开挖条件下抗拔桩的承载力损失比分析

Loss ratio of bearing capacity of uplift piles under deep excavation

基坑开挖不可避免地引起地基应力场变化,导致桩–土接触面法向应力发生改变,从而降低抗拔桩承载力,而由于实际工程情况限制,开挖后抗拔桩的承载力又无法通过现场试桩获得,因此采用理论方法分析开挖条件下抗拔桩的承载力就显得尤为重要。基于此,首先采用极限平衡法,应用经典Mindlin应力解考虑土体开挖卸载在开挖面下引起的附加应力,推导出开挖条件下抗拔桩承载力的简化计算公式;其次利用弹塑性有限元法分析了开挖条件下抗拔桩的承载力并和极限平衡法计算结果进行对比,验证了该方法的合理性及实用性。在此基础上分析了开挖条件下抗拔桩的承载力损失比与开挖半径和桩长的变化规律,分析表明:抗拔桩承载力损失比随开挖半径的增大而增大,随有效桩长增长而减小;相同开挖条件下,扩底桩的承载力损失比等截面桩少。

Bearing capacity of uplift piles would inev.itably decrease owing to the change of the ground stress field and the normal stress of pile-soil interface under deep excavation. Unfortunately, due to the limitation of field conditions, it was almost impossible to obtain the bearing capacity of uplift piles under deep excavation from field tests, and theoretical study became the only feasible way to solve this problem. By use of the limit equilibrium method and the simplified Mindlin＇s stress solution, a simple formula for predicting the bearing capacity of uplift piles in soft clay under deep excavation was deduced. The calculated results by the proposed formula were in well accordance with the simulated results of FEM. The influence of radius of excavation and effective length of pile on the loss ratio of bearing capacity was f＇mally discussed, and some useful conclusions for engineering design of uplift piles under deep excavation were drawn.