Bearing capacity failure of shallow foundations in earthquakes

This paper addresses the behaviour of buildings on shallow foundations in sand under earthquake loading. Centrifuge tests were used to investigate building performance on dry or saturated sand foundations as a function of aspect ratio. The tests showed good repeatability and illustrated clearly the mechanism of failure by rotation and vertical settlement. Displacement of the structures during successive earthquakes showed a gradual accumulation at a rate dependent on the stiffness of the foundation soil. Clear evidence was seen of the sudden onset of bearing capacity failure near the end of the final earthquake when the rotation of the structure was at its peak but with the ground shaking well below its maximum value. Such failures were observed in two independent tests; these data are compared with the results of pseudo-static type calculations, A new theoretical analysis which shows the degradation of bearing capacity as the result of accumulation of rotation is validated by the experimental data. The limitations of pseudo-static type methods and implications for design are discussed.