Multi-Scale Experimental Investigation of Soil-Abutment Interaction in Integral Bridges Under Cyclic Thermal Loading

Integral Abutment Bridges (IABs) are widely adopted in modern infrastructure due to their construction simplicity and reduced maintenance relative to ordinary bridges. However, their long-term performance remains uncertain due to the complex soil-structure interaction (SSI) effects under seasonal cyclic thermal loading. This study investigates the evolution of lateral earth pressures and backfill densification through a coordinated multi-scale (large and small) experimental programme and numerical modelling. Small-scale 1g tests and large-scale physical experiments were conducted at the University of Bristol’s Soil Foundation Structure interaction laboratories (SoFSI) and the Earthquake and Large Structures (EQUALS) facilities, simulating thermal expansion and contraction over up to 120 cycles. Results reveal a progressive increase in lateral earth pressure, driven by cyclic densification of the backfill—particularly near rigid abutments. Peak pressure locations were observed to shift with loading cycles, closely associated with local compaction patterns captured using Ground Penetrating Radar (GPR) and Particle Image Velocimetry (PIV). Complementary numerical simulations using different computational platforms (PLAXIS, ABAQUS, and OpenSees) reproduced key behavioural trends and confirmed the critical influence of abutment stiffness, displacement amplitude, and cumulative loading. Compared with static predictions from analytical methods and design guidelines (e.g. PD6694-1), the herein reported experimental and numerical results demonstrate significantly higher lateral pressures, underscoring the limitations of current design assumptions. By bridging small-scale precision, large-scale realism, and numerical insight, this study advances the understanding of SSI in IABs under realistic thermal loads. The findings provide a foundation for improving design practices to ensure the long-term resilience and reliability of integral bridges.