Interaction of caisson foundations with a seismically rupturing normal fault:Centrifuge testing versus numerical simulation

Dramatic failures have occurred in recent earthquakes as a result of the interplay of surface structures with out-cropping fault ruptures, highlighting the need to account for fault-induced loading in seismic design. Current research into the mechanisms of fault rupture-foundation-structure interaction has revealed a potentially favourable role of caissons in comparison with other foundation types. This paper explores the mechanisms of normal fault rupture interaction with rigid caisson foundations, with an integrated approach using both experiments and analysis. A series of centrifuge model tests were first conducted to study the response of a square (in plan) caisson foundation of dimensions 5m X 35m X 310m, founded on a 15 m thick layer of dry dense sand. A nonlinear three-dimensional numerical simulation of the problem was then developed, and adequately validated against centrifuge test results. Depending on its position relative to the fault, the caisson interacts with the fault rupture, sometimes modifying spectacularly the free field rupture path. Acting as a kinematic constraint, the caisson 'forces' the rupture to divert on either one, or both, of its sides. The numerical study was subsequently extended to gain further insight into the effect of the exact position of the caisson relative to the fault outcrop. Different mechanisms taking place for different caisson positions are identified, and their effect on the response of the system is discussed