Bearing Capacity of Shallow Foundations in Unsaturated Soil Considering Hydraulic Hysteresis and Three Drainage Conditions

Most shallow foundations are built in unsaturated soils above the groundwater table. Being able to evaluate the bearing capacity of these foundations is important in foundation design because the conventional bearing-capacity equations, which were developed for soils in saturated or dry states, are not directly applicable. This paper presents an effective stress-based finite-element formulation and shows how it can be used to compute the bearing capacities of shallow foundations in unsaturated soils. The mechanical behavior of unsaturated soil is described using the elastic-perfectly plastic Mohr-Coulomb model. It is shown that suction and hydraulic hysteresis directly impact the bearing capacity of unsaturated soil foundations. The effects of three different drainage conditions are also investigated, namely, a constant suction, a constant moisture content, and a constant contribution of suction to the effective stress and strength condition (also referred to as a constant s condition). It is shown that the constant s condition provides a good approximation to the other two and may simplify calculation of bearing capacity. When foundation loading occurs quickly, meaning a condition of constant moisture content prevails, a constant s condition can be assumed without loss of significant accuracy. The s magnitude can be taken as being equal to the initial in situ value.