical structures in the vicinity, and the estimated depth to bedrock from the subsurface study was used to create the profile for the slope stability analysis. The eleven (11) critical manmade structures were identified within 200 feet of the proposed Tucson South, East, and West Pit mine boundaries were: • Tucson St. • Colorado Highway 7 (E 1601h Avenue) • Power poles to the west of the proposed West Pit boundary • South Platte River • Gas well to the west of the proposed West Pit boundary • Todd Creek waterline north of the proposed mine boundary • Brighton Ditch • Brighton Return Ditch • A gravel road and building to the west of the South pit • Challenger Pit to the north of the West pit • City of Aurora waterline A total of fifteen (15) slope stability models were prepared to evaluate the risk of damage to critical manmade structures for this evaluation as shown on Figure 1. Below is a description of the 15 model sections: Section A—Tucson St. Setback from the West Mine Pit Section A, (static analysis Figures A-1; seismic analysis Figures A-2) evaluated the proposed setback from the Tucson St. easement from the West Pit boundary. A 500 psf load was applied to the road to represent live traffic loads. A mud lens was present and modeled based on previous studies in the area. The area is modeled with high water table assuming wet season. The model includes the transmission lines adjacent to the road as the critical structure. Section B —Tucson St. Setback from the East Mine Pit Section B (static analysis Figures B-1; seismic analysis Figures B-2) evaluated the proposed setback from the Tucson St. easement from the East Pit boundary. A 500 psf load was applied to the road to represent live traffic loads. No mud lens was modeled based on previous studies in the area'. The model uses the edge of the Right of Way as the critical location. Section C—South Platte River Section C (static analysis Figures C-1; seismic analysis Figures C-2) evaluated the proposed setback for the South Platte River from the East Pit boundary. The section is modeled where the South Platte River comes closest to the proposed mine boundary. A mud lens is not modeled based on the information available from previous studies in the areal. The proposed slurry wall is modeled 20 feet from the river bank. This alignment should be verified by the slurry wall designer. Offset regulations may dictate a greater minimum setback. The model uses the edge of the riverbank as the critical structure. Section D— Pipeline (North Side of East Cell) Section D (static analysis Figures D-1; seismic analysis Figures D-2) evaluated the proposed setback for the Todd Creek waterline (pipeline) north of the proposed East and West Pit boundaries. A mud lens is not modeled based on the information available from previous studies in the areal. The proposed slurry wall alignment is modeled 15 feet inside of the existing easement. The model identifies the pipeline as the critical structure with the setback measured from the edge of the 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. TETRA TECH