2023-10-31_REVISION - M1973021 (4)

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depth of 3 feet was set. Slip surfaces less than 3 feet below the ground surface were assumed to be a maintenance issue but if not addressed, could lead to progressive slope failures. Material Properties A geotechnical investigation of the highwall and channel foundation soils has not been performed. For the stability analysis the bedrock highwall was considered to be a high strength material that would not impact stability of the channel and the geometry of the highwall was based on available topographic mapping prior to reclamation. Calibration modeling was performed to develop material properties of the channel foundation soils (reclaim fill) based on the assumption that the current reclaim fill has been placed at a factor of safety on the order of 1.0. Because of the unknown material properties, engineering judgement was applied in addition to the calibration model results to arrive at the following model parameters for the reclaim fill that was identified in the field as a low plasticity gravelly clay. Table 1. Material Strength Parameters I Material Name I Unit Weight I Effective Friction Effective • - on Bedrock 130 N/A(1) N/A(1) Reclaim Fill 125 35 E Iq 25 'Bedrock modeled as"High Strength" Assumptions The following assumptions were made for the slope stability analysis: • The mined highwall was assumed to be stable and is not part of the stability analyses except for the geometry provided by Holcim. • Reclaim fill above the diversion channel will be removed so that no loading from uphill fill is exerted on the diversion channel foundation soils. • The veneer of riprap in the channel was not included for modeling purposes. • Groundwater was assumed to be below the depth of influence and not infiltrating the fill soils from the highwall. The channel will be lined with an impermeable geosynthetic resulting in no infiltration from the diversion channel. • No guidance is available from DRMS for temporary construction factor of safety and therefore it is assumed that the same minimum factor of safety of 1.3 can be applied for temporary construction conditions. Holcim or the contractor responsible for construction will be responsible for equipment and personnel safety during construction. This may require a long reach excavator or other specialized equipment to keep loads away from the exterior slope and reduce vibrations. Slope Stability Analysis Results Stability modeling was performed to design a slope that meets or exceeds a factor of safety of 1.3. Stability model results for cross-sections at Stations 1+00, 2+00, 10+00, 11+00, and 14+00 hdrinc.com 1670 Broadway, Suite 3400, Denver, CO 80202 (303)764-1520