The influence of the nonlinear behaviour of connecting bolts on the shear stiffness of circular joints in a tunnel segmental lining

The segmental lining is a very common type of lining of tunnels, which is adopted when the TBM (Tunnel Boring Machine) is used as an excavation machine. It has a very important static task, because it has to support the loads transmitted by the ground or rock around the tunnel. This segmental lining has a complex mechanical behaviour, conditioned by the presence of joints (circumferential and longitudinal ones) that exist between the different segments. The joints are connected to each other through different types of devices: the most used are steel bolts, which can be linear or curved. In this paper, the mechanical behaviour of the circumferential joints has been studied in detail, in the presence of bolts to ensure the connection between the different rings of the lining. More specifically, the influence of the bolts on the shear stiffness of the joints was analysed; it has a fundamental role in the development of the stresses and displacements of the segmental lining along the tunnel axis. To analyse the behaviour of the connecting bolts of the lining rings, specific numerical FEM models have been developed: they are able to evaluate the shear stiffness of the circumferential joint during the relative displacement between the rings facing the same circumferential joint. Some practical applications referring to real cases have made it possible to detect which are the geometric and mechanical parameters of the bolts that have a greater role on the shear behaviour of the circumferential joint. The comparison of the results obtained from the developed numerical models with the measurements of the laboratory tests allowed to validate the numerical models. Thanks to the achieved results, it is therefore now possible to define the characteristics of the connecting bolts of the segmental lining rings, also in relation to the possible consequences on the tensional and deformative state in the segmental lining and on the risk of damage due to possible high stress values in the concrete or in the steel constituting the bolts.