Numerical Modelling of Shallow Foundations in Unsaturated Soils

Foundations are ubiquitous to geotechnical engineering. Most shallow foundations are built in unsaturated soils above the ground water table. Methods for evaluating the bearing capacity and settlement of foundations in unsaturated soils are not yet well established. Methods developed for foundations in saturated/dry soils are not directly applicable. This study addresses this problem.It presents an effective stress based finite element formulation that can be used to predict the bearing capacity and consolidation settlement of unsaturated soil shallow foundations. For the first time the influences of hydraulic hysteresis, the location of the hydraulic state on the soil-water characteristic curve and three different drainage conditions are considered when modelling bearing capacity. The three drainage conditions are a constant suction condition, a constant moisture content condition and a constant contribution of suction to the effective stress and strength condition (also referred to as a constant χs condition). The elastic-perfectly plastic Mohr-Coulomb model is used to simulate the mechanical behaviour of unsaturated soil. The effects of the location of the hydraulic state on the soil-water characteristic curve and the degree of saturation are considered when determining the rate and magnitude of consolidation settlement.The study also includes a program of experiments including plate load tests (PLTs) on saturated and unsaturated soil samples, along with cone penetration tests (CPTs) in the samples. Good agreement is obtained between computed and measured bearing capacities. It is shown by the numerical results and the experimental data that suction and hydraulic hysteresis directly impact the bearing capacity of unsaturated soil foundations. It is also shown that the constant χs condition provides a good approximation to the other two drainage conditions and may be assumed to simplify calculation of bearing capacity. When foundation loading occurs quickly, meaning a constant moisture content condition prevails, a constant χs condition can be assumed without loss of significant accuracy. The χs magnitude can be taken as being equal to the initial in-situ value for this silty sand. Further numerical analyses are performed to study the settlement of unsaturated soil foundations by simulating the consolidation of unsaturated soils. It is found that hydraulic hysteresis and degree of saturation have major influences on the consolidation of unsaturated soils. The results indicate that in engineering practice most settlement of a footing on soil with a low degree of saturation takes place during a short time. Long term settlement is insignificant and can be ignored under certain conditions