Strain ratcheting mechanisms in the backfill of integral bridges

Integral bridges use a jointless and bearingless design to minimise long-term maintenance. Interaction between the abutments and backfill due to thermal movements of the deck causes an accumulation of strain and an associated increase in earth pressure over time, with the mechanisms underlying this increase not being well understood. This study aimed to elucidate strain ratcheting behaviour through centrifuge tests in which various thermal movement magnitudes, backfill densities, and loading histories were simulated. The results suggest that the evolution of earth pressure depends upon backfill densification and dilation, driven by settlement at the backfill–abutment interface. Other findings include a logarithmic relationship between deck axial force and thermal cycles, the ability of foundation sliding to limit earth pressures, and the negligible influence of daily movements on the ratcheting response. A comparison of available modelling and field data was carried out based on these results, from which physical modelling was shown to justify the limited earth pressure increase observed in the field due to small backfill shear strains. Furthermore, from an overestimation of the design temperature range and thermal movements reported from three bridges, it was found that using temperatures with a low return period offers a means to reduce over-conservative design.