by F. Makdisi, C. Chang,
Z. Wang and C. Mok
March 1994, 60 pp.
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The recordings at Lexington Dam, due to at least three different levels of shaking, provided
excellent data for examining the validity of commonly used dynamic analysis procedures as well
as examining nonlinear stress-strain behavior of the dam material due to earthquake shaking.
The properties of the embankment materials including cross-hole shear wave velocity measurements
were well documented during earlier safety evaluation studies.
Amplification ratios of peak ground acceleration between the crest and abutment rock recordings
decreased with increasing levels of ground shaking. For abutment rock acceleration levels of
about 0.03 to 0.04 g, the amplification ratio is about 3 to 4; at acceleration levels of about
0.l g, the amplification ratio is about 1.5 to 2, and at acceleration levels of about 0.4 to
0.45 g, the amplification is about 1.0.
This report summarizes the results of Fourier spectral analyses of the recordings at the dam
crest and rock abutment for the three earthquakes described above; and one- and two-dimensional
dynamic response analyses to evaluate the nonlinear strain-dependent behavior of the embankment
materials at various levels of earthquake shaking. Response and Fourier spectral ratios (crest
to abutment) of the recorded ground motions showed a definite shift in the fundamental natural
period of the embankment with increased level of shaking. Two-dimensional finite element analyses
of the embankment response provided predictions of motions at the crest of the dam that are in
reasonable agreement with recorded motions. The results of the finite element computations showed
a non-linear strain dependent behavior of the embankment materials for levels of shaking
associated with the Loma Prieta earthquake.