Effects of Complex Surface Conditions on the Seismic Response of Caisson Foundations

Soil-structure interaction phenomena under seismic loads can be studied by means of several approaches, including full dynamic models that require the use of 2D and 3D FE techniques. Through the latter, it is possible to consider and investigate the effect of the three-dimensionality of the ground on the response of the foundation. In this work, we analyzed the response of caisson foundations for an ideal viaduct, subjected to a seismic load, when the viaduct is located over topographical hollows, simulating configurations that are encountered very frequently in reality (e.g., for the presence of an incise watercourse channel). First, a 3D prototype model was prepared, representative of recurrent conditions mirrored by real-world case studies, adopting simplified conditions of a single caisson and horizontal surface. Then, four different 3D models were considered, reproducing topographical convergences, progressively increasing the lateral slopes’ steepness (5°, 10°, 15°, and 20°). The presence of a topographic convergence can produce an amplified seismic acceleration, both at the top of the foundation, both in free-field conditions. The results illustrated that the maximum displacements at the caisson top increase with the increase in the hollow’s lateral slope angle. More specifically, going from the horizontal surface case to the case of convergence with 20° of lateral slope angle, the increase is up to 25% in the vertical displacement, while the horizontal displacement increases up to 4 times the one predicted in planar conditions.