Characterization of Rocking Shallow Foundations on Cohesive Soil Using Field Snap-Back Tests

Abstract

This paper presents a series of field snap-back experiments of a soil-footing-structure system equipped with rocking foundation on a cohesive soil. The objective was to investigate the nonlinear dynamic behavior of the rocking system subjected to particularly pulse-like motions. During the snap-back tests, an initial drift ratio was applied to the deck, at a maximum of 8.5%, and then released using a quick release mechanism to enable the free vibration of the system. Effects of initial factor of safety against the bearing failure, initial drift amplitude, and snap-back directions on the dynamic properties of the rocking system were investigated. Test results show that the rocking moment versus footing rotation is highly nonlinear and the moment capacity can be well predicted on cohesive soil. The shear capacity of footing did not significantly change with the number of trials or amplitude of initial drift. The damping ratio observed during the oscillations after snap-back release of the shallow foundations ranged from 8% to 30%. The average measured period of the rocking system was elongated by approximately 235% compared with the period of a fixed-base structure. An acceptably small residual settlement was observed even at a high cumulative rotation. The rocking system on clay exhibited a good recentering ability, which is even better than on sand. Finally, the increase in the density and shear strength of soil beneath the footing due to rocking cycles was observed.