深层开挖条件下自平衡试桩法上段桩承载力损失简化计算方法

针对在深层开挖条件下进行自平衡法试桩时，通过采取有效措施消除开挖段的侧摩阻力影响后，由于深层覆土开挖卸荷导致工程桩桩周土体围压减小，从而导致荷载箱上段桩抗拔承载力降低这一问题，通过借鉴目前在抗拔桩方面已有的相关分析方法、负摩阻力、基底附加应力以及相关土力学基础理论，提出了基于Mindlin解的简化分析方法以及负摩阻力-附加应力法两种简化分析方法，并以北京某地铁车站中间柱下桩为例，分别估算深层开挖土体卸荷引起的上段桩承载力的损失值。两种方法所得出的估算值存在较大差异，对其原因进行了初步分析。两种简化分析方法的合理性及准确性还有待于进一步完善和实践验证。

Simplified calculation method for the bearing capacity loss of the upper part of the deep excavation

In the self-balanced pile test in the deep excavation, effective measures are taken to eliminate the side friction effect after excavation. The unloading of deep overburden excavation caused decrease in soil confining pressure surrounding engineering pile and as a result reduce of the upper load box pile pullout capacity. Based on the additional correlation analysis method, the negative friction, base and relevant soil mechanics theory, a simplified analysis method of Mindlin and negative friction-additional stress solutions are proposed. The column pile in a subway station in Beijing is studied to estimate the loss of upper pile bearing capacity caused by deep excavation soil unloading. The results of the two methods show significant difference and the reasons are discussed. The applicability and accuracy of the two simplified analysis methods require further study..