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CSMIP 93-03

"Investigation of the Response of Cogswell Dam in the Whittier Narrows Earthquake of October 1, 1987"

by R. Boulanger, R. Seed and J. Bray

December 1993, 53 pp.

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The Whittier Narrows Earthquake of October 1, 1987 (ML ≈ 5.9) shook Cogswell Dam, a concrete faced rockfill dam, which had previously been instrumented as part of the California Strong Motion Instrumentation Program (CSMIP). The maximum recorded crest acceleration was 0.15 g. The resulting recorded accelerograms provided a valuable opportunity to investigate and evaluate the accuracy and reliability of conventional geotechnical procedures for evaluation of dynamic response characteristics of earth and rockfill dams. In particular, the recorded accelerograms provided a rare opportunity to investigate the in-situ dynamic properties of rockfill materials. Presented in this report are the results of dynamic analysis studies of the response of Cogswell Dam to the 1987 Whittier Narrows Earthquake and the subsequent back-calculated estimates of the in-situ dynamic properties of the rockfill materials which comprise Cogswell Dam. Dynamic moduli and damping factors back-calculated from this valuable field case history were found to be in good agreement with similar values back-calculated from other recent case histories. Dynamic modulus degradation relationships, as a function of shear strain, were found to be better modelled by recent relationships proposed specifically for gravelly soils (Seed et al., 1984), than by the heretofore more widely used relationships proposed for sandy soils (e.g. Seed and Idriss, 1970, Seed et al., 1984). In addition to providing an opportunity for evaluation of in-situ dynamic properties of rockfill, this case study also provided an opportunity for evaluation of the ability of two-dimensional (plane strain) dynamic finite element analyses to model the fully three-dimensional behavior of a relatively tall dam in a narrow, steep-walled, V-shaped canyon. The results of these analyses suggest that three-dimensional effects are significant for this type of geometry, and that these are not well modelled by two-dimensional analysis methods.