Unified Method for Estimating the Ultimate Bearing Capacity of Shallow Foundations in Variably Saturated Soils under Steady Flow

This study presents a unified method for estimating the ultimate bearing capacity of shallow foundations resting on variably saturated soils under a steady flow. The effective stress approach originally employed for predicting the bearing capacity of saturated soils is extended by incorporating a suction stress-based representation. A closed-form equation is used to define the suction stress characteristic curve, capturing changes in the effective stress because of varying soil saturation and matric suction. The proposed method uses the classic effective shear strength parameters and two fitting parameters to represent the soil water characteristic curve. The method can be used for different soil types, various degrees of saturation, and different surface flux boundary conditions. The results from the proposed formulation were compared, and good agreement was observed against three sets of experimental results from model footing and plate load tests. A parametric study was performed, and the ultimate bearing capacity profiles are presented for various surface flux boundary conditions and depths of the water table. It is shown that different flow conditions can significantly affect the ultimate bearing capacity of clay. However, different flow conditions have negligible effects on the ultimate bearing capacity of sand.