Physical modelling of soil structure interaction in stone column foundations

The development of transparent synthetic soil has enabled non-intrusive measurement of internal soil displacement and detection of pre-failure strains using laser aided imaging and Particle Image Velocimetry. The work described in this paper applies this novel methodology to evaluate the deformation and failure behaviour of stone column foundations in reduced scale physical models. Four tests have been conducted on 18mm diameter isolated columns of area replacement ratios (Ar) equal to 5, 3.3 2.5 and 1, installed in a 220mm test bed. Visualisation of real-time internal displacement has indicated that an isolated stone column fails mostly through a combination of compression and bulging with a negligible amount of end bearing failure. At all levels of foundation displacement, it was seen that the geometry of the footings interacted with the geometry of the individual columns in protecting the upper column from bulging and producing an eventual failure mechanism for the composite foundation, therefore governing the ultimate bearing capacity of the foundation too. Increasing footing size and correspondingly increasing Ar from 1 to 2.5, to 3.3 and then 5 was seen to increase load capacity by 21%, 54% and 108% respectively. These findings have important implications for design as current design practice is based on cylindrical cavity expansion theory whereby the size of the foundation itself is not considered.