A semi-analytical approach for estimating active earth pressure of unsaturated narrow backfills considering strength nonlinearity and arching effects

Active earth pressure theories often ignore matric suction in unsaturated backfills behind retaining walls, yet suction enhances soil arching and alters pressure distribution, especially in narrow spaces. This study extends unsaturated soil shear strength models to soil–structure interfaces, validates them with experimental data, then develops an improved horizontal slice method using arch-shaped elements and two suction profiles to account for infiltration. A semi-analytical solution for wall rotation about the base (RB) is obtained via the fourth-order Runge–Kutta method. The framework agrees well with numerical simulations, experiment data, and existing theories. Parametric studies show: suction reduces active pressure, producing S- or M-shaped profiles; its effect fades beyond a critical change rate; pressure depends on suction type and soil properties; and the critical aspect ratio varies with displacement mode, suction, and strength model. These findings underscore the importance of unsaturated soil mechanics in retaining wall design, particularly for narrow backfills.