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CSMIP 94-03

"Analysis of the Recorded Response of Lexington Dam during Various Levels of Ground Shaking"

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.