PBA法暗挖施工是复杂地面环境下城市地铁车站施工的重要方法之一，开展PBA法暗挖地铁车站竖井及导洞施工过程中围岩变形、支护结构变形、受力特性研究具有重要意义。本文以西安地铁八号环线某车站为背景，采用室内试验、数值模拟及现场监测相结合的手段开展研究工作，主要工作及结论为：

（1）采用室内宏观试验和核磁共振细观试验研究了不同围压、不同含水率下地铁车站黄土的力学特性规律，给出了地铁车站黄土的宏细观破坏规律。含水率增加会对地层强度有一定的削弱作用，围压升高可以增强围岩的强度；土样T₂谱曲线峰值随含水率的增加而增加。

（2）研究表明，黄土地区地铁车站PBA施工诱发地表沉降的主要影响因素包括：车站埋深、黄土性质、地下水位、地下管线、地面条件、开挖方法与步序、导洞的支护结构等七大因素。基于控制PBA施工诱发的地表沉降处于合理范围的理念，给出了PBA法暗挖施工地铁车站的施工工序。

（3）建立暗挖车站竖井三维FLAC模型，模拟现浇式与预制式支撑竖井施工对周围环境、内支撑及围护桩的影响，分析两种不同工况施工引起竖井及周围环境变形的影响规律，得到结论：采用钻孔灌注桩联合预制混凝土支撑施工，能有效克服现浇钢筋混凝土内支撑的施工缓慢、污染严重等缺点。

（4）利用FLAC数值计算方法研究双向导洞施工引起的空间效应及导洞施工过程中马头门处的力学特征，分析三种不同双向导洞开挖顺序下的地表沉降变形规律及马头门处应力变化规律，研究马头门最大主应力与最大剪应力随导洞开挖进尺的演化规律，并将模拟结果与现场监测数据进行对比分析，给出了最优的双向导洞施工方案。

（5）建立地铁PBA暗挖车站二维FLAC数值模拟分析模型，模拟地铁车站各阶段施工过程，预测各施工阶段地表沉降、地层变形以及支护结构变形规律、车站围岩应力、主要结构应力变化规律。地表沉降主要影响区域在距车站20m范围内；地层沉降量随着地层埋深的增加而增加；导洞开挖后拱顶下沉，拱底隆起，两帮向洞内收敛；边桩水平位移曲线呈现臌胀形式；车站施工完成后，车站中部扣拱受拉应力较大。

（6）完成了围岩及支护结构变形、受力监测方案设计并进行了现场监测，分析了竖井及导洞施工阶段围岩及支护结构变形及受力变化规律。现场监测结果表明：装配式支撑能有效抵消支撑的轴力；装配式支撑能有效降低围护桩桩体倾斜程度；导洞施工阶段要注意先行导洞拱顶和净空收敛变形随施工的变化情况，还要注意相邻两导洞之间的净空收敛变形的互相影响；开挖断面越大，上部荷载越大，扰动越大，围岩、支护结构受力就越复杂。

PBA method is one of the important methods of urban subway station construction under complex ground environment. It is of great significance to carry out the research on surrounding rock deformation, supporting structure deformation and stress characteristics of PBA method in the construction of shaft and guide hole of subway station. In this paper, a station in Xi 'an Metro Line 8 is taken as the background. Laboratory tests, numerical simulation and field monitoring are used. The main work and conclusions are as follows:

(1)The mechanical properties of subway station loess under different confining pressure and moisture content were studied by indoor macroscopic test and nuclear magnetic resonance microscopic test, and the macroscopic and microscopic failure law of subway station loess was given. The increase of water content will weaken the formation strength to some extent, and the increase of confining pressure can enhance the strength of surrounding rock. The peak value of soil T₂ spectrum curve increased with the increase of water content.

(2)The research shows that the main influencing factors of ground settlement induced by PBA construction of subway station in loess area include seven factors : station buried depth, loess property, groundwater level, underground pipeline, ground conditions, excavation method and sequence, and support structure of pilot tunnel. Based on the idea of controlling the surface settlement induced by PBA construction in a reasonable range, the construction process of PBA method for underground excavation of subway station is given.

(3)The three-dimensional FLAC model of underground excavation station shaft is established to simulate the influence of cast-in-place and prefabricated support shaft construction on the surrounding environment, internal support and retaining pile. The influence law of shaft and surrounding environment deformation caused by two different working conditions is analyzed. The conclusion is that the construction of bored pile combined with prefabricated concrete support can effectively overcome the shortcomings of cast-in-place reinforced concrete internal support such as slow construction and serious pollution.

(4)The FLAC numerical calculation method is used to study the spatial effect caused by the construction of the two-way guide hole and the mechanical characteristics of the horsehead gate during the construction of the guide hole. The surface settlement deformation law and the stress variation law of the horsehead gate under three different excavation sequences of the two-way guide hole are analyzed. The evolution law of the maximum principal stress and the maximum shear stress of the horsehead gate with the excavation footage of the guide hole is studied. The simulation results are compared with the field monitoring data, and the optimal construction scheme of the two-way guide hole is given.

(5)The two-dimensional FLAC numerical simulation analysis model of subway PBA underground excavation station is established to simulate the construction process of subway station in each stage, and to predict the surface settlement, stratum deformation, deformation law of supporting structure, surrounding rock stress of station, and variation law of main structural stress in each construction stage. the main influence area of surface subsidence is within 20m from the station; the stratum settlement increases with the increase of stratum depth; after the pilot tunnel excavation, the vault sinks, the arch bottom rises, and the two sides converge to the tunnel; the horizontal displacement curve of side pile shows bulging form; after the completion of the station construction, the tensile stress of the middle buckle arch of the station is large.

(6)The deformation and stress monitoring scheme design of surrounding rock and supporting structure are completed, and the field monitoring is carried out. The deformation and stress variation of surrounding rock and supporting structure during shaft and guide hole construction are analyzed. The field monitoring results show that the assembled support can effectively offset the axial force of the support; fabricated support can effectively reduce the inclination of the retaining pile; attention should be paid to the change of vault and clearance convergence deformation with construction in the construction stage of pilot tunnel, and the mutual influence of clearance convergence deformation between adjacent two pilot tunnels should also be paid to. The greater the excavation section, the greater the upper load, the greater the disturbance, the more complex the surrounding rock and supporting structure.

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