The role of seismic intensity on the performance of caisson foundations supporting bridge piers: preliminary results from dynamic centrifuge

Seismic performance of caisson foundations supporting bridge piers may take advantage of soil inelastic response when subjected to strong seismic events, thanks to the soil nonlinear and hysteretic behaviour. This can bring to a substantial optimisation in caisson design and major cost savings. In the framework of Capacity Design extended to geotechnical systems, temporary attainment of plastic mechanisms may be permitted provided that the resulting permanent displacements are lower than given threshold values, which in turn depend on the considered limit state and performance level required to the structure. Clearly, this new design approach needs to be validated against physically-sound numerical and experimental simulations. A campaign of dynamic centrifuge tests was therefore recently carried out at the Schofield Centre, University of Cambridge, where the seismic performance of caisson foundations was assessed. In this paper, a preliminary interpretation of the experimental results is given, shedding some light on the interplay between seismic intensity and mechanical soil properties. Specifically, the results obtained in two tests are discussed, where a caisson-pierdeck system was subjected to earthquakes of increasing intensity. In the two tests, a soft and very soft clay layer was reproduced, to either avoid or promote the plastic soil behaviour. It is shown that the highly nonlinear and hysteretic response of the very soft clay limits the inertial forces transmitted to the superstructure, thus validating the above-mentioned design approach. The beneficial effect of inelastic soil behaviour entailed permanent displacements increasing with earthquake intensity, which should be checked against limit state prescriptions.