Slip line theory extended to unsaturated soils and applied to the retaining wall-unsaturated soil interaction problem

Analyses of retaining walls usually assume dry or saturated soils. However, walls in practice retain unsaturated soils with an internal suction and which exhibit higher strengths. This paper presents an extension of the slip line theory to unsaturated soils. A term accounting for suction is introduced into the governing stress equilibrium equations. A displacement slip line field is obtained from its stress equivalent by assuming coaxility of stress and increment of strain and the soil dilating at a constant ratio of volumetric strain increment to shear strain increment. The theory is applied to a boundary value problem of a rigid retaining wall rotating about its toe into unsaturated soils. The influence of suction on limiting passive stress and soil displacement is studied. The results show clear differences between steady state evaporative and infiltrative conditions.