Safety of a foundation under seismic loading is strongly dependent on the inertial forces transmitted by the superstructure, exchanged with the surrounding soil and acting into the foundation itself. The latter contribution, typically neglected for shallow and pile foundations, should be considered for caisson foundations, much more massive and rigid than the foundation soil.
Substructure method is widely used to evaluate the seismic performance of caisson foundations supporting bridge piers subjected to strong ground motions, mainly because of its simplicity. However, the strongly-simplifying assumption of linear viscous-elastic behaviour for the foundation soil limits its applicability to flexible systems subjected to low-intensity earthquakes, for which irreversible strains and pore water pressure build-up are not anticipated.
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