Estimating Vibration Response of East Canyon Dam, Utah, From P-, S-, and Surface-Wave Measurements

Compressional and shear reflection, compressional and shear VSP/check shot, compressional<br>refraction tomography, and multi-channel analysis of surface waves (MASW) techniques were evaluated<br>and determined effective and accurate in defining and delineating the seismic wave velocity structure of<br>rocks supporting a thin-arch cement dam in north-central Utah. A reliable measure of seismic properties<br>as a function of depth is important to the comprehensive and accurate appraisal of site response and<br>vibration modes in concrete dams. Models used to predict dam performance during earthquakes are only<br>as realistic as the material attributes incorporated into those simulations. Proven correlation between<br>seismic properties and stiffness/rigidity is the basis for highly detailed measurements of the seismic<br>wavefield at this dam site. Optimal 30-fold CMP seismic reflection profiles provided images from<br>within the massive conglomerate supporting the dam. The conglomerate possessed bedding plains<br>dipping upward of 20 degrees and visible fractures both along bedding plains and at right angles to<br>dominant bedding surfaces. Of particular interest was the right abutment of the dam, which was not only<br>most accessible but the strongest influence on the dam performance during ground shaking events. VSP<br>profiles through both the cement dam and downstream toe provided excellent velocity control and<br>identified changes in rock types within the first 200 ft below ground surface. Calculations of Poisson’s<br>ratio based on continuous, detailed, coincident measurements of compressional and shear-wave<br>velocities at each significant geologic contact and within each major geologic unit were critical to<br>realizations of site response at this site where failure potential is known to exist.