Bearing capacity of strip footings on unsaturated soils by the slip line theory

The bearing capacity of strip footings on unsaturated soils is studied using slip line theory. The suction profiles considered are non-uniform with depth and correspond to vertical flow of water by infiltration or evaporation. The slip line theory assumes a plastic equilibrium state of a Mohr–Coulomb soil in which suction influences are included using the effective stress concept. This paper shows the similar and independent effects of cohesion and the contribution of suction to the effective stress in the governing equations. It shows that the influence of a non-uniform suction profile on bearing capacity is significant, and the depth to the ground water table and the footing width have significant roles in how much suction influences the bearing capacity. By approximating the contribution of suction to the effective stress as a function that varies linearly with depth, the effect of suction on bearing capacity is represented in dimensionless form in separate charts for smooth and rough footings. Using the charts the bearing capacity can be determined for any combination of friction angle, footing width, surcharge, soil unit weight and linear profiles of cohesion and the contribution of suction to the effective stress. An example is given which highlights the significant influence suction can have. These charts also permit assessment of bearing capacity changes that may occur when changes to suction are expected, due to seasonal fluctuations of soil moisture, drought or flooding.