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3.4 Stability Analysis <br /> To assess the stability of the existing access road cut slopes, the following activities were <br /> performed: <br /> • Reviewed the results of previous geotechnical evaluations performed to evaluate the <br /> stability of the access road (Section 1.3); <br /> • Observed and mapped the conditions exposed in the cutslopes; <br /> • Performed subsurface investigations in 2010 and 2012; and <br /> • Performed numerical stability analyses. <br /> The most common form of slope failure in rock cut slopes is planar failure. Planar <br /> failure can result when a competent block of rock lies along a planar discontinuity that dips in <br /> the same direction as the slope (referred to as unfavorable bedding). When the dip angle is <br /> greater than the peak friction angle for the discontinuity surface, the block tends to slide. As <br /> noted in the 2010 geotechnical reconnaissance report (R Squared, 2010), the rock bedding in new <br /> access road rock cuts is dipping favorably from a stability standpoint. <br /> 3.4.1 Numerical Stability Anal <br /> Stability analyses were performed for each of the geologic conditions: the through cut <br /> exposing terrace deposits/colluvium, and the cutslopes in bedrock. The analyses were performed <br /> using Slide 5.0, developed by Rocscience, Toronto, Canada. The design roadway surcharge <br /> loading used for the cut-fill analysis was 900 pounds per square foots. <br /> For terrace deposits/colluvium, strength testing was not performed as the fraction of <br /> oversize particles precluded obtaining a relatively undisturbed sample that is required for such <br /> tests, as described in Appendix B. Consequently, strength parameters were used for soils with <br /> similar moisture/density and Atterberg Limits which are believed to be conservative. For the cut <br /> slope rock, as discussed above, sliding along bedding planes is unlikely due to the favorable <br /> orientation of the rock bedding. To develop strength parameters for the numerical slope <br /> stability model, presumptive values were evaluated. These values, for sandstone and claystone, <br /> indicate unconfined compressive strengths greater than 5,000 pounds per square inch (SME, <br /> 2002). Because of weaker claystone layers and sandstone fractures observed in rock exposed on <br /> the cut slope, it was judged that these presumptive values may overestimate the strength of the <br /> rock mass. Thus, the parameters of dense clayey gravel were used to evaluate the stability of the <br /> 3 The stability of the fill slopes retained by the existing timber cribbing was not considered as this material will be <br /> removed as part of the improvements planned for the access road. <br /> d Slide 5.0 is a software program that performs 2D Limit Equilibrium Slope Stability Analysis using several <br /> approaches for evaluating circular and non-circular failure surfaces. <br /> 5 900 pounds per square foot is equivalent to a 20-ton, 3-axle vehicle. A surcharge load equivalent to a 40-ton, 3- <br /> axle vehicle resulted in no change in the safety factor. <br /> Rule 6.5 Geotechnical Stability Technical Revision April 2,2012 <br /> New Access Road Page 4 <br />