开挖诱发坑内既有基桩附加内力的模型试验

既有建筑下挖改造引起的基坑被动区土体侧移会对坑内基桩承载性产生重要影响. 通过室内模型试验研究坑内基桩在被动区土体侧移作用下的桩身受力特性，重点分析支护结构与坑内基桩距离、开挖深度、桩顶竖向荷载及承台约束高度对基桩弯矩和剪力的影响. 试验结果表明，在悬臂式支护开挖条件下，被动区土体位移模式呈倒三角形，基桩弯矩和剪力沿桩身分布具有多个异号峰值，桩身自上而下可分为开挖裸露段、被动受荷段和主动作用段. 基桩与支护水平间距越小、基坑下挖深度越大，基桩各部位弯矩和剪力越大，且竖向受荷和桩身侧向变形的耦合效应将使桩身弯矩变大. 桩顶约束高度的改变会对基桩弯矩和剪力产生影响，在其他条件相同时，约束高度越大，基桩弯矩和剪力越小. 研究结果可为地下增层工程的设计提供支撑.

Chamber tests for investigating additional internal forces in existing foundation piles induced by excavation

The lateral displacement of passive soil induced by the excavation of existing buildings has significant impact on the bearing capacity of foundation piles within the pit. The load-bearing characteristics of foundation piles under the lateral movement of the passive soil were investigated through indoor model experiments. Focus was paid on the influences on the pile’s bending moment and shear force of a few factors such as the spacing between retaining and foundation piles, the excavation depth, the axial loading level at the top of the pile and the height of cap restraint. Results showed that under the cantilevered supported excavation condition, the displacement pattern of the passive soil resembled an inverted triangle. The bending moment and the shear force of the foundation pile had several heterogeneous peaks distributed along the pile. Based on this, the foundation pile from top to bottom was divided into three sections, i.e. the excavation-exposed section, the passive-load section and the active-effect section. The smaller the distance between the supporting and foundation piles and the deeper the excavation depth, the greater the bending moment and the shear force induced along the foundation pile. The coupling effect of axial loading and lateral displacement will further increase the bending moment of the foundation pile. The change of the height of the pile top restraint will affect the bending moment and the shear force of the pile. With other conditions being the same, the greater the height of the pile top restraint, the smaller the bending moment and the shear force of the pile. The research results can provide support for the engineering design of underground-storey supplement.