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• <br />• <br />• <br />(I) Calculated factors of safety utilizing slope stability analysis. <br />(2) Landslide velocity classifications (LNG's) based on movement rates measured in the field. <br />Utilizing Bayes' Rule, which combines the probability of outcomes determined from these two methods, a <br />weighted average of the probabilities was determined to further refine the estimation of probability. A <br />comparison of these two sets of probabilities, along with the Bayes' Rule analysis, provides a reasonable <br />estimate of the probability of future landslide outcomes under existing conditions. <br />H7.2.1 Probability Based UpoB Factor of Safety ADalyses. <br />Probabilities where assigned to landslide outcomes based upon the factor of safety against sliding <br />determined by slope stability analysis. To simplify the analysis, the factors of safety listed in Section H4.2 <br />were rounded to the lowest 5/I OOths so that factors of safety of 1.0, 1.25, and 1.3 were assumed for the <br />1997, 1998, and 1999 conditions. <br />Mean Shear Strength. The mean shear strength of soil was assumed [o be [ha[ strength which produced a <br />factor of safety of 1.0 against sliding in the 1997 condition. In the 1997 condition, the slide was moving <br />and accelerating to rapid failure as evidenced by the landslide movement figures. in this condition, the <br />slide was azbitrarily given a 50/50 chance of rapid failure, and assigned a factor of safety of 1.0. The mean <br />strength was calculated to be a fiction angle of 12.1 and cohesion of 200 psf. <br />Coefficien! ojVariation of Shear Strength. The mean shear strength parameters were assumed to have a <br />coefficient of variation of 30 percent (Lee, White and Ingles, page 199). This is validated by Sylvester <br />Gulch Borings that indicate a coefficient of variation of 32% for the friction angle of clay. <br />Assigning factors ojsajety to landslide movements. The landslide outcomes were related to the factor of <br />safety of sliding as follows: <br />(1) rapid movement, a factor of safety of less than 1.0. <br />Barr Engineering Company <br />PCDOCSl208220/CAK <br />July 31, 1999 <br />24 Appendu H <br />1999 Addendum to <br />1997/99 Landslide Corrective Measures Report <br />