Effect of structure backfill on stiffness and capacity of bridge abutments

Bridge abutments provide resistance to deformation and earthquake induced inertial forces from the bridge superstructure. In order to limit the inertial forces transmitted into the abutment walls and piles, the abutment walls are designed to be sheared off in major seismic events. Therefore, the force-resistance mechanism of bridge abutments in the longitudinal direction is mainly provided by backwall-soil interaction. Current design practice in California makes use of bi-linear load- deformation curve and does not account for the structure backfill properties. An experimental and an analytical research program were conducted at UCSD to further investigate such structure backfill interaction characteristics. In order to meet the objectives of this research project, a field investigation was conducted to develop a proper characterization of the soil types used for abutment structure backfills. The experimental program included five large-scale tests. In the first phase of the experiment, an abutment wall (without a foundation) was built at 50% scale of a prototype diaphragm abutment. The second phase of this research program was performed on a backwall sheared off from wingwalls and stem in seat-type abutments. The specific aims of the experimental program were to examine the effect of structure backfill soil type, backfill height, vertical movement of the wall, and pre- existing cut slope in backfilling on stiffness and capacity of the abutments in the longitudinal direction. An analytical model was developed for evaluating the response of bridge abutments loaded longitudinally. The approach involves calculating maximum passive resistance of the structure backfill material, and creating p-y curves to predict the force-displacement relationship of longitudinally loaded bridge abutments. The finite element program Plaxis 2D was used to model the abutment wall experiments. The procedure was validated by comparing the finite element results with the experimental results. In conclusion, the study indicates that the response of bridge abutments in the longitudinal direction is nonlinear and a function of several factors which need to be considered. The passive resistance of the structure backfill is controlled by the soil shear strength and the interface friction angle. Finally, the vertical movement of the wall has a significant effect on post-peak behavior of abutments