考虑深基坑开挖效应的超长灌注桩桩身压缩综合系数的理论分析

对于深基坑坑底以下的工程桩，受开挖效应影响，桩侧摩阻力分布与常规条件下差异较大。基于超长钻孔灌注桩的足尺试验和数值分析结果，研究了常规和基坑开挖条件下试桩不同侧摩阻力分布导致的沉降特性差异。研究结果表明，超长钻孔灌注桩在工作荷载下一般端阻比较小，桩侧摩阻力的分布形式直接决定了桩身压缩综合系数。无深基坑开挖影响时，规范推荐的桩身压缩综合系数计算超长桩桩身压缩量是可行的。但在深基坑开挖条件下，钻孔灌注桩桩身压缩综合系数取值可比现行规范基于不考虑基坑开挖效应的推荐值大20%，不考虑深基坑开挖影响可能导致桩顶沉降被低估。此外，在深开挖条件下，超长钻孔灌注桩的桩身压缩综合系数随桩身长径比l/d的增大而减小；而常规条件下，桩身压缩综合系数随桩身长径比l/d变化的规律不明显。

Theoretical analysis of comprehensive coefficient of pile compression for super-long bored pile considering effect of deep excavation

For the piles beneath deep excavations, the distribution of pile shaft resistance is different from that under conventional condition. Based on the field tests and numerical simulation of super-long bored piles, theoretical analysis is conducted to investigate the different settlement behaviors caused by the shaft resistance distributions under both conventional and excavation conditions. The results show that due to the low tip resistance of the super-long pile under working load, the comprehensive coefficients of pile compression depend on the distributions of shaft resistance. The comprehensive coefficients of pile compression recommended in the current code are reasonable for super-long bored pile under conventional condition. However, it is found that the comprehensive coefficient of pile compression under excavation condition is 20% larger than the recommended value in the current code. Therefore, the settlement of pile may be underestimated without considering excavation effect. Furthermore, for the pile under deep excavation condition, the comprehensive coefficient of pile compression decreases with the increase of the slenderness ratio l/d; while for the pile under conventional condition, there is no direct relation between the comprehensive coefficient of pile compression and the slenderness ration.