开挖过程对桩基影响的工程实例对比分析

Metro Project based Analysis of Excavation Impact on Pile Bearing Capacity

摘要:

采用3维快速拉格朗日法（FLAC3D）建立了考虑基坑分步开挖对桩基础影响的动态计算模型.该模型考虑了桩土之间的相对滑移作用，土体采用摩尔 库伦计算模型.计算分析了桩身轴力、桩身最大拉力值及其位置随开挖步的变化，通过与实测值对比验证了模型的准确性，进而对比分析了开挖前后桩基础承载性能的变化，并探讨了开挖后桩基承载力的损失机理.结果表明：基坑开挖使桩身轴力发生明显的变化，开挖后使桩基处于受拉状态，桩身最大拉力值随开挖深度的增加而增大，且其位置随开挖深度的增加而下移；开挖使得桩基的承载性状发生了显著的变化，由于土体卸荷回弹，桩身上部土体对桩周的约束作用明显减小，使得桩顶的荷载向下传递量明显增加；开挖降低了桩基承载力，其损失主要来自桩侧摩阻力.

Abstract:

Fast Lagrangian analysis of continua in three dimension (FLAC3D) was employed to investigate the effect on pile foundation after a staged excavation, which considered the relative slip between pile and soil and also realized the dynamic process of excavation, and the model was verified by the results of axial force changing in pile shaft, the maximum tension force in pile and its location through the comparative analysis between the measurement and calculate after the excavation, and then the impact on the bearing capacity of pile foundation was discussed. Results show that pile foundation is in tension state after excavation, the maximum tension is increasing and the location is moving downward with the excavation depth; the bearing capacity changes significantly after excavation, the normal stress along pile shaft decreases sharply after excavation, especially on the pile top, which will cause the loss of bearing capacity in pile foundation.