Abstract: To consider the effect of nonlinear changes of the shear strength of unsaturated soil in the two dimensions of net normal stress and matric suction on the passive earth pressure, a calculation method for the passive earth pressure on the back of retaining walls for unsaturated backfill is proposed based on the nonlinear shear strength envelope shell and the classical Rankine earth pressure theory. First, employing the generalized Mohr-Coulomb failure criterion, we applied the variable tangent technique to determine equivalent cohesion, equivalent friction angle, and their correlations at arbitrary points on the envelope. Subsequently, through iterative analysis combining Mohr’s circle stress states, we determined the equivalent cohesion and friction angle under passive limit equilibrium conditions. Finally, we derived Rankine passive earth pressure expressions for unsaturated soils that maintain formal consistency with classical soil mechanics formulations. The method’s reliability was validated through comparative case studies with established models. Parametric analysis revealed that increasing matric suction and overburden pressure enhances equivalent cohesion and passive pressure, while reducing equivalent friction angle. In the case study, the envelope shell model yielded lower passive earth pressures than the plane model at low matric suction levels, but higher pressures at elevated suction levels.

Key words: unsaturated soil, Rankine passive earth pressure, shear strength envelope shell, generalized Mohr-Coulomb criterion, soil-water retention curve (SWRC)