Abstract: Generally, special requirements on the tunnel lining structure are needed for swelling rock. This study investigated the lining structure improvements for Lirang tunnel of Liangzhong-Chongqing highway composed of swelling rock. A new numerical simulation model was proposed based on the Noorany modified swelling model. Then we studied the optimum design of EPS buffer layer thickness and the stability of the tunnel structure by using the established model. FLAC software was employed to comparatively analyse mechanics states of three kinds of structures after excavation, including the primary support, buffer layer and secondary lining of the excavated tunnel. At last, the optimum thickness of buffer layer was obtained for the certain tunnel condition. In addition, the EPS buffer layer with a thickness of 40 cm was applied in practical supporting structure between the first and the second lining. The field monitoring results showed that the actual values of lining contact pressure were less than those of standard warning, which indicated that the reliability of optimum proposal for support structures was verified. Therefore, theoretical and applied studies demonstrate that the EPS buffer layer can be an effective improvement for lining structure and a control measure for preventing potential disasters of tunnels in rich-water and swelling rock. The optimisation scheme and the design method also have essential guidance for similar projects.

Key words: tunneling, swelling rock, swelling constitutive, lining optimum structure, buffer layer