基于大直径盾构隧道扩挖地铁车站结构安全分析

北京地铁14号线将台站主体结构基于大直径盾构隧道扩挖形成，设计提出的扩挖方案为CRD法。在充分考虑扩挖车站部位地质条件及环境条件的基础上，分析了设计方案的弊端，提出采用洞桩法(PBA工法)作为扩挖大盾构隧道建造地铁车站施工方案，为有效控制地面沉降及扩挖施工对周围环境的影响，取消地面施工降水方案，扩挖前洞内采用超前深孔注浆堵水。并以将台站作为工程背景，采用有限元数值模拟方法，针对车站扩挖施工的优化方案，研究了关键施工阶段的结构体系受力转换规律。计算结果表明：在扩挖施工中，结构受力转换频繁，封顶块管片两侧的小管片拆除后，中洞拱部的初期支护压应力数值急剧增大，且超出了初期支护强度允许值，根据计算结果对中洞的初期支护参数和开挖方案进行了调整。从侧导洞开挖到中洞开挖支护期间，管片最大主应力为压应力，且未超出管片结构强度允许值，在封顶块两侧的管片拆除后，尽管管片结构的最大主应力由压应力变为拉应力，但由于拱部已经完成开挖支护，两侧管片的应力状态并不制约结构的整体稳定性；中隔墙最大主应力也出现在封顶块两侧的管片拆除后，但未超出中墙结构强度允许值。数值模拟结论和施工实践均表明基于本工程地质条件及环境条件所提出的车站扩挖施工优化方案能够保证车站结构安全。

Safety analysis for metro station constructed by enlarging large diameter shield tunnel with mining method

The Jiangtai metro station of Beijing Metro line 14 is constructed by enlarging the large-diameter shield tunnel with the mining method. A CRD (cross diaphragm) construction scheme of enlarging the shield tunnel is put forward in the preliminary design. By considering the geological conditions and environmental conditions, the problems of the CRD scheme are analyzed and a PBA method (Pile-Beam-Arc method) is proposed. In order to control the ground settlement and the influence on the surrounding environment, the dewatering scheme is replaced with deep the hole grouting in the excavation face. Three-dimensional finite element models are used to simulate the ground-structure interactions. The numerical results show that the structural stress state varies frequently in the enlarging process. Due to excavation of the two sides of middle pilot tunnels and dismantling of the K segment, the stress of the primary lining of the middle pilot tunnel increases dramatically. The primary lining parameters and the excavation method for the two sides of the middle pilot tunnels are adjusted according to the analysis results. Although the stress state of the segments shifts from compressive stress to tensile stress in the dismantling process of both sides of the small segment, the overall stability is not affected. The maximum principal stress in the middle partition wall appears after dismantling both sides of the small segment. The calculation results show that the construction optimization scheme is feasible and the structural safety can be guaranteed.