Basal anti-heave stability of foundation pits in unsaturated soils

It is known that the soils surrounding a foundation pit are commonly in an unsaturated condition and the displacement of a retaining wall routinely rotates around wall bottom (i.e., RB displacement mode). The bearing capacity of strip foundations at wall bottom with a single-sided sliding surface and the vertical anti-sliding force of wall-soil system under RB displacement mode with an overall down-sliding are first derived in the framework of the two-stress state variable theory for unsaturated soils. For uniform and linear suction profiles, the limit equilibrium method is then adopted to provide a solving process on the safety factor against basal heave of foundation pits in unsaturated soils, respectively. Finally, based on the verifications of the theoretical solutions reported in the literatures with an engineering case of foundation pits, the influences of different wall displacement modes are explored. The results show that the proposed safety factor against basal heave of foundation pits accounts for unsaturated characteristics of soils and RB displacement mode of a retaining wall. The proposed safety factor compares reasonably well with the available theoretical solutions of both the foundation bearing capacity model and the limit upper bound approach, and it can be naturally degraded to that of saturated soils. The vertical anti-sliding force and the safety factor against basal heave of foundation pits under RB displacement mode of retaining walls are smaller than those under an in-situ state. The safety factor against basal heave of foundation pits is conservative for simplified calculation of unsaturated soils taken as saturated soils ignoring unsaturated strength characteristics.