既有建筑物地下室增层开挖桩基屈曲稳定研究

既有建筑物地下室增层开挖可有效利用地下空间，是目前改善停车难问题的主要手段之一。既有建筑地下室增层开挖将改变原有桩基础的受力性状，从而可能引起原有桩基础屈曲失稳。提出一种预测地下室增层开挖既有桩基失稳的临界荷载及稳定计算长度的理论解法。首先，结合浙江饭店地下室增层开挖工程，简单介绍增层开挖方案。之后，通过Winkler弹性地基梁理论建立增层开挖工况下单桩基础桩-土体系总势能方程。最后，利用最小势能原理导得增层开挖工况下单桩基础屈曲失稳的临界荷载及稳定计算长度的表达式。在此基础上，研究了桩身临界荷载的影响因素。计算结果表明：随着半波数n的增加，既有桩基的临界荷载逐渐收敛；既有桩基的临界荷载随开挖深度的增大而急剧减小。分析结果可为类似地下室增层开挖工程的设计提供依据，减少开挖引起既有桩基础失稳隐患。

Buckling stability analysis of pile foundation for excavation beneath the basement of existing building

Excavation beneath the original basement can effectively utilize the underground space, which is one of the popular ways to improve the parking problem in the dense urban areas. Excavation beneath the original basement will change the behavior of original pile. If the excavation depth exceeds a certain value, it may lead to buckling instability of the original piles due to the reduction of ground confining pressure around piles. In this paper, a theoretical analysis is performed to estimate the bucking critical load and the effective length of a single pile. At first, the scheme of floor-addition of basement is briefly introduced with the case of Zhejiang Hotel Extension Project. Secondly, the total potential energy of the pile-soil system under the condition of excavation beneath the original basement is set up based on the Winkler elastic beam theory. Finally, the expressions of critical load and effective length of single pile are deduced by using the minimum potential energy principle. Based on the proposed theory, the influence factors of critical load, including half-wave number n and excavation depth, are analyzed. It is shown that the buckling critical load of pile shaft converges with an increase in half-wave number n; by increasing the excavation depth, the buckling critical load decreases rapidly. The proposed theory may provide guideline to estimate the supported pile behavior of excavation beneath original basement under existing buildings.