EXPERIMENTAL STUDY ON BEARING PERFORMANCE OF STEEL PIPE PILE COMPOSITE FOUNDATION CONSTRUCTED BY TOP- DOWN METHOD UNDER VERTICAL LOADS

Model tests were designed to investigate the bearing capacity properties of composite steel pipe pile foundations in sandy soil. Results indicated that:To a single pile composite foundation,the load was totally carried by ground under the cap before sealing piles. Then,after sealing,the pile started to work and the pile-soil load share ratios were between 73. 5% ～ 92% before reaching ultimate bearing capacity. As vertical load increased continually to the capacity limit,the ratio reduced obviously. To a composite foundation with 8 steel pipe piles was tested,the attenuation gradient of axial load was larger in the middle and lower parts rather than the upper part during axial load transfer. The rates of the corner piles were biggest,then the side piles. The side frictional resistances developed gradually from the top to the bottom and the maximum value reached at the distance of 1. 55 m below the top. At every load step,the resistances of the corner piles were greater than that of side piles. Moreover,the ultimate bearing capacity of the steel pipe pile composite foundation was significantly higher than that of the high-rise cap pile group foundation.