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

Abstract

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