在黄土地区城市地铁建设中，由于黄土本身固有的大孔隙、水敏性等工程特点，易导致施工出现地面沉降、坍塌等问题，造成较大的经济损失。洞桩法车站施工时，扣拱施工工序和受力转换复杂，对整体结构的稳定性和安全性影响较大。鉴于此，本文以西安地铁2号线何家营站项目为依托，采用室内试验、数值模拟、现场监测相结合的研究方法，对黄土地区洞桩法车站扣拱施工受力特性进行研究，主要工作及结论如下：

（1）为了更准确地掌握施工区域黄土物理力学性质，采集何家营地区原状土制样，进行相关力学试验，研究不同含水率对黄土的物理力学参数的影响规律并进行拟合分析。试验表明：黄土抗压强度、变形模量、黏聚力、内摩擦角皆会随着含水率的升高而降低，且抗压强度、变形模量的减小速度在含水率较低时最快，表明干燥状态下的土体遇水后，其抗压能力急剧降低，容易出现失稳破坏。因此建议扣拱施工过程中应加强对给排水管线和地下水的监测。

（2）利用有限元软件PLAXIS 3D对不同扣拱初支、二衬施工顺序进行模拟，研究结果表明：先施工中跨初支扣拱，错开安全步距后再施工边跨初支扣拱，是较为合理的初支施工顺序，能够有效控制地表变形、初支结构内力、顶纵梁底部水平位移；两边跨施工错距需控制在3m内，有利于地表沉降与结构受力。初支扣拱施工完成后，再按照先中后边的顺序施作二衬扣拱，且边跨错距也应控制在3m以内。

（3）分析土层在不同含水率条件下地表沉降和扣拱力学特性的影响规律，研究结果为：随着含水率的增大，地表沉降的变形逐渐变缓。当含水率从8%增加到35%，地表沉降增大了357.26%，扣拱初支弯矩增大了69%～94%，轴力增大了20%～30%，扣拱二衬应力增长了14%～15%。因此，土层含水率对洞桩法扣拱施工的影响较大，在工程建设中应对工作面的围岩含水率进行实时监控，避免因含水率增加而导致的围岩强度降低或变形增大。

（4）为验证数值模拟的可靠性，对何家营车站施工现场地表沉降、扣拱围岩压力和初支内力变化情况进行现场监测。监测结果表明：扣拱初支及二衬施工完成后，最大地表沉降值分别为 11.01 mm和15.37 mm，五个主要监测点的最大沉降量的实测值与数值模拟相差约11.57%~17.29%，表明数值模拟的研究方法可靠。围岩压力快速上升主要发生在扣拱上台阶开挖期间，最大值达到53.2kPa；当边跨上台阶开挖时，中跨扣拱围岩受到扰动而受力重分布，围岩压力增大了50%。随着扣拱二衬施工，围岩压力趋于稳定，但由于拆除导洞初支使得本该由其承担的部分荷载全部由扣拱初支承担，扣拱初支内力急剧增长。通过对数据进行深度分析发现：在黄土地区洞桩法施工中，全土柱法计算的理论土压力值与现场监测值存在较大差异，表明使用全土柱法计算得到的土压力值进行支护结构设计偏于安全。

In the construction of urban subways in loess areas, due to the inherent large pores and water sensitivity of loess, it is easy to cause problems such as land subsidence and collapse during construction, resulting in large economic losses. In the construction of the Pile-Beam-Arch method station, the construction procedure and force conversion of the buckle arch are complex, which has a great impact on the stability and safety of the overall structure construction. In view of this, based on the Hejiaying Station project of Xi'an Metro Line 2, this paper adopts the research method combining indoor test, numerical simulation and field monitoring to study the force characteristics during the construction of the PBA method station in the loess area. The main work and conclusions are as follows:

(1) Undisturbed soil samples were collected in Hejiaying area for uniaxial and triaxial soil compression tests. The influence of different water contents on the mechanical properties of the samples was studied, and the law of the influence of water contents on the physical and mechanical parameters of loess was obtained and fitting analysis was carried out. The test shows that the compressive strength, deformation modulus, cohesion and internal friction angle of loess all decrease with the increase of the moisture content of loess. It shows that the compressive capacity of soil in dry state decreases sharply when it encounters water, and it is prone to instability failure. Therefore, it is suggested that the monitoring of water supply and drainage pipelines and groundwater should be strengthened during the construction of the buckle arch.

(2) The finite element software PLAXIS 3D is used to carry out numerical simulation of different buckle arch construction sequences. The research results show that it is a more reasonable construction sequence to construct the initial support of the middle span first, and then to stagger the safety step distance and then construct the initial support of the side span, which can effectively control the surface deformation, the internal force of the primary support structure, and the horizontal displacement of the top longitudinal beam. In addition, the staggered distance between the two sides of the span construction should be controlled within 3 meters, which is beneficial to the formation deformation and the stress of the structure. After the construction of the primary buckle arch is completed, the second lining buckle arch is applied in the order of "first middle and back side", and the side-span stagger distance should also be controlled within 3m.

 (3) The influence law of stratum deformation and buckled arch mechanical properties of soil layer under different moisture content conditions is analyzed, and the research results are as follows: with the increase of water content, the deformation of surface subsidence gradually slowed down. When the water content increases from 8% to 35%, the surface settlement increases by 357.26%, the bending moment of the primary support of the buckle arch increases by 69% to 94%, and the axial force increases by 20% to 30%. The stress of the secondary lining structure increases by 14% to 15%. Therefore, the moisture content of the soil layer has a great influence on the construction of the PBA method. In the construction of the project, the moisture content of the surrounding rock of the working face should be monitored in real time to avoid the decrease in the strength of the surrounding rock or the increase in the deformation caused by the increase of the moisture content.

(4) In order to verify the reliability of the numerical simulation, on-site monitoring was carried out on the changes of the surface settlement, the surrounding rock pressure of the buckle arch and the internal force of the initial support at the construction site of Hejiaying Station. The monitoring results show that: after the completion of the primary support of the buckle arch and the construction of the secondary lining, the maximum surface settlement values are 11.01 mm and 15.37 mm, respectively. The measured value of the maximum settlement of the five main monitoring points differs from the numerical simulation by about 11.57%~17.29%. It shows that the research method of numerical simulation is reliable. The rapid rise of the surrounding rock pressure mainly occurs during the excavation of the upper bench of the buckle arch, and the maximum value reaches 53.2kPa; when the side span is excavated on the upper bench, the surrounding rock of the middle-span buckle arch is disturbed and the force is redistributed, and the surrounding rock pressure increases 50%. After the construction of the second lining of the buckle arch, the surrounding rock pressure tends to be stable. The internal force of the main reinforcement of the primary support of the buckle arch surged during the construction of the secondary lining, because the removal of the primary support of the pilot hole caused a part of the external load have been borne by it to be transferred to the buckle arch. Through the in-depth analysis of the data, it is found that in the construction of the hole pile method in the loess area, there is a big difference between the theoretical earth pressure value calculated by the whole soil column method and the field monitoring value, indicating that the earth pressure value calculated by the whole soil column method is used for supporting structures. The design is biased towards safety.

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