PBA法暗挖施工是复杂地面环境下地铁车站施工的重要方法之一，西安地区某地铁车站首次采用PBA法施工，开展黄土地区PBA法暗挖地铁车站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。本文以西安地铁六号线二期工程某车站为背景，采用室内试验、理论分析、数值模拟及现场监测的手段进行研究，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、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）分析诱发地表沉降、支护结构变形的主要影响因素，基于随机介质理论研究了PBA法暗挖车站施工主要阶段地表沉降规律，建立了导洞开挖和扣拱施工阶段地表沉降理论计算公式。 （3）建立了地铁暗挖车站三维FLAC数值模拟分析模型，采用室内试验所得的土层物理力学参数，分析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鼓胀形式；车站施工完成后，车站中部扣拱具有较大的拉应力。 （4）完成了围岩及支护结构变形、受力监测方案设计并进行现场监测，分析了各施工阶段围岩及支护结构变形、受力变化规律。现场监测结果表明：扣拱施工阶段是引起地层沉降的主要原因之一；施工过程中，要注意先行导洞拱顶和净空收敛随施工的变化情况，还要注意相邻两导洞之间的净空收敛变形的互相影响；迎土侧和中性面轴向钢筋均处于受压状态，背土侧呈现受拉状态；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 most important construction methods for subway station under complex ground environment. A subway station in Xi 'an was constructed with PBA method for the first time. It is of great significance to study the deformation and the stress characteristics of surrounding rock and supporting structure during the construction of subway station with PBA method in loess area. Taking a station of Xi 'an Metro Line 6 Phase II Project as the background, this paper adopts the means of laboratory test, theoretical analysis, numerical simulation and on-site monitoring to conduct research. The main work and conclusions are as follows:

 (1) The mechanical properties of loess in subway stations under different confining pressures and water content were studied by indoor macro test and nuclear magnetic resonance (NMR) micro test. The macro and micro failure rules of loess in subway stations were given. The increase of water content will weaken the formation strength to a certain extent, and the increase of confining pressure can enhance the strength of surrounding rock. The peak value of T₂ spectrum curve of soil samples increases with the increase of water content.

(2) The main influencing factors of surface settlement and deformation of supporting structure are analyzed. Based on the random medium theory, the law of surface settlement in the main stages of construction of underground excavation station with PBA method is studied, and the theoretical calculation formula of surface settlement in the construction stage of guide tunnel excavation and buckle arch is established.

(3) The three-dimensional FLAC numerical simulation analysis model of underground excavation station is established. The soil physical and mechanical parameters obtained from laboratory tests are used to analyze the rules of surface settlement, stratum deformation and supporting structure deformation, as well as the change rules of station surrounding rock stress and main structure stress at each construction stage. The main affected area of ground subsidence is within 20m from the station. The subsidence of stratum increases with the increase of the depth of stratum. When the guide tunnel is excavated, the vault sinks, the arch bottom uplifts, and the two sides converge to the cave. The horizontal displacement curve of side pile presents bulging form. After the construction of the station, the buckle arch in the middle of the station has a large tensile stress.

(4) The design of deformation and stress monitoring scheme of surrounding rock and supporting structure is completed and field monitoring is carried out. The deformation and stress change rule of surrounding rock and supporting structure in each construction stage was analyzed. Field monitoring results show that the buckle arch stage is one of the main reasons for ground settlement. In the process of construction, attention should be paid to the variation of the convergence of the vault and headroom of the pilot tunnel with the construction, and the mutual influence of the convergence deformation of the headroom between the adjacent two pilot tunnels. The axial reinforcement stress of the soil-facing side and the neutral side is in the state of compression, while the soil-back side is in the state of tension. The larger the excavation section is, the greater the upper load and disturbance are, and the more complex the force of surrounding rock and supporting structure is.

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