2010-12-17_REVISION - C1981019 (94)

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SHANNON 6WILSON, INC. We performed pseudo- static analysis assuming bedrock dip of 3 and 5 and computed a minimum FS of 1.4. This exceeds the regulatory minimum for seismic analysis of FS 1.3. 8.0 CONCLUSIONS AND RECOMMENDATIONS The results of our stability analysis as described above indicate that temporary spoil valley fill constructed with the geometries shown in Figures 2 and 5 has calculated long term Factors of Safety in of 1.5 or greater as required by CMLRB Regulations (2005). Our stability analysis was performed to evaluate long -term conditions in accordance with the requirements of the regulations. The recommendations discussed below, primarily related to the underdrain and surface water control on the fill faces, also consider the temporary nature of the fill and the limited consequences should a slope failure occur. Short-term stability conditions during fill placement will vary depending on the location within the fill. We made assumptions regarding the sequence of construction and bedrock dip in the area which should be confirmed prior to construction of the fill. The following paragraphs provide our design and construction recommendations to address these issues. 8.1 Spoil Fill Placement 1 0 We recommend that the fill be placed in approximately horizontal lifts. To maintain comparatively high permeability within the fill, the use of thick lifts is desirable. It is our opinion that lifts on the order of 50 to 100 feet can be used because of the insensitivity of the spoil site to post - construction settlement. Previous experience with the Streeter, Section 16 and West Pit Fills has shown that fills intended for open rangeland following construction perform satisfactorily with thick lifts. Performance of the temporary spoil fill is dependent on following the recommendations presented in this report and good construction practice. We recommend observation of the fill placement be performed quarterly as required by CMLRB Regulations to observe and document stripping, drain construction, fill placement, and surface drainage conditions. 8.2 Spoil Fill Underdrain We recommended the drain location begin at the highest elevation spring location below the fill and extend downstream, exiting at the northern toe of the fill (Figure 4). The drain size and materials should be in accordance with the regulations. Lateral drains should be constructed in locations where seeps or springs are mapped (or discovered during stripping), and should tie into 23 -1- 01101 -400 -R I .docx/wp /Ikn 23 -1- 01101 -400 12 Exh. 23 -1 -19 Revision Date: 08/05/09 Revision No.: PR -03