Bearing capacity under increasing undrained shear strength

The undrained shear strength of naturally deposited, consolidated and saturated clay usually increases with depth due to the overburden pressure and the decreasing water content. Nonetheless, calculations of the bearing capacities of foundations under undrained conditions are usually done using the approach of Prandtl, which assumes constant values of the undrained shear strength. Analytical models using the kinematic element method show that the failure mechanism and the bearing capacity are both influenced by the increasing undrained shear strength; therefore, Prandtl's method might overestimate the bearing capacity of a shallow foundation assuming an average value of the undrained shear strength. Centrifuge tests of shallow strip footings on kaolin clay were conducted to validate the analytical model in terms of bearing capacity and failure mechanism. The estimated bearing capacity of the shallow foundation based on analytical models show good agreement with the centrifuge test results. However, the shape of the failure mechanisms is considerably different. In the analytical models, the development of the failure mechanism is estimated based on the assumption of a perfectly plastic behaviour, developing a rigid-body collapse. This behaviour was not observed in the physical models due to the development of the failure mechanism.