港珠澳大桥岛隧工程水下深厚软土采用挤密砂桩进行加固，设计了一套大型原位载荷试验系统对水下挤密砂桩 （sand compaction pile，SCP）复合地基的力学和变形特性进行研究，通过在水下设置高精度静力水准系统获得了良好的沉降观测结果。试验结果表明，水下挤密砂桩复合地基在同一置换率下的应力分担比与载荷水平以及时间效应密切相关；应力分担比比值随荷载级别的升高表现出先减小后增大的趋势，在同一荷载级别下比值随着时间逐步衰减，并最终趋于某一定值。对比分析了几种散体材料桩复合地基承载力理论的计算结果，表明按被动土压力法得到的复合地基承载力与载荷试验结果较为接近。对于高置换率的挤密砂桩复合地基，采用建筑地基处理技术规范提出的沉降折减系数计算得到的复合地基沉降与实测值最为接近，采用日本经验公式计算得到的沉降量偏大。研究结果对于水下挤密砂桩复合地基的设计计算和深水原位测试具有一定的参考意义。

The thick soft marine clay of Hong Kong-Zhuhai-Macau Bridge(HZMB) ground was reinforced by sand compaction pile (SCP) method. A large scale plate load test was designed for evaluate the bearing capacity and deformability of the SCP composite ground. Better settlement result was obtained by utilizing the high sensitivity liquid level sensor system underwater. The research results show that the pile-soil stress ratio of SCP composite ground is closely related to load level that decreased with the load level increasing. On the same level, the ratio gradually reduced to a certain value finally. Four computing theory including Brauns’ method, Wong formula, Hughes & Withers formula and passive earth pressure method were discussed in bearing capacity calculation of SCP composite ground. The results by passive earth pressure method agreed best with the field experimental data. For this high-replacement ratio SCP composite ground, it is demonstrated that the settlement calculated with a reduction factor which was derived from Technical code for ground treatment of building is most close to the measured result; while there is significant difference when the overseas Coastal Area Development Institute of Japan(OCDI)’s experiential formula is employed. This research results provide useful data for the marine SCP composite ground design and deep sea in-situ test.