Full-scale experimental investigation of a modern earth dam

Abstract

An extensive investigation has been made based on the results of full-scale dynamic tests performed on a modern earth dam, Santa Felicia Cam in Southern California. This dam was chosen for the experimental studies because it had been subjected to strong shaking during two earthquakes: the strong, 6.3 local Richter magnitude San Fernando earthquake of 1971, and a 1976 earthquake of magnitude 4.7. The records recovered from these two earthquakes provided usable information on the dynamic characteristics of the dam which was instrumented with motion sensors to yield data on the structural response as well as the input ground motion at the site. In the test programs, various types of dynamic excitations were used including mechanical vibration, ambient vibration, hydrodynamically generated forces, and the two strong seismic ground motions; thus, the imposed dynamic forces varied greatly in their time-history characteristics, spatial distributions, and intensities. For the forced vibration tests, the dam was excited into resonance in the upstream-downstream direction and in the longitudinal direction by a coupled pair of mechanical vibration generators (200 feet apart) capable of producing force up to 10,000 lbs. Symmetric and antisymmetric vibrations were separated by synchronizing the two shakers to run in-phase and 1800 out-of-phase, respectively, with the aid of control units. During the ambient vibration tests, the naturally occurring vibrations of the dam caused by strong wind and the spilling of the reservoir were measured. The test of dam response to hydrodynamic forces involved the use of pressure waves (to impinge upon the upstream face of the dam) originating from a controlled, submerged release of gas under pressure in the reservoir water. During the dynamic tests, three-dimensional measurements of the motions of approximately 25 stations along the crest and seven stations on the downstream slope were recorded and then analyzed in both time and frequency domains. Modes of vibrations and associated natural frequencies as well as damping ratios were determined in the frequency range from 0.0 to 6.0 Hz. The reliability of the existing analytical techniques for earth dams, which are restricted to horizontal shear deformation in the upstreamdownstream direction, was examined. Finally, in order to reveal any change in the dynamic properties of the dam, the dam's natural frequencies, mode shapes, dynamic shear moduli and damping factors (the latter two as functions of the induced strains) estimated from the measured responses to the two earthquakes were compared with those determined from the full-scale dynamic excitation tests.

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