Analysis of the Safety and Performance of Scott Dam Using FLAC

This paper describes the methodology used to model the behaviour of Scott Dam and its foundation corresponding to a rising reservoir. The laboratory testing for Scott Dam had ascribed to the melange foundation the unusually low modulus of 20,000 psi. The rock mass deformability, evaluated by comparing the response of the model to instrumental measures of Scott Dam’s tilting and deformation under the load increment produced by flood conditions, was found to be fifteen times greater, but still unusually soft for a rock foundation. We wished to learn how the softness of the foundation might alter conventional concepts about sliding stability, particularly since the contact surface was very rough such that failure could not occur by any simple rigid-block sliding mechanism. A method of calculating the reserve of safety conforming to stability evaluations in rock masses led to the conclusion that the dam had a sufficient margin of safety under all conceivable loading conditions. Particularly germane to this was the finding tha:t the concrete dam would not debond from its foundation under the heel, as might have been expected had the foundation been harder. Introduction Scott Dam, on the Eel River, California, has been the object of numerous safety evaluations since its construction in 1920/21. An overview of its difficulties and engineering studies was presented in companion papers by Goodman, Ahlgren, and Lindquist, and McManus & Bro in these same Proceedings. Additionally, a paper by Hendron 6~ Hendron evaluates the traditional factor of safety for the dam and the implications for dam safety. * Senior Engineer, Jacobs Associates, San Francisco **Consultant, Mendocino, CA, and Emeritus Professor, Univ. Calif. Berkeley