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• BACK CALCULATIONS AND SENSITMTY ANALYSES <br />If it can be determined that a slope has failed in the recent past and that pre-failure conditions can <br />be estimated, then a common tool for establishing some of the parameters is a technique known as <br />`back calculation' or `sensitivity analysis'. Pre-failure slope geometry can be reconstructed in <br />most cases, groundwater conditions can be assumed, and slope failure mechanisms can be <br />hypothesized. The shear strength parameters c (cohesion) and ~ (phi, angle of friction) can be <br />estimated by this method. The process involves two iterative steps: <br />1. The best possible estimates must be made of the soil strengths and units weights using the <br />available information. Laboratory tests and strength correlations provide an effective basis for <br />these estimates. The slope geometry and groundwater conditions at the time of failure must <br />also be established. <br />2. The slide should be analyzed using the estimated properties. If the calculated factor of safety <br />is equal to 1 the properties and conditions represent a reasonable model of the slide. If the <br />calculated factor of safety is not equal to 1, the strength aze adjusted until a factor of safety of <br />1 is achieved. The adjustment ratio need not be the same for all materials involved in the slide. <br />Logically, larger adjustments should be made for strengths that are considered to involve <br />greater degrees of uncertainty. Further adjustments must be made to account for hypothesized <br />failure mechanisms. <br />The following table shows the iterations for stability sections A-A', D-D' and F-F' that were made <br />• in the process of performing a sensitivity analysis. The assumptions for this analysis were that the <br />slope consisted of a homogeneous and infinite slope of a single material type with groundwater at <br />the elevation of Sylvester Gulch. <br />• <br />