FREE-VIBRATION OF EMBEDDED FOUNDATIONS - THEORY VERSUS EXPERIMENT - CLOSURE

Comprehensive experimental results of 54 free-vibration tests on model footings embedded at various depths in moist dense 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 in this paper to verify the homogeneous-half-space solutions given in a companion paper published by Gazetas in 1991 in the form of algebraic formulas and graphs for dynamic stiffnesses and damping factors. Particular attention is accorded to selecting appropriate values of the effective S-wave velocity, using either Erden’s original measurements and recommendations of 1974 or the measured response of the corresponding surface footings. It is demonstrated beyond significant doubt that: (1) The lower value of shear modulus around the footing 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, primarily at the vertical interface between the sand and the high-damping soft boundary layer, are responsible for at least 75% of the observed discrepancies, having essentially eliminated the contribution to radiation damping from the swaying component of motion.