2010-12-17_REVISION - C1981019 (94)

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SHANNON &WILSON. INC. CMLRD Regulations require a long term FS of at least 1.5. We performed a drained (long -term) analysis and defined pore water pressures by a piezometric line. As previously discussed in Section 6, we considered two failure mechanisms: a smaller -scale toe failure and a larger -scale crest failure. The results of the parametric analysis are presented in Figure 10 with details in Appendix C. Prior to publication of our report, Colowyo informed us the spoil slope will be decreased from the 3H:1 V slope used in our models to a slope of 3.5H:1 V. The flatter slope will in all cases increase the calculated FS for the cases we modeled. In Case 22, we did use the flatter 3.5H:1 V slope and this case resulted in a FS of 1.6 for the flatter slope. to Using the soil and rock shear strength parameters discussed in Section 6, we analyzed baseline cases for both models. For the Clay Layer at Base of Spoil model we computed an FS of 2.2 for a larger -scale crest failure and a FS of 2.4 for a smaller -scale toe failure. For the baseline case for Carbonaceous Mudstone /Claystone at Base of Spoil model we computed a FS of 2.1 for a larger -scale toe failure and a FS of 1.7 for a larger -scale crest failure. In addition to analyzing the baseline cases for Sections A -A' and B -B', we also performed a parametric study in which we varied shear strength parameters, ground water conditions, and bedrock dips. 0 For the Clay Layer at Base of Spoil model, the results of our parametric study indicate the static FS against slope instability is greater than or equal to 1.5 for all combinations of shear strength parameters and ground water conditions, as required by CMLRB regulations. Additionally, we performed a pseudo- static analysis and computed FS greater than 1.5 for both the smaller -scale toe failure and larger -scale crest failures. Depending on groundwater conditions and shear strength parameters, the smaller -scale toe failure tended to be more critical than the larger -scale crest failure. For Trials 5, 7, and 9, the FSs for the two different failures were similar as the failure surface began to approach an infinite slope condition. The FS for larger -scale crest failures tended to be relatively insensitive to changes in groundwater elevation up to 40 feet above the base of the spoil. Our parametric study of the Carbonaceous Mudstone /Claystone at Base of Spoil yielded similar results. A static FS of 1.5 or greater was computed for larger and smaller scale failure modes for all combinations of shear strength parameters, bedrock dip, and groundwater conditions. 23 -1- 01101 -400 -R l .docx/wpAkn 23-1-01101-400 11 Exh. 23 -1 -18 Revision Date: 08/05/09 Revision No.: PR -03