Numerical Evaluation of Forces on Piled Bridge Foundations in Laterally Spreading Soil
The response of piled bridge foundations to liquefaction-induced lateral soil deformation is an important design consideration in seismically active regions. Recent research and case history data suggest that three-dimensional deformation of the approach embankment can significantly influence the loads placed on the embedded foundations during a flow failure or lateral spreading event. For example, the 2010 Maule earthquake in Chile caused widespread lateral spreading in the soil surrounding the Mataquito river bridge, however, only insignificant structural damage was observed in the bridge itself. The discrepancy between the amount of soil deformation and structural damage suggests that design procedures for this load case that do not make adequate consideration for 3D soil deformation mechanisms may lead to overly conservative and expensive design solutions. Finite element models of the Mataquito river bridge are created using the OpenSees computational framework to investigate the reduction in foundation loads during lateral spreading implied by the minimal structural damage at the site. These models include beam on nonlinear Winkler foundation models, dynamic effective stress models of the bridge-foundation-soil system in plane strain, and 3D models of the southern bridge abutment, approach embankment, and surrounding soil. This numerical work focuses on the development of efficient element formulations and appropriate mesh configurations to minimize computational effort, and seeks to frame the load reduction mechanisms in the context of a simplified analysis procedure for the lateral spreading load case. The results of the numerical models for the Mataquito bridge, along with a parameter study conducted using a second set of 3D finite element models, indicate that consideration for the 3D geometry of the bridge site results in tangible reductions in foundation bending demands and abutment displacements compared to those returned by a plane strain description of the problem. This reduction increases as the depth of the liquefiable layer and the effective width of the approach embankment are decreased. An approach is proposed to estimate the reductions in abutment displacement and associated foundation bending demands for a given site geometry, and an existing simplified analysis procedure is modified to better consider the findings of this work.
