Abstract：The bearing capacity of shallow foundations obtained by the conventional approaches，which assume fully saturated conditions ignoring the influence of the matric suction，may not be reliable and lead to uneconomical designs. On the assumption that the shear strength formula of unsaturated soils is the extension and expansion of the shear strength formula of saturated soils，and based on the linear failure criterion and associated flow rule，the bearing capacity of strip footing on unsaturated soils under the action of ultimate loads is studied by means of the kinematical approach of limit analysis theory. A planar knematically admissible failure mechanism is proposed for the calculation schemes. The objective functions of bearing capacity are obtained by equating the work rate of external force to internal dissipation along the velocity discontinuities，and then the objective function is transformed as a upper-bound mathematic optimization model for bearing capacity as low as possible. The objective functions are obtained by use of the sequential quadratic programming. The bearing capacity of unsaturated soil is predicted by using the approach presented above，in which not only the effective internal friction angle，effective cohesion and matric suction，but also the soil-water characteristic curve and the engineering practice are analyzed synthetically. Numerical results show that the existence of matric suction，the distribution mode of matric suction and under water level in unsaturated soils have significant effects on the ultimate bearing capacity. The parametric analysis indicats that preferences possessing nonlinear effect on the bearing capacity. If the yield criterion of unsaturated soils turns into the yield criterion of saturated soils，the presented solution provides better upper-bound result.
