2007-08-10_PERMIT FILE - C1980007 (3)

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or equal to 20%. From Barr (2007), the average colluvium liquid limit and clay-size fraction are 38% and 22%, respectively. Some of the data points from Stark and Eid have been added to the plot in Figure 10 for comparison. They show that the drained failure envelope selected for the colluvium is somewhat conservative compared to estimated values from the literature. It should be noted that the lower colluvium, or the material below the failure surface, was assigned infinite strength for the USSA and ESSA so that the slip surfaces for factor of safety calculations would be forced through the known failure surface to create a block failure. This is the type of failure surface geometry that has been observed at the site with inclinometers. 8.2.2.5 Failure Surface Material The shear strength of the material from the failure surface in the landslide, modeled in Cross- Sections Aand Conly, was measured in the reversal direct shear apparatus. Figure 11 shows the reversal direct shear results at the three normal stresses at which the test was run. It can be seen from Figure 11 that the measured residual friction angle was approximately 20 degrees. The material in the failure surface has a higher liquid limit than the other colluvium material surrounding it. The recovered samples show soils with liquid limits as high as 51% and other portions of the failure surface may contain even higher plasticity material. However, the sample tested (at SB-06-06 from 45 to 47 ft.) only has a liquid limit of 43%. As is well known, the residual shear strength is a function of the clay mineralogy of the material and correlations have been developed to correlate residual shear strength with index soil properties. Stark and Eid (1994) develop a correlation between residual secant friction angle, liquid limit, and clay-size fraction. Use of this correlation with a liquid limit of 43% and clay-size fraction of 25 to 45% yields a friction angle of approximately 20 degrees, which is similar to the value measured in the test. Assuming that the material in the failure surface has a liquid limit in the range of 50 to 60% and a clay-size fraction greater than 50%, use of the Stark and Eid correlation yielded a secant friction angle in the range of 14 to 16 degrees. To further evaluate the strength of the failure surface, aback-analysis was also performed to estimate the shear strength of the failure surface because this is a critical piece of information for the design. The back-analysis was performed using the groundwater conditions at Cross-Section C associated with the water levels measured on July 21, 2006. While these measured water levels are not associated with landslide movement, they provide a basis for the back-analysis because they represent steady-state groundwater flow under full reservoir conditions. Using these porewater pressures, aback-analysis provides a minimum value for the strength of the failure surface material because any lower value would result in a factor of safety less than one. The back-analysis was performed by changing the shear strength in the failure surface until a factor of safety of unity was P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 28