Effect of partial saturation on the stability of retaining diaphragms above the water table

Granular ‘cohesionless’ soils above the water table are partially saturated but are commonly assumed to be dry in geotechnical practice. Accordingly, ‘drained’ shear strength is calculated by replacing the ‘saturated’ effective stress with the total stress. The ‘dry soil’ assumption neglects the effect t of suction on shear strength and, as a result, geo-structures are over designed. To investigate the implications of this assumption, this paper presents an approach to design retain-ing diaphragms in unsaturated soils above the water table. This approach is based on the lower bound theorem of plasticity. As an example, the embedment depth of retaining diaphragms in ‘cohesionless’ granular soils under unsaturated conditions is calculated and compared with the solution obtained from the classical ‘dry’ ap-proach