| Immediate ring closure, based on full face excavation with small bench-cut, is achieved by installing an inverted arch at a short distance from the excavation face to stabilize the tunnel structurein poor geological conditions. However, the relationship between the effects of immediate ring closure and ground conditions is not fully understood. Our case study of past construction data shows that the technique has been used in conditions of deep overburden, low competence factor, and stratified rocks with strong cohesion and low internal friction angle compared with massive rocks. In this study, a three-dimensional numerical analysis was carried out to examine the influence of ground conditions in a construction with immediate ring closure. Cohesion, internal friction angle, and closure length (CL) from face to inverted arch were varied while keeping the competence factor of ground constant at a small value. As a result, immediate ring closure had the potential to constrain the convergence of tunnel cross section and to reduce ground-loosening. In addition, the shortening of CL had a large effect on constraining the convergence of tunnel cross section and reducing the ground loosening area because the inverted arch could bear a large axial force and generate large inner pressure acting on the ground. The results also showed that the instability of the tunnel structure was more likely to occur in stratified rocks because a larger plastic zone formed in stratified rocks than in massive rocks for the same competence factor. However, the effect of the plastic zone reduction by the shortening of CL was more prominent in stratified rocks than in massive rocks. Therefore, immediate ring closure is potentially effective, especially in stratified rocks. |
