软土地区深埋桩筏基础沉降实测与反分析

对上海环球金融中心（Shanghai World Financial Center，简称SWFC）实测沉降结果进行研究，分析坑底的回弹、沉降随时间的发展规律，并通过沉降推测上部结构刚度的变化情况。软土地区深埋桩筏基础沉降与加荷速率、基坑回弹、附加应力在桩长及桩下土中的传递、基坑降水与停止降水等多种因素有关。基础沉降-时间关系曲线并非理想的双曲线，即使是施工加荷速率相对稳定时。建筑物施工初始阶段沉降速率较小，与深基坑回弹密切相关，建筑物接近封顶时往往沉降速率较大，可能与附加应力传至桩端以下土中有关。沉降过程反映4.5 m的厚筏为弹性体，上部结构刚度对筏板刚度具有贡献性和有限性，上部刚度是随着施工的进程逐步形成。采用3种方法对SWFC进行沉降计算反分析，通过计算结果与实测数沉降数据进行对比得出，运用上海规范采取合适的经验系数（0.40～0.55）、考虑70%的水浮力，利用明德林解得出的计算结果与实测值吻合很好；深埋桩筏基础共同作用分析计算中不能过大考虑上部结构刚度的贡献。

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.