山岭隧道浅埋段盖挖支护设计方法研究

山岭隧道的进出口或穿越峡谷地区常存在浅埋段，如何在保证安全的前提下经济、高效地完成隧道开挖支护施工成为工程建设的关键问题，兼具明挖与暗挖优点的盖挖法已初步应用于山岭隧道浅埋段，但其支护设计方法尚需完善。为此，基于山岭隧道浅埋段盖挖施工特点及其盖拱几何模型，首先提出盖拱承载受力简化分析模型；其次，采用结构力学方法建立出盖拱支护结构内力简化计算方法，获得盖拱安全厚度确定方法，并考虑盖拱与拱脚过渡段的平滑缓和作用，构建出拱脚扩大基础的承载力与稳定性分析方法；然后，采用所建立的盖挖支护设计方法探讨隧道埋深、盖拱矢高、圆心角、半径与拱脚宽度等因素对盖拱支护结构承载特性的影响规律，提出了山岭隧道浅埋段盖挖优化设计原则；最后，采用所建立的盖挖支护设计方法对工程实例进行分析，验证了工程实例典型断面盖拱设计参数的合理性，同时探讨了山岭隧道浅埋段盖挖支护设计方法及其优化设计原则的合理性。研究结果表明：浅埋段盖拱宜与隧道支护结构完全接触，盖拱设计厚度不宜大于0.6 m、内侧圆心角不宜小于120°；盖拱与拱脚应设置平滑缓和的过渡段，提高拱脚地基承载力能有效减小拱脚扩大基础的宽度；隧道初衬钢拱架浇筑于盖拱内不仅能保证盖挖时隧道初期支护封闭成环，还能提高盖拱稳定性；地基注浆加固锚杆不仅能提高地基承载力，还能增强拱脚基础的水平抗滑移稳定性。

Supporting Structure Design Method for the Shallow-buried Section of a Mountain Tunnel by Covered Excavation

A mountain tunnel often has shallow-buried sections at the entrance, exit, or in the region where it passes through the canyon. Completing tunnel excavation in an economical, effective, and safe manner has become a key consideration in engineering construction. The covered excavation method, which has the advantages of the open-cut method and subsurface excavation method, has been employed for shallow-buried sections of mountain tunnels. However, supporting structure design for the covered arch still needs to be improved. Therefore, first, a simplified bearing capacity analysis model for the covered arch was proposed based on the construction characteristics of the arch and a geometric model of the shallow-buried section of the mountain tunnel. Second, a simplified method for the calculation of the internal force of the supporting structure was established using structural mechanics. Further, a process for the determination of the thickness of the covered arch safety was obtained. Then, a bearing capacity and stability analysis method for the spread foundation of the covered arch foot, which considered the smooth nature of the transition section between the covered arch and its foot, was proposed. Third, the relationships between factors such as tunnel depth, arch rise, central angle, radius, and arch foot width and the bearing characteristics of the supporting structure were discussed using the proposed method. Moreover, a covered excavation optimization design principle for the shallow-buried section of the mountain tunnel was proposed. Finally, the covered excavation design method was used to analyze the engineering example, and the rationality of the design parameters of typical section covered arch was verified. At the same time, the rationality of the design method and the optimization design principle were discussed. The results show that the covered arch needs to be in full contact with the tunnel supporting structure. The thickness of the covered arch should not be more than 0.6 m, and the central angle should not be less than 120°. The smooth transition section between the covered arch and its foot should be designed to improve the force condition of the foundation, and the spread foundation width of the arch foot can be effectively reduced by increasing the bearing capacity of the foundation soil. The steel arch frame of the initial liner placed in the covered arch not only ensures that the initial support forms one closed ring but also improves the stability of the covered arch. The grouting anchors can improve the bearing capacity of the foundation soil and can enhance the horizontal antisliding stability of the arch foundation.