Field test on composite foundation with thick cushion and sand pile group

Base on the foundation treatment of south anchorage caisson of Wenzhou Oujiang Estuary Bridge, nine groups of field static loading tests on composite foundation with thick cushion and sand pile group of different cushion materials, cushion thicknesses and pile spacings were performed to study the application of this composite foundation and influencing factors of its bearing capacity. The interaction test on sand piles installation was conducted to investigate the impact of sand pile installation on completed sand piles. The results show that thick cushion and sand pile group, which can strengthen the soft foundation very well, is an ideal way of foundation treatment of super-large caissons. Water content of cushion, material of cushion and thickness of cushion have greater influence on bearing capacity than pile spacing. Because of the differences between the impact on sand pile and the impact on soil which are caused by installation of sand pile, it is inaccurate to use compaction effect of static-press pile to analyze the impact on existing sand pile. The impact on existing sand pile is very large and the maximum impact concentrates on the 1/3 pile length below earth surface. At the same time, the better of soil property, the smaller of this impact. A big pile spacing and blocking effect of existing sand pile can effectively reduce the impact.