Comparative experimental research on the bearing and deformation character of composite foundations with different pile materials

The pile-soil load share ratio, pile-soil stress ratio and inter-pile soil deep-seated deformation of compos-ite foundations, which include sand-gravel columns, rammed soil-cement columns and CFG piles composite ground, were measured by indoor model test. The bearing and deformation character of composite foundations were comparatively analyzed between the pile materials. An effective pile length or effective composite soil thickness exists in composite foundations with sand-gravel columns and rammed soil-cement columns. When the pile length of sand-gravel columns exceeds the effective pile length, the effect of improving the load bearing capacity, increasing the compression modulus, and decreasing the deformation is not obvious. It is inadvisable to design the pile length exceeding the effective pile length, except for treating liquefiable foundations, but the sand-gravel columns should not be too long. There is a significant correlation between the effective pile length of rammed soil-cement columns and the compressive strength of pile body; the design of pile body is dependent on the pile strength, and the load bearing capacity of a single pile derived from the pile strength should be not less than that provided by the resistance of soil around piles and pile-end soil. The CFG piles have high pile strength and small compression, and the side resistance can play along the whole pile length. The tip resistance works well when the pile tip penetrating into a good bearing layer. It is advisable to prioritize selecting a good pile-tip bearing layer for design.