PERMFILE52052 - Laserfiche WebLink

Home Browse Search

shown on Table 2 and, based upon the results of the stability analyses (as shown on Figure 6), the • landslide will have along-term factor of safety greater than 1.1 and is stable. Therefore, the analyses indicate that the overall landslide is stable in the long-term with the existing corrective measures. Comparison of Measured Movements A comparison of the results of the surface movement monitoring information from April 1997 with April 1998, as shown on Figures 14 and 15 from HLA, indicate that the toe of the landslide has been stabilized where corrective measures have been implemented and that the landslide movement in this region will remain constant. The piezometer responses in the stone columns and the dewatering wells are inconclusive at this time to evaluate long-term conditions but will continue to be monitored and evaluated (consistent with the observational approach). If short-term corrective measures are implemented (i.e., buttress or portal retaining structure) they will tend to further stabilize portions of the landslide. Probability of Future Landslide Outcomes The risk analysis method presented in Appendix F provides a first approximation of the probabilities for different landslide outcomes using only the calculated factor of safety and the • measured movements. The method described in Appendia F can be used as part of the observational approach where the probability of landslide outcomes can be estimated using measured performance and judgement. In addition, the probability of future landslide outcomes may be further refined by the use of other methods of analysis, such as: 1) probability of climatic events; 2) comparison of predicted deformation using the FLAC model with the measured deformation; and/or 3) judgement of qualified persons. Future Monitoring The measured movements and pore pressure changes within the landslide provide the most accurate estimate of the landslide's performance. Long-term monitoring, together with the method presented in Appendix F (including Bayes Theorem) and other methods (i.e., climatological, FLAC, and Judgement) will provide a basis for estimating long-term stability. The period of monitoring should continue until a level of confidence has been reached that the predicted probability of a desired landslide outcome is within a specified tolerance level, say 95 percent. However, since this time period cannot be determined at this time, future monitoring should continue for a period of at least five years, through several climatic conditions, and the probability of landslide outcomes 0626024\58991-1/YI~II-I 16 Revised September 21, 1998