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6.0 Discussion of Stability for Existing Conditions <br />• -Revleed September 21, 1998 <br />Based upon the analyses and the monitoring information, the existing landslide is stable from <br />catastrophic movement. As shown on Figure 6, the relative factor of safety with the pins is about <br />1.2'. When the dewatering trenches aze installed, the factor of safety will increase further, but it is <br />not likely that the water table will decrease the full 30 feet of the trench depth. The continued <br />dewatering by the stone columns and pond lining, however, will further increase this factor of safety. <br />Therefore, the actual factor of safety is expected to be between 1.2 and 1.3, when the dewatering <br />trenches are installed. A factor of safety of 1.3 is a reasonable value for design with aback- <br />calculation of analready failed surface and the observational approach to landslide stabilization. <br />Based upon the FLAC analysis in Appendix D, the landslide is stable. <br />The FLAC analyses does not provide a Factor of safety but based upon the analysis, the landslide is <br />stable with the pins installed. As shown on Figure 7, the toe of the landslide where the pins are <br />located will undergo about 1.8 feet of additional deformation before the sheaz strength is fully <br />mobilized. This analysis is based upon aback-calculated sheaz strength with the existing water <br />. pressures acting upon the landslide so that any dewatering that takes place will serve to reduce the <br />calculated deformation. The two methods of analyses (i.e., factor of safety approach and the FLAC <br />method) both predict that the landslide is stable from a catastrophic movement with the pins <br />installed. Both methods will predict more stable conditions (less movement) as the landslide becomes <br />dewatered. <br />Another approach to evaluate stability is using the probability of different landslide outcomes. One <br />outcome is rapid movement or failure of the slope, and others are slow, very slow, and extremely slow <br />movements. This approach is useful with the observational approach because it incorporates <br />previous probabilities of movement with current movement measurements. Bayes Theorem of <br />probability can `test" each measurement and successively upgrade the existing stability estimate over <br />time using the following equation: <br />Upgraded Probability of Stability (Previous probability of stability) x (curtent probability of stability) <br />• summaries of orobabiliN of sfabiliN <br />Note: The factor of safely computed by the slope stability model will vary between about 1.16 and 1.23 depending upon <br />• slight changes in groundwater elevations from the piezometers at the head of the landslide. <br />K:\0626024\58991_I.WPD/CAK 13 Revised September 2l, 1998 <br />