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<br />. <br /> <br />. <br /> <br />(2) Concur with the recommendation that <br />pressure relief wells could provide a safe exit <br />gradient. NOTE: Coombs used the words "could <br />provide a safe exit gradient." What if it doesn't? <br /> <br />(3) Two-3 dimensional models are not suf- <br />ficient to provide a detailed description to judge <br />the predictive capability of the models used by <br />the Bureau. NOTE: Coombs was not satisfied with <br />the description of the model used by the Bureau <br />to determine seepage. . <br /> <br />(4) A 3 dimensional model should be used <br />to determine the beneficial effects of a cutoff <br />wall operating simultaneously with pressure relief <br />wells. NOTE: This has not been done as yet and <br />therefore there is no way to predict whether <br />pressure relief wells will handle the seepage <br />around the right end of the slurry trench to avoid <br />inadequate Bureau design which has heretofore <br />contributed to dam failure. The TetOn Dam failure <br />should be a warning. "'" <br /> <br />(5) Coombs commented on the fact that the <br />permeability values in the lower alluvial sands <br />and gravel are quite high. He felt that the value <br />should be checked and that if the values were <br />lower, the hydraulic gradient in the buried channel <br />would be higher, resulting in greater uplift <br />pressure. NOTE: This demonstrates that Coombs <br />has reservations about the accuracy of the data <br />provided to him regarding the permeability of <br />the lower alluvial sands. Uplift pressure is <br />a well known cause of earth-fill darn failure. <br /> <br />Again, it should be emphasized that Coombs did not <br />talk about the cost of using pressure relief wells and other <br />safety measures mentioned in his report. <br /> <br />Thomas M. Leps, Inc. had the following comments to <br />make regarding safety aspects of the feasibility stage <br />design of the proposed Narrows Dam: <br /> <br />a. Leps clearly defined the scope of his review <br />as follows: "In the context intended, the assessment <br />is restricted to the principal atructural and geotech- <br />nical features of the proposed cross-section of the <br />dam, and of its foundation, in regard to their adequacy <br />against possible catastrophic failure due to natural <br />or operational events. Additionally, the assessment <br /> <br />-17- <br />