A branching-out tunnel is a special underground structure, which contains four sections of separated, small-spaced, multi-arched, and large-span structural sections. In order to study the stability of the branching-out tunnel construction process, a finite difference software Flac3D was used to study the whole construction process. The construction from the large-span structure to the multi-arched structure is difficult, which was simulated by Flac3D numerical model in this study. The following conclusions can be made after the numerical simulation: (1) the whole structure is stable because of the existence of sound surrounding rock, (2) stress concentration develops at the feet of the large-span structure and the middle wall of the multi-arched structure, (3) the tension concentrates on the roof and the floor, (4) vertical displacement is the key displacement and the maximum displacement is approximately 5mm without any support and 2mm with supports, and (5) less plastic zone distributes in the middle wall.
The recent western development strategies in China have promoted a rapid development of highway constructions in western areas. More and more tunnels need to go through mountains. Due to the limitations of terrain conditions and construction cost, a Baziling tunnel in the Shanghai-to-Chengdu National Expressway is under construction adopting a form of branching-out tunnel. So far, only a few branching-out tunnels have been constructed and put into operation in China and other countries. This kind of tunnels involves the construction of separated, small-spaced, multi-arched, and large-span structural sections. The complex structures and the changes of the working sequences often bring the difficulties for construction. It is crucial to ensure the rapid construction and safety for the branching-out tunnels. Numerical simulation has become an important tool in the design of underground excavation in geotechnical engineering projects. Numerical analysis can reasonably represent field conditions by taking into account key elements of excavation such as geo-mechanical characteristics of the ground and in-situ stress conditions. The most widely used technique in the numerical simulation of tunneling is the finite element method (FEM) as shown in a recent survey by Negro and de Queiroz . However, the Fast Lagrangian Analysis of Continua (FLAC) in 3 Dimensions (Flac3D), a three-dimensional explicit finite-difference program, has been increasingly used in the recent years. This software offers a explicit solution scheme, which follows arbitrary non-linearity in stress/strain laws and can accurately model plastic collapse loads and plastic flow .
2. PROJECT DESCRIPTIONS
The Baziling branching-out tunnel is located along the Shanghai-to-Chengdu National Expressway. Because a large number of mountains exist between the western areas of Hubei Province and the city of Chongqing, a number of tunnels have to be constructed through the mountains. This Baziling branching-out tunnel is one of such tunnels. The entrance of Baziling branching-out tunnel is in the county of Changyang in the city of Yichang while the exit is in the city of Enshi. Therefore, typically two parallel tunnels are required to cross a 400m wide river. If a branching-out tunnel is used, one bridge is needed to cross the river.