中文摘要：

目前对于超深基础下的桩的设计尚未考虑超深开挖、坑底隆起对桩产生的作用。现场采用静载试验确定单桩承载力时也仍采用常规的地表加载并扣除开挖深度范围内桩侧摩阻力的方法，没有考虑深开挖效应的影响。采用经实际工程算例验证的土参数、桩身材料参数和桩土接触面参数，在均质土中建立了轴对称有限元模型，对三种不同试桩方法进行了模拟。通过开挖模拟，首先揭示了深开挖对桩的影响效应，在此基础上，对超深开挖对桩在开挖后再加载的荷载传递机理和沉降机理进行了研究。与常规试桩法和套管试桩法中的基坑底以下相同桩长的桩相比，超深开挖产生的影响效应使桩的极限承载力降低，竖向刚度减小，相同荷载下沉降加大，且其沉降中桩整体刺入所占的比例显著大于其它两者。桩侧摩阻力完全发挥时需要的桩土滑移量增大。此外，超深开挖可在桩身中产生较大的拉力，桩身中下部钢筋不能随意减少。

英文摘要：

The effect of deep excavation and heave of soil beneath pit bottom on the pile supporting extra-deep foundation has not been considered. The approaches adopted to determine the bearing capacity of a single pile in extra-deep foundation still employ conventional pile load tests on ground surface first and then deduct the skin friction along the pile shaft in the range of excavation from the results obtained by the conventional tests, where the effect of deep excavation is not taken into account. Here three different methods of pile load tests are simulated through the axisymmetric finite element model of a pile in homogeneous soil adopting parameters of soil, pile material and soil-pile interface verified by a case history. Through the simulation of deep excavation, its effect on the pile is investigated and on this basis the mechanism of load transfer and the settlement of the pile loaded after deep excavation are studied. Compared with the conventional load tests and load tests with sleeve, the bearing capacity and vertical stiffness of the pile loaded after excavation are reduced. Under the same load the pile experiencing deep excavation has a larger settlement and a larger proportion of pile rigid penetration in settlement than that of piles in the other two tests. Deep excavation also leads to a large pile-soil relative displacement when the skin friction is totally mobilized. In addition, deep excavation can cause considerable tension force in the pile, which means that the reinforcing steel bars in the lower part of the pile should not be reduced at discretion.