Observation and back analysis of settlement of piled-raft foundation in soft soil

The settlement of Shanghai World Financial Center (SWFC), which is piled-raft foundation, is analyzed in this study. Then the corresponding foundation pit resilience, settlement developing with time and structure stiffness are investigated, and the variation of superstructure stiffness is predicted according to the settlement as well. It is found that the settlement progress of deep piled-raft foundation in soft soil is depending on a various of factors such as loading rate, pit resilience, additional stress transferring in the scopes of shaft and its embedded soils, dewatering and so on. The settlement-time curve of foundation is not an ideal hyperbola, even if the loading rate of construction is relatively stable. At the initial stage of building construction, the low settlement rate is mainly related to the rebound of soil deep foundation pit. While the high settlement rate is found when the building reaches completion, which is probably caused by the additional stress that has transferred to the soil layer under pile toe. It is found that 4.5 m thick raft in settlement development is still considered as an elastic body, and the contribution of superstructure stiffness to the raft is limited. Moreover, the superstructure stiffness is gradually formed with the progress of construction. Three methods are employed to conduct the back analysis of settlement of the SWFC, and calculated results are compared with the settlement data. It is shown that in-situ data is fitted well with the results obtained by the Mindlin solution, referring to "Shanghai code", in which the proper experience coefficient is considered as 0.40-0.55 and the water buoyancy is set as 70%. By analyzing interaction of deep piled-raft foundation in soft soil, the superstructure stiffness should not be overvalued.