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by J. Bray, R. Seed and R. Boulanger
December 1993, 60 pp
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Abstract
The Whittier Narrows Earthquake of October 1, 1987 (ML ≈ 5.9) shook
Puddingstone Dam, a primarily cohesive, homogeneous section, compacted
earth dam which had previously been instrumented as part of the California
Strong Motion Instrumentation Program (CSMIP). The resulting maximum
(transverse, horizontal) crest acceleration was 0.19 g, and maximum
accelerations recorded at abutment stations were on the order of 0.04
to 0.08 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. Presented in this report
are the results of dynamic analysis studies of the response of Puddingstone
Dam to the 1987 Whittier Narrows Earthquake performed using both (a) simple
one-dimensional columnar analyses, and (b) two-dimensional (plane strain)
dynamic finite element analyses. Nonlinear, strain-dependent dynamic shear
moduli and damping characteristics were modelled using the "equivalent
linear" method. Nonlinear modulus degradation and damping relationships
for the compacted sandy silty clay which comprises a majority of the
embankment were modelled based on the relationships proposed by Sun et al.
(1988) for clays of low plasticity. The results of the two-dimensional
finite element analyses were found to be in good agreement with the observed
(recorded) field response, providing good support for these modelling and
analysis techniques. The simpler, one-dimensional columnar analyses were
found to significantly underestimate the crest response, as a result of
their inability to model geometric effects or topographic amplification,
but were also found to produce reasonably good agreement between calculated
and recorded response at the center of the downstream face.
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