Failure Mechanism and Active Earth Pressure of Narrow Backfills behind Retaining Structures Rotating about the Base

A series of model tests were conducted to simulate the active failure of narrow cohesionless backfills behind retaining walls rotating about the base (RB mode). The tests aimed to investigate the effect of wall displacement magnitude, backfill widths, and the inclinations of retaining walls and existing structures on the failure mechanism and earth pressure. The test results revealed that the rupture propagation follows a progressive top-down failure pattern and does not extend to the base of the wall under RB mode, contrasting with the assumptions of the existing theoretical solution. Notably, a narrower backfill exhibited multiple parallel shear bands in contrast to the semi-infinite backfill, highlighting the significant impact of backfill geometry on the orientation of these shear bands. Furthermore, the active earth pressure distribution under RB mode displayed an approximately linear trend, slightly reducing earth pressure near the base. The development of earth pressure suggested that the backfill reached the active limit state after the wall had experienced a displacement equal to 0.35% of its height (H). It was observed that the active earth pressure for a backfill width-to-height ratio (B/H) of 0.5 closely corresponded to values obtained through the Coulomb method. Moreover, the results indicated that the active earth pressure increased proportionally with an increase in the B/H ratio and a decrease in the inclinations of both the retaining structures and existing structures.