Group effects in stone column foundations: model tests

Model tests have been performed to determine the mechanisms of response for beds of clay reinforced with stone columns subjected to surface footing loads. An exhumation technique has been used to discover the deformed shapes of the model stone columns and thence to deduce the way in which the columns have transferred load to the surrounding clay. Tests have explored the effect of varying the diameter, length and spacing of the model stone columns. These parameters control whether the columns act as somewhat rigid inclusions transferring load to their tips and eventually deforming either by bulging or by the formation of a failure plane, or whether they are able to compress axially or even, if sufficiently slender, to ‘bend’ and undergo significant lateral deformation. Miniature pressure transducers have been used to reveal the distribution of contact pressure between columns and clay at various stages during the loading of the footings. It appears that the columns at mid-radius of the footing are typically the most heavily loaded. Results from numerical analysis are used to provide qualitative support for some of the findings from the physical model tests.