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Mr. Lee Sampson <br />March 28, 2022 <br />Page 4 <br />Agapito Associates, Inc. <br />planned worst-case geometric conditions using Spencer’s method of analysis. These conditions <br />include maximum slope height and overall slope angle. <br />As shown in Figure 1, Section A represents the north-facing slope of the dump pit and the final <br />reclaimed surface. In this area, the height of the proposed slope will be approximately 195 ft and <br />will comprise two benches with inter-slope angles of 38. The overall slope angle is proposed to <br />be 23. The final reclaimed surface will comprise a 14 slope at a height of 107 ft. <br />Also shown in Figure 1 is Section B, which bisects the valley fill. In this area, the height of the <br />proposed dump slope will be 155 ft with a proposed slope angle of around 37. The final claimed <br />surface will comprise a 19 slope at a height of 225 ft. <br />A generalized clay thickness of 35 ft has been used for both sections and a 5-ft-thick impenetrable <br />layer of gravel has also been used at the contact with the bedrock. The bedrock was also modeled <br />as impenetrable as deep-seated failures through the bedrock are not considered likely below the <br />base of the fill. The modeled cross-sections are shown in Figures 2 to 5. <br />United States Geologic Survey (USGS) data indicates that the peak ground acceleration with a <br />10% probability of exceedance in 50 years is about 0.05g (acceleration of gravity).5 On this basis, <br />a version of the analysis was performed for the anticipated seismic conditions using a pseudo- <br />static analysis with a horizontal acceleration 0.05g. <br />In terms of the modeled groundwater conditions, a base case of 15 ft below the existing ground <br />surface was assumed. Fluctuating groundwater conditions were considered for Section A, and a <br />secondary set of modeling was conducted using a groundwater level of 40 ft above the clay surface. <br />These levels are similar to those assumed in the 2009 assessment.2 The rock lined drainage ditch <br />in Section B will limit the groundwater table to the base of the spoil. <br />In line with CMLRD requirements, a minimum long-term static factor of safety criterion of 1.5 <br />has been used. For seismic analysis, a minimum factor of safety of 1.3 will satisfy the regulatory <br />requirements. <br />Similar with the 2009 assessment, 2 two general failure modes were considered for the analysis. <br />The first mode involves failure paths near the toe of the slopes. This is considered the most likely <br />mechanism for the assessed slopes. The second mode involves failure paths further up-slope, <br />typically near the crest. <br />RESULTS AND RECOMMENDATIONS <br />The results of the stability assessment are provided in Attachment and summarized in Tables 2 and <br />3. <br />5 USGS (2002), “Interpolated Probabilistic Ground Motion for the Conterminous 48 States by Latitude-Longitude,” <br />2002 data from USGS web data.