2012-04-10_REVISION - M1981185 (43)

Home Browse Search

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