Long-term mechanical analysis of tunnel structures in rheological rock considering the degradation of primary lining

Rock load on lining structures increases over time for tunnels buried in rheological rock, and in addition deterioration of primary lining is common due to its structural characteristics and service environment attack, where these delayed features affect the mechanical response of tunnels. However, accounting for these delayed features in long-term stability assessment of tunnel structures is complex and has not attracted enough attention. In this paper, an analytical approach is proposed for investigating long-term mechanical response of tunnel structures in rheological rock influenced by degradation of primary lining. For this purpose, degradation of primary lining, characterized by decreasing concrete stiffness over time, is quantitatively described by an exponential model. The rheological characteristic of surrounding rock is simulated by the Burgers model. The time-varying solutions for rock deformation and support pressure are obtained by considering the coordinated interaction between surrounding rock and linings, and their correctness is verified by comparing them with numerical results. The results revealed that the pressure imposed on linings due to the rheological behavior of surrounding rock increases over time. As the primary lining degrades, the rheological load is transferred from primary lining to secondary lining, leading to increasing pressure on secondary lining; and a faster degradation rate of primary lining leads to greater pressure on secondary lining. Therefore, the primary lining should not be overlooked in long-term safety assessment of operation tunnels because of its role in bearing and transmitting load. Finally, the tunnel's design and operational maintenance strategy are discussed when the delay effects of surrounding rock and lining are taken into account.