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|The Vaiont Slide : a geotechnical analysis based on new geologic observations of the failure surface, Volume 2 : appendices A through G
|Hendron, Alfred Joseph.
Patton, F. D.
|Geotechnical Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
|Technical report (U.S. Army Engineer Waterways Experiment Station) ; GL-85-5 v.2.
Abstract: The large volume and high velocity of the Vaiont Slide combined with the great destruction and loss of life that occurred make it a key precedent landslide, particularly for slides caused by reservoir filling. Engineers and geologists are now generally obliged to examine the slopes of proposed reservoirs for the owners. When the identified slides are large and the effects on the project could be significant, there is an obligation to explain why such slopes are different from and safer than the Vaiont slopes. Such technical evaluations and comparisons require detailed knowledge of the Vaiont Slide, its geology, and the geotechnical evaluations made prior to and following the slide. If the engineers cannot give a reasonably complete and consistent explanation for the Vaiont Slide, in terms of currently available stability analyses, then it is difficult to see how they can feel confident about their comparative evaluation of other reservoir slopes. The disturbing aspect of previous reviews of the Vaiont Slide is that there are gross inconsistencies when the field data, slide behavior, and the results of analyses are compared. This report describes the efforts to confirm the existence and nature of clay seams in the slide mass and to confirm the possible existence of an "old" slide at the site. These efforts were made by (A.) firsthand field observations of the geology, (B.) an examination of preslide and postslide airphotographs, (C.) laboratory testing of samples of failure plane materials, and (D.) an examination and translation of geologic and other documents related to preslide and postslide conditions. Stability analyses of the Vaiont Slide are presented in the report which are relatively consistent with all the observed facts. The study confirmed that the Vaiont Slide was a reactivation of an old slide. The slide moved upon one or more clay layers which were continuous over large areas of the surface of sliding. Three-dimensional stability analyses were required due to the magnitude of the upstream inclination of the clay layers forming the base of the slide. The angle of shearing resistance of the clay layers was determined to be about 12 degrees. The fluid pressure distributions used were consistent with the only piezometric data available before the 1963 slide and with an interpretation of the local groundwater flow system including the presence of karstic terrain above the slide. Results of the analyses completed for key periods in the history of the slide agree with the known slide behavior during these periods. The results also indicate that the reduction in the factor of safety caused by reservoir filling alone was approximately 12 percent, while the reduction caused by rainfall or snowmelt ranged from 10 to 18 percent. Correlations made between cumulative precipitation, reservoir levels, and slide movement records provide a well-defined "failure" envelope. These results explain why the slide was unstable at a given reservoir level and later stable at the same level. Conclusions from these correlations are consistent with the results of the stability analyses. The results of the study also suggest that the slide could have been stabilized by drainage.
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