Experimental Evaluation and Design of Narrow Backfill Retaining Walls under Combined Sliding and Rotational Mode

This study investigates the active earth pressure behaviour and design implications for retaining walls constructed under narrow backfill conditions. Large-scale physical model tests are conducted to evaluate wall response under two distinct movement modes: rotation about the base and horizontal sliding. The active earth pressure coefficient (Ka) and normalized point of application of thrust (ys) were measured for aspect ratios (b/H) ranging from 0.1 to 0.7. Results show that Ka decreases from 0.28 to 0.14 in the rotation mode and to 0.16 in the sliding mode. The point of application of thrust shifts from the classical H/3 to approximately 0.4H at b/H = 0.2, indicating a nonlinear pressure distribution. A combined-mode design methodology is proposed wherein Ka from sliding is used for checking sliding stability, while Ka from rotation is applied for overturning and bearing capacity checks. This approach reduces the required base width from 0.49H at b/H = 0.7 to 0.33H at b/H = 0.1, achieving a 32.7% reduction while satisfying all safety requirements. Overturning FOS decreases from 3.65 to 2.73, and the bearing FOS from 3.3 to 2.52 as b/H reduces from 0.7 to 0.1. For b/H > 0.3, the safety factors largely align with those from the sliding-based design. For b/H < 0.3, the values gradually approach those predicted using only the rotation-mode. These findings highlight the limitations of classical design theories in narrow backfills and demonstrate that incorporating both movement modes yields a more rational and efficient design framework.