2018-11-21_REVISION - M2004044 (22)

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a � OL TETRA TECH Updated Tucson South Proposed Gravel Mine Slope Stability Analysis ■ti November 9, 2018 Groundwater was encountered while drilling in 2004 at depths between eight and 12 feet below the ground surface. Monitoring well readings from December 2003 to August 2004 have water levels between four and 16 feet below ground surface. 3.0 PREVIOUS EVALUATIONS A similar evaluation was completed for the Tucson South mine in November 20043. The evaluation used the (previously required)factor of safety value of 1.0 and analyzed a total of 14 critical structures surrounding the Tucson South Resource footprint. No seismic analysis was conducted. A mud lens was modeled in each section. The analysis found that several of the minimum mining setbacks needed to be increased to meet the minimum required factor of safety of 1.0. The previous evaluation was used as a basis for this evaluation. 4.0 STABILITY ANALYSIS The stability analysis was conducted using GeoStudio 2012 (Slope/W Define) computer software4. The software is capable of computing factors of safety for a range of materials using finite element analysis. Spencer's method of slices5 was used to calculate the safety factor for the individual failure surfaces. Several assumptions were made for the models including: • Surficial geology was modeled based on stability models and borehole data from 2004. • Geologic layers are horizontal in the subsurface. • The West and East pits will be mined after the slurry wall is installed and will be dewatered as needed. • The slurry wall will be built around the perimeter of the East and West Pits, crossing Tucson Street on the north and south ends. No slurry wall will divide the East and West Pits. • The slurry wall will be keyed six feet into bedrock. • No significant berms of stockpiled materials would be placed on the surface. 4.1 ANALYZED SECTIONS Critical structures within 200 feet of proposed disturbance were modeled. A critical structure is defined by the DRMS as a structure that poses a potential human safety risk, major environmental impact, and major repair cost if the slope were to fail'. No critical structures were analyzed for the Southwest Pit since the pit will be mined via wet mining and the slopes will not be vertical. Also, contrary to the previous stability modeling effort, a mud lens was not modeled in each section. Seven critical structures were identified within 200 feet of the proposed Tucson South, East, and West Pit mine boundaries: • Tucson St. • Colorado Highway 7 (E 160'h 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 • Pipeline north of the proposed mine boundary • Brighton Ditch 3 Goss, C. (2004, November 22). RE: Revised Tucson South Proposed Gravel Mine Slope Stability Analysis; Tetra Tech RMC Job No. 19-3919.019.00. (T. M. Refer, Ed.) Longmont, CO: Tetra Tech. 4 GEO-SLOPE International, Ltd. (2014). SLOPE/W 2012. December 2014 Release. 5 Spencer, E., 1967. A Method of Analysis of the Stability of Embankments Assuming Parallel Inter-Slice Forces." Geotechnique, Vol. XVII, No. 1, pp. 11-26. Page 215