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by R. Boulanger, R. Seed and J. Bray
December 1993, 53 pp.
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Abstract
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.
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