Laserfiche WebLink
changing the shear strength in the failure surface until a factor of safety of unity was achieved. The <br />resultant back-calculated shear strength was a friction angle of 14 degrees. It is thought that this <br />value represents a conservative estimate of the shear strength along the landslide failure surface as <br />compared to the measured value from the reversal direct shear testing and the Stark and Eid (1994) <br />correlation, as described in Section 8.2.2.5. As a result, the failure surface was modeled with a <br />friction angle of 14 degrees. <br />It should be noted that the back-calculated residual friction angles are different at Cross-Sections A <br />and C. The difference can likely be attributed to complex three-dimensional geometry and <br />groundwater conditions and significant heterogeneity which is inherent in colluvial material. Both <br />back-calculated values are within the range of the tested value and values from the literature (Stark <br />and Eid, 1994). <br />9.1.3.4 Buttress <br />The buttress strength parameters were not changed from those described in Section 8.2.2.5. Thus, <br />~'=32.2 degrees and c=0 for the ESSA. <br />9.1.4 Modeling <br />As with the analysis of dam stability, the landslide was modeled using the limit equilibrium software <br />SLOPE/W developed by GEO-SLOPE International, Ltd. Instead of importing computed porewater <br />pressures from SEEP/W as was done in the dam analysis, piezometric lines were drawn into the <br />model to account for measured and expected porewater pressures. The materials in Cross-Section A <br />are almost entirely colluvium, thus it is expected that the permeability will be nearly the same <br />everywhere in the cross-section. Further, no significant upward or downward flow was observed in <br />the historical standpipe piezometer readings reported by Norfleet (1999). Therefore, it was <br />considered acceptable to use piezometric lines to match piezometer readings. Piezometric lines <br />assume that porewater pressures are hydrostatic. <br />' 9.1.4.1 Existing Conditions <br />As discussed above, the correlation between landslide movement and measured porewater pressures <br />' is well documented by Norfleet (1999). Piezometer readings at which slope movement was observed <br />were termed the threshold values. Using the threshold water levels in the slope stability model.and <br />assuming that a factor of safety equal to one corresponds to slope movement, the residual shear <br />strength of the failure surface material and the upper colluvium was back-calculated by changing the <br />strength until a factor of safety of one was achieved. As noted in Section 8.1.2.3, the resulting <br />' residual friction angle was 14 degrees. This configuration was considered the base case, and the <br />output can be seen in Appendix C. <br />P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc <br />42 <br />