Investigating the changing deformation mechanism beneath shallow foundations

Abstract

The design of shallow foundations has traditionally used a mixture of plasticity-based solutions to find the ultimate limit state and either a factor of safety on the plasticity solution or a linear elastic solution to attempt to design for the serviceability limit state. The serviceability limit state is intrinsically linked to the deformation mechanism that occurs beneath the shallow foundation in service. A better understanding of these soil movements can pave the way for more rational design approaches. In this paper, small-scale experimental work is used to show that the deformation mechanism beneath strip and circular foundations continuously changes as the footing is displaced. The mechanisms observed at intermediate settlements, noted to be typical design points, are best described by a mixture of solutions. Linear mixes of idealised fields were analysed using an upper bound approach to determine the load–displacement behaviour of each mix. The envelope of lowest upper bounds indicated that the optimal mix of fields changes depending on the footing settlement. At typical design points for shallow foundations mixtures dominated by ellipsoidal cavity expansion mechanisms were found to be optimal for both axisymmetric and plane strain cases. Comparison of theoretical and experimentally measured predictions indicated that using linear mixes of fields gives a good approximation to the true behaviour and may be used for settlement-based design approaches in the future.