Rigid Retaining Walls with Narrow Cohesionless Backfills under Various Wall Movement Modes

Spaces for backfills are often constrained and limited to a narrow width when retaining walls must be built close to existing stable walls in urban areas or near rock faces in mountainous areas. Estimation of lateral earth pressure acting on the back of the retaining wall has been typically based on classical earth-pressure theories such as Coulomb’s theory or Rankine’s theory, which assumes that the backfill is sufficiently wide to allow the full development of failure planes in the backfill. It has been generally recognized that the classical earth-pressure theories tend to significantly overestimate the lateral pressures for retaining walls with narrow backfill width. However, data from experimental studies are scarce, preventing a close examination of how narrow backfill spaces and various wall movement modes impact the forming of the failure surfaces and the distribution of lateral pressure acting on the retaining wall. In this study, a rigid retaining wall model was custom designed and built to investigate the effects of narrow backfill spaces on lateral pressures and backfill failure under active earth-pressure conditions. A series of tests was conducted on combinations of different backfill widths and wall movement modes [translation (T) mode, rotation about bottom (RB) mode, and rotation about top (RT) mode]. The failure surfaces in the backfill and distribution of lateral pressures acting on the back of the wall were recorded for each of the tests. Results of the tests demonstrated that the failure surfaces were continuous and nonlinear and limited by the width of narrow backfills. It was also found that the mode of retaining wall movement plays a role in forming the failure surfaces and mobilizing the lateral pressures. The coefficient of active pressure K_a based on measured lateral pressure decreases along with the depth. Whereas the RT mode showed much higher K_a values at the upper part of the wall, K_a values for the T mode and RB mode were smaller than those found by Coulomb’s theory. The resultant of lateral pressure for all three wall movement modes was significantly smaller than that of Coulomb’s solution, with its point of application generally higher than one-third from the wall base. A simple and practically useful analytical solution of the lateral pressure based on arching theory is presented in this article and shows a reasonably good agreement with the measured lateral pressure for the translation and rotation about the bottom modes of retaining wall movements.