2009-09-16_REVISION - C1980007 (3)

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To: Chris Nyikos From: Philip B. Solseng Subject: Stability Analysis Results Date: 16 September 2009 Project: Pond MB5E Evaluation c: Jed Greenwood, Brad Lindsay GEOTECHNICAL PROPERTIES OF MATERIAL The geotechnical properties assigned to each layer in the analysis are based on field and laboratory testing, and the assumed values were based on engineering judgment. The selected parameters for the slope stability analysis are shown below in Table 2. The laboratory testing results are included in the Appendix. Table 2. Parameters Used in Analysis Sat. Moist Unit Unit USSA3 ESSA4 Weight Weight Friction Friction Material Description [pcf] [pcfl2 Cohesion Angle Cohesion Angle s [degrees] s [degrees] Foundation Material 127.4 115 1,000 0 0 Non-linear Compacted 1252 115 1,000 0 0 Non-linear Foundation Material Embankment 1252 115 1,000 0 0 Non-linear Material Clay Blanket 125 115 1,000 0 0 Non-linear Notes: 1. Computed based on wsat=22.9% (lab value, TP#6 48-64 in., assumed saturated) and G,=2.68 (assumed) 2. Assumed 3. Based on pocket penetrometer values during test pitting (lowest value was QP 1.0 tsf) 4. Non-linear fully-softened failure envelope (after Stark and Eid, 1994) assumes liquid limit (LL)=37% and clay-size fraction (CF)>25%. These values were from highest plasticity laboratory sample (TP#1 0-16 in.). Envelope included in table below. Normal Stress s Shear Stress s Secant Friction Angle de 1044.3 665.3 32.5 2088.5 1205.8 30.0 8354.2 4256.7 27.0 The same permeability value was assigned to all materials included in Table 1. An assumed value of 1x10-6 cm/s was used. This value is somewhat higher than might be expected for a lean clay material, but it accounts for possible dessication and material variability. SLOPE STABILITY RESULTS The results of the analysis indicate minimum static factors of safety of 1.36 against downstream slope failure at the maximum pool elevation, 1.43 against upstream failure at the normal pool elevation, and