Supports for tunneling in squeezing ground

The Jiyoshi road tunnel is about 3000 m in length and runs through the Shikoku karst. Geologically, the Shikoku Mountains consist of an accretionary prism in the Chichibu belt. This accretionary prism is a typical mlange. The squeezing phenomenon occurred because the tunnel passed through very weak serpentinite. This serpentinite had an unconfined compressive strength of under 0.5 MPa and was composed of 90% clay minerals. The tunnel was over 250 m deep and the competence factor (the ground strength - overburden ratio) was estimated to be under 0.05. Consequently, tunnel convergence exceeded 30 cm. Our analysis showed a strong correlation between the initial convergence of the tunnel, when its face advanced 3 m, and the final convergence. It was predicted from this correlation that the final convergence in tunneling through the serpentine would be approximately 30-50 cm. The results of probabilistic and predictive analyses revealed that it would be difficult to control the convergence. In order to keep the support system sound, the tunnel required very rigid supports (H-250 + H-250, 60 cm steel fiber reinforced shotcrete with a compressive strength of 36 N/mm2) and pre-supports (forepiling L=12 m, face bolts, L=12 m). This was the first time that these measures had been applied to a tunnel in Japan. This paper analyzes the relationship between the actual ground pressure and the rigidity of the supports. It also reports on a study in which it was determined that the most suitable supporting members for squeezing rock were very rigid supports, and compares the results of the study with actual measurements of stress and displacement. It was found through stress measurements that the double supporting system shifted the neutral axis and decreased the stresses to below the ultimate stresses, and that the system was sound. The initial convergence rate per 3 m was reduced drastically and we could gradually decrease the rigidity of the system. A new problem heaving of the invert occurred in this tunnel section. This report describes the results of geological explorations that were carried out to find a solution to this problem. (A) "Reprinted with permission from Elsevier". For the covering abstract see ITRD E124500.