Bridge-Pier Caisson foundations subjected to normal and thrust faulting: physical experiments versus numerical analysis

Surface fault ruptures can inflict serious damage to engineering structures built on or near them. In the earthquakes of Kocaeli, Chi-chi, and Wenchuan a number of bridges were crossed by the emerging normal or thrust faults suffering various degrees of damage. While piles have proved incapable of tolerating large displacements, massive embedded caisson foundations can be advantageous thanks to their rigidity. The paper explores the key mechanisms affecting the response of such bridge foundations subjected to dip-slip (normal or thrust) faulting. A series of physical model experiments are conducted in the National Technical University of Athens, to gain a deeper insight in the mechanics of the problem. The position of the caisson relative to the fault rupture is parametrically investigated. High-resolution images of the deformed physical model is PIV-processed to compute caisson displacements and soil deformation. A novel laser scanning technique, applied after each dislocation increment, reveals the surface topography (the relief) of the deformed ground. 3D finite element analyses accounting for soil strain-softening give results in accord with the physical model tests. It is shown that the caisson offers a kinematic constraint, diverting the fault rupture towards one or both of its sides. Depending on the caisson's exact location relative to the rupture, various interesting interaction mechanisms develop, including bifurcation of the rupture path and diffusion of plastic deformation.