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Table 14 Summary of Unit Weights for Landslide Stability Modeling <br /> Unit Wei ht <br />Material sac c moist c <br />Existing Materials <br />Colluvium 130 108 <br />Failure Surface Material 130 108 <br />Weathered Bedrock" - - <br />Pro osed Materials <br />Buttress 132 110 <br />'-This material was assumed to be infinitely strong in the model, <br />thus unit weights were not assigned. <br />~ 9.1.3.2 Colluvium <br />' The undrained shear strength of the Colluvium was not used in the landslide analysis because failed <br />slopes typically mobilize their drained residual strengths. A description of the drained residual <br />strength for the failure surface in the Colluvium is given in the following section. <br />The drained strength for the non-failed Colluvium, however, was assigned based on modeling <br />constraints. The lower Colluvium, or the material below the failure surface, was assigned infinite <br />strength so that the slip surfaces for factor of safety calculations would be forced through the known <br />failure surface to create a block failure. This is the type of landslide geometry that has been observed <br />at the site with inclinometers. <br />The upper Colluvium, on the other hand, was given the same strength properties as the failure surface <br />material. This was done because the model could not be extended all the way to the landslide scarp <br />for scaling reasons. It is known that the failure surface daylights somewhere on the upper portion of <br />the slope and that the weaker failure surface material must be present somewhere above the planar <br />failure surface shown in the model. As long as aback-analysis is performed and the modifications to <br />the landslide are based on a baseline model that has a factor of safety of one, this course of action is <br />deemed appropriate. <br />9.1.3.3 Failure Surface Material <br />The shear strength of the material from the failure surface in the landslide was measured in the <br />reversal direct shear apparatus as described in Section 8.2.2.5. <br />As at Cross-Section C, aback-analysis was also performed at Cross-Section A to estimate the shear <br />strength of the failure surface in the landslide. The back-analysis was performed using the <br />groundwater conditions associated with the threshold water levels, which have been correlated with <br />the triggering of landslide movement (Norfleet, 1999). The back-analysis was performed by <br />P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 41 <br />