Abstract
The platform of the Badaling Great Wall Underground Station consisted of large span triple tunnels. The triple tunnels passed through several major faults. Based on preliminary in situ monitoring, the interaction behavior among triple tunnels was significantly severe, and the original supports failed to meet the safety requirement. Therefore, pipe roof pre-support (PRPS) and curtain grouting pre-reinforcement (CGPR) were used as the pre-construction techniques. By numerical investigation, the pipe diameter, the arrangement angle of PRPS, and the grouting thickness of CGPR were optimized as 108 mm, 150°, and 3 m, respectively. According to numerical results, PRPS predominantly bore loose rock and reduced mean crown settlement (MCS), while CGPR primarily improved the mechanical properties of rock and decreased the pillar plastic ratio (PPR). PRPS and CGPR had complementary advantages in time, space, and mechanical properties. PRPS could timely form a strong arch structure far ahead of the tunnel face, CGPR could effectively restrict the development of plastic zone. They mutually and actively formed a strong permanent ring in front of the tunnel face. After the field application of PRPS and CGPR, the mean surrounding rock pressure reduced by 33.4%, the MCS reduced by 58.7%, and no support damage was observed. The excavation safety was guaranteed.
Highlights
For large span tunnels excavated in the weak and broken surrounding ground, excessive deformation and potential damage may take place ahead of the excavation face.Pre-construction techniques have been extensively used to minimize pre-deformation and pre-failure of the rock mass
After the field application of Pipe roof pre-support (PRPS) and curtain grouting pre-reinforcement (CGPR), the mean surrounding rock pressure reduced by 33.4%, the mean crown settlement (MCS) reduced by 58.7%, and no support damage was observed
The positive value indicates a compressive force between the primary lining and rock mass
Summary
For large span tunnels excavated in the weak and broken surrounding ground, excessive deformation and potential damage may take place ahead of the excavation face.Pre-construction techniques have been extensively used to minimize pre-deformation and pre-failure of the rock mass. For large span tunnels excavated in the weak and broken surrounding ground, excessive deformation and potential damage may take place ahead of the excavation face. Pre-construction techniques can be divided into pre-support techniques and pre-reinforcement techniques. The pipe roof and curtain grouting are commonly used as the representative techniques of pre-support and pre-reinforcement, respectively. Pipe roof pre-support (PRPS) consists of grouted steel pipes with large moment inertia. These pipes are circumferentially inserted from the tunnel face with the pipe diameter of 50–200 mm and the advanced length of 20–50 m. With the great stiffness of the steel pipe, the PRPS system could redistribute the stress state ahead of the tunnel face and eventually decrease the ground settlement [1].
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