Dynamic Stiffness of Circular Foundations

A series of parametric studies have been presented investigating the effects of internal soil damping, Poisson's ratio, layer depth, and embedment on the stiffness functions of circular footings subjected to dynamic forces. The effect of having a finite layer of soil on rigid rock is to introduce valleys in the stiffnesses at the resonant frequencies of the stratum. These valleys are smoothed by the presence of internal damping and their position depends on the value of Poisson's ratio. Embedded foundations have an increased static stiffness, but the frequency variations of the stiffness coefficients is not very different from the corresponding curve for surface footings. The most important factor in reproducing adequately the effect of embedment is the evaluation of the static stiffnesses. They are, however, very sensitive to the assumed conditions at the vertical edges (sidewalls welded to the surrounding soil, degree of disturbance of the backfill, etc.). Experimental work to assess these conditions is necessary.