Experimental and Theoretical Study on Passive Earth Pressure: Effect of Backfill Width

This study combines model tests and a theoretical solution to investigate the influence of backfill width on passive earth pressure. The results indicate that backfill width has a negligible influence on the displacement threshold for attaining the passive limit state, but strongly governs the magnitude of the mobilized passive thrust. Both the experimental and theoretical results demonstrate that narrowing the backfill intensifies soil-wall interaction and leads to a nonlinear increase in total passive thrust. Using node-based time-series analysis, the initiation of shear strain localization is detected before the peak total passive thrust. In narrow backfills, shear bands develop within the soil mass and propagate along the adjacent interface, while earth pressure distributions shift from linear to superlinear as the backfill width decreases, which is attributed to interface-friction-induced soil arching. This interpretation is supported by the transition from linear to superlinear earth pressure distributions with decreasing backfill width, together with the principal stress redistribution and coupled shear band propagation along soil-structure interfaces observed using PIV measurements. Compared with existing theoretical solutions, the proposed method demonstrates reliable accuracy in predicting passive earth pressure under non-limit states, and provides a practical tool for designing of retaining structures with narrow backfills.