Abstract: The sand compaction pile which is an efficient foundation treatment method under water is used in Hongkong-Zhuhai-Macao Bridge and Shenzhen-Zhongshan Link. A large number of in-situ tests such as underwater plate load tests and surcharge preloading tests under water are implemented. The in-situ values of pile-soil stress ratio in the underwater plate load tests are discussed, and the values of pile-soil stress ratio are kept to 6 in the process of loading. The settlement of the sand compaction pile ground in the process of surcharge preloading under water is monitored, and the values of settlement are comparatively analyzed to those by the available calculation method recommended in the Japanese standard. It is illustrated that the available calculation method in the Japanese standard is conservative when the replacement rate is less than 50%, and is not suitable for the practice when the replacement rate is more than or equal to 50%. A modified settlement calculation method based on the in-situ measurement is proposed and illustrated, and the values of pile-soil stress ratio are suggested in the range of 5 to 6.3.