Progressive failure of backfill behind the integral bridge abutment due to seasonal temperature variation

Seasonal expansion and contraction of integral abutment bridges (IABs) impose cyclic abutment movements on the backfill, potentially causing progressive soil failure. This study conducted model tests integrating particle image velocimetry and particle tracking velocimetry to investigate backfill behavior under cyclic abutment movements. By monitoring the earth pressure-deformation response and soil particle kinematics, and by comparing with separate monotonic loading tests under active and passive conditions, the failure evolution and mechanisms were revealed. Results show that under cyclic abutment movements, the backfill undergoes an evolution from initial strain localization to the formation of multiple failure surfaces. A three-stage progressive failure process (i.e., active failure, alternating active and passive failure, and failure surface migration stages) was identified. Compared with monotonic tests, cyclic perturbations induce uneven settlement, earth pressure ratcheting, and failure surface evolution, with volumetric fluctuating as cycles increase. Larger amplitudes accelerate the backfill failure development, while the long-term influence of movement paths remains negligible. Backfill failure under cyclic abutment movement arises from the accumulation of residual particle displacements, mesoscopically manifesting as a vortex-like soil flow from active to passive zone. These findings provide mechanistic insights into mitigating earth pressure ratcheting and excessive ground deformation in IAB backfills.