Free Vibration of Embedded Foundations: Theory versus Experiment

Abstract

Comprehensive experimental results of 54 free‐vibration tests on model footings embedded at various depths in sand and having circular, square, and rectangular base shapes were reported by Erden and Stokoe in 1974 and 1985.The results for vertical and coupled swaying‐rocking oscillations are used herein to verify the homogeneous‐halfspace solutions of the companion paper by Gazetas (1991) in the form of algebraic formulas and graphs for dynamic stiffnesses and damping factors. Particular attention is accorded to selecting appropriate values of S‐wave velocity, using either Erden's original recommendations of 1974 or the measured response of the corresponding surface footings. Close agreement is found between theory and experiment for vertical oscillations (in both frequency and damping values) and for swaying‐rocking (only in frequency values). Measured swaying‐rocking damping averages only half the theoretical estimates. It is demonstrated that: (1) The lower value of modulus around the sides and slippage at the footing side‐soil interface may have contributed to not more than 25% of the shortfall in the experimental values; and (2) boundary wave reflections, at the interfaces between sand and high‐damping soft boundary layer are responsible for at least 75% of the observed discrepancies, having essentially eliminated the damping contribution of the swaying component of motion.