2022-09-26_PERMIT FILE - M2022042 (23)

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Slope Stability and Setback Exhibit – Windsor East Mine September 21, 2022 TETRA TECH 3 Section 9 Section 9, (Figures 9a to 9f) evaluated the proposed setback from the southwestern property boundary to Cell D. This section also included analysis of a proposed temporary stockpile shown on Figures 9c and 9d. Section 10 Section 10, (Figures 10a to 10d) evaluated the proposed setback from the Cache la Poudre River and south permit boundary to Cell C. Section 11 Section 11, (Figures 11a to 11d) evaluated the proposed setback from the Cache la Poudre River and south permit boundary to Cell A. Section 12 Section 12, (Figures 12a to 12h) evaluated the proposed setback from the water pipeline to Cells C and D. 3.2 MATERIAL STRENGTH PROPERTIES Material properties used in the analysis were based on boring logs in the area from the 2017 Deere & Ault report and 2019 Martin Marietta borings and typical published strength values. Table 1. Model Material Input Parameters Location Unit Weight (pcf) Cohesion (psf) Friction Angle (deg) UCS (psf) Overburden 110 200 23 - Sand and Gravel 120 0 30 - Silty Sand 120 0 28 - Compacted Overburden 112 100 28 - Claystone 140 - - 70,000 *pcf = pounds per cubic foot; psf = pounds per square foot; UCS = unconfined compressive strength Static and seismic stability analyses were performed for each of the sections described above. Seismic analysis accounts for the effects of horizontal acceleration experienced during an earthquake. The horizontal acceleration used was 0.08 g and corresponds to an event with a 2% probability of exceedance in 50 years (return interval of 2,475 years). The acceleration value was obtained from United States Geological Survey Unified Hazard Tool which considers the soil/rock classification of the upper 100 feet (Classification C selected) and location of the site. The following minimum factor of safety criteria requirements from the DRMS were used: The minimum factor of safety for the static analysis is 1.5. The minimum factor of safety for the seismic analysis is 1.3. 3.3 SLOPE STABILITY RESULTS The tables below present stability model results with graphical output presented in Attachment C. The setbacks listed are the entry/exit of the minimum factor of safety failure circle for the reclaimed slope and the location of the entry/exit of the failure circle for the minimum factor of safety as required by the DRMS for the mine slope. Model results indicate that the minimum required factor of safety is achieved for the reclamation slope models (without and with stockpile where applicable) and appropriate setbacks are presented for the relatively steeper mine slope. Stockpiles should be constructed at least 8 horizontal feet from the crest of the highwall and after the reclaimed slope has been constructed.