桩基础托换开发地下空间不均匀沉降的数值分析

桩基础托换法开发既有建筑物地下空间的施工过程可分为桩基托换前、托换后和土方开挖至设计标高3个工况。施工过程会使既有建筑物地基产生二次沉降变形，由此产生的附加内力会影响到上部结构的安全。为了研究一个3层框架结构建筑物增加一层地下室桩基础托换过程的地基沉降规律，建立了3个工况的ANSYS三维有限元模型，并进行数值分析。通过定义路径的方法，可以得到3个工况的沉降曲线和柱脚处地基的沉降差。从沉降分析的结果可以看出：桩基托换前，地基总的沉降趋势是中间部位柱脚大于周边部位柱脚的“盆式沉降”；桩基础托换后，柱脚处的沉降差进一步增大；土方开挖后，由于下层土被“卸载”，中间部位柱脚处的地基明显回弹，使得中间部位柱脚与周边部位柱脚的沉降差出现了减少的趋势。最大的沉降差出现在桩基托换后土方开挖前，此时由沉降差引起的上部结构的附加弯距可以通过ANSYS有限元分析求出。

Numerical analysis of settlement difference for pile foundation underpinning for constructing underground space

Pile foundation underpinning method can be used to construct the underground space of the existing buildings .The construction processes are constituted three steps: before the pile foundation underpinning, after the pile foundation underpinning and soil excavation. The construction processes produce secondary settlement deformations, which will result in the additional internal forces affecting superstructure safety. Numerical models of three-dimensional finite element analysis based on Ansys code are established to study the settlement of socles on pile foundation underpinning construction for a three-floor frame structure, on which a flat basement is built. The settlement curve and the settlement difference are obtained by defining paths method. Some conclusions are drawn as follows:The total tendency of socles is basin-shaped settlement which is bigger in the middle position socles than the peripheral position socles before the pile foundation underpinning. Pile foundation underpinning augments the differential settlements. After soil excavation, middle position socles rebound obviously owing to unloading action, which lessens the differential settlements between middle and peripheral position socles.The most differential settlement is produced after pile foundation underpinning but before soil excavation. The additional moment of the superstructure can be gained from the finite element model based on Ansys.