Centrifuge testing of bridge piers on caisson foundations subjected to strong ground motions

Consideration of the instantaneous bearing capacity of caisson foundations supporting bridge piers and subjected to strong ground motions can lead to a substantial optimisation in caisson design and major cost savings. In the framework of the newly-emerging paradigm of Capacity Design extended to geotechnical systems, temporary attainment of limit conditions may be permitted provided that the resulting permanent displacements are smaller than a given threshold value. To validate this new design approach, the seismic performance of caisson foundations was assessed through dynamic centrifuge tests performed on reduced-scale models at the Schofield Centre, University of Cambridge. In this paper, some results obtained in two out of the three tests that were carried out are presented, where a caisson-pier-deck system embedded in a typical alluvial deposit was subjected to a series of earthquakes of increasing intensity. In the two tests at hand, a soft and very soft clay layer was reproduced, so as to either avoid or induce the attainment of the plastic soil behaviour. It is shown that both yielding and failure of the layer of very soft clay limit the inertial forces transmitted to the superstructure, thus validating the design approach which has been taken as a reference in this study and in previous numerical analyses. However, the beneficial effect of soil inelastic and hysteretic behaviour implies the accumulation of permanent rotation of the pier, which increases with earthquake intensity