An investigation has been made to analyze observations of the effect of two earthquakes (with M L= 6. 3 and 4. 7) on Santa Felicia Dam, a rolled-fill embankment located in Southern California. The dam is 236.5 ft. high and 1,275 ft. long by 30 ft. wide at the crest. The purpose of the investigation is: (1) to study the nonlinear behavior of the dam during the two earthquakes, (2) to provide data on the in-plane dynamic shear moduli and damping factors for the materials of the dam during real earthquake conditions, and (3) to compare these properties with those previously available from laboratory investigations.
From the recorded motions of the dam, amplification spectra were computed to indicate the natural frequencies of the dam and to estimate the relative contribution of different modes of vibrations. A comparison between these natural frequencies and those obtained by two elastic shear -beam models was made to obtain representative dam material properties. In addition, field wave-velocity measurements were carried out as a further check as well as to study the variation of shear wave velocity at various depths in the dam. The amplification spectra showed a predominant frequency of 1.45 Hz in the upstream/ downstream direction; in this direction the response was treated as that of a single-degree-of -freedom hysteretic structure. Three types of digital bandpass filtering of the crest and abutment records were used to enhance the hysteresis loops which show the relationship between the relative displacement of the crest with respect to the abutment and the absolute acceleration of the dam. A method is described which, using some of the existing elastic -response theories, enables the shear stresses and strains, and consequently the shear moduli, to be evaluated from the hysteresis loops. The equivalent viscous damping factors were calculated from the areas inside the hysteresis loops. The shear moduli and the damping factors were determined as functions of the induced strains in the dam. Finally, the shear moduli and damping factors obtained for the dam were compared with previously available laboratory data for sands and saturated clays
