Analysis of mechanical behavior of primary lining in double-arch tunnel based on distributed fibre optic monitoring

Abstract

Cracking and crushing were observed in the first tunnel lining of the Kaidagu double-arch tunnel (China), excavated in weathered surrounding rock. This concrete damage occurred during excavation of the second tunnel. It was necessary to monitor the continuous strain distribution along the entire cross section of the lining. Optical fibres were attached to the inner and outer surfaces of the steel arch (primary lining) of the first lining, and monitoring was carried out during the second tunnel excavation. The overall strain distribution was obtained by the fibre optic monitoring, and the internal force and external load distribution of the primary lining were calculated using the proposed inversion analysis method. The calculation results indicated that the axial compression between the right arch shoulder and the right arch foot decreased, which could cause the internal forces of the middle partition wall to increase. The damage mechanism of the lining was inferred. For the first time, optical fibres were installed inside the lining instead of on the inner surface, and the inverse analysis method, which considered the combined effect of axial force and bending moment, was applied to the mechanical analysis of the primary lining. The calculations and analyses provided valuable information for the design and construction of similar projects.