The support load analysis of deep-buried composite lining tunnel in rheological rock mass

The loads on primary support and secondary lining in composite lining tunnel are one of the key problems in support design. The primary support and secondary lining which as the main bearing structure determines the long-term operation safety of tunnels. In this study, three kinds of tunnel support conditions are established and the rheological model, stress releasing coefficient and additional radial body force of bolts are considered. Also, the analytical formulas of displacement on the primary support and secondary lining are derived. According to the relationship between displacement difference and support stiffness, the support loads are obtained. The calculated displacements and loads of the support structure are basically consistent with the field measurement, which verifies the reliability of the theoretical method. In addition, the influences of support parameters on the support load and bearing ratio are studied. Finally, the variation of the time-history curves of the measured support loads are discussed. The results show that the installation time of primary support, installation time of secondary lining, bolt length, thickness of the secondary lining, cohesion and rheology all may influence the support load. Meanwhile, the thickness of the secondary lining and the installation time of secondary lining have greater impact to the load bearing ratio. The combination of the weak rock mass, the early installation of the secondary lining, and the insufficient thickness of the secondary lining will make the load ratio of the secondary lining significantly higher. The stress paths of support loads are different depending on whether the support structure is in tension or in compression.