Centrifuge tests to assess the impact of unsaturated soils on seismic bridge performance

Abstract

Rocking shallow foundations have seismic performance advantages over conventional fixed-base foundations; they can limit the inertial load transmitted to the structure as a function of the capacity of the foundation. Although previous experimental campaigns have highlighted the effectiveness of rocking foundations in practice, these have tended to focus on standalone simplified single-degree-of-freedom (SDOF) systems. Structures incorporating rocking foundations, such as bridges, are more complex than these simplified designs. In addition, limited experimental work has been performed to assess the influence of the soil’s degree of saturation on the seismic performance of rocking foundations, with most studies focusing on soil layers in dry or fully saturated conditions. The purpose of this research is to appropriately model and assess the behavior of a prototype bridge system, built to incorporate rocking foundations, and placed on unsaturated sandy silt layers. A series of dynamic centrifuge tests were performed on a single-span bridge with two foundations, subject to seismic motion in the longitudinal direction. The experimental program consisted of tests on dry, saturated, and two mixed saturated-unsaturated soil layers where the water table depth was lowered to a certain depth below the soil layer surface. The soil-structure system was subjected to a suite of scaled earthquake motions of varying intensities while the response was recorded. As the depth of the water table decreased, the settlement experienced by both foundations decreased, becoming less than the settlement experienced when the structure was placed in the dry soil layer. Furthermore, as the depth of the water table decreased, bridge deck drifts were found to reduce. This study highlights the performance of rocking foundations when implemented in bridge structures resting on soils with varying water table depths.