Model testing for pipe-reinforced tunnel heading in a granular soil

One of the most popular pre-reinforcement methods of tunnel heading in granular soils would be the grouting with pipes, which is so-called the umbrella arch method. This technique allows safe excavation even in poor ground conditions by creating longitudinal arching parallel to the tunnel axis. The complexity of boundary conditions of the methods, however, imposes difficulties in representing the tunnelling procedure in laboratory and analytical models. Full-scale study to identify the reinforcement mechanism of the tunnel heading, therefore, has rarely been carried out. In this study, a large scale model testing for the pipe-reinforced tunnel heading in a granular soil was performed to understand reinforcing mechanism and improve design practice. Reinforcing patterns considered are non-reinforced, crown-only-reinforced, face-only-reinforced and crown and face-reinforced cases. The behaviour of ground and pipes was instrumented during tunnelling. The effects of reinforcing pattern and pipe length were investigated, particularly in terms of ground displacement and stress changes. Supplemental numerical analyses were carried out to investigate the effect of pipe length which cannot be covered by the model tests. It is revealed that the pipe reinforcement of heading increases longitudinal arching along the tunnel axis, decreases settlements significantly, and consequently improves the face stability.