Analytical Solution and Shaking Table Test on Tunnels through Soft-Hard Stratum with a Transition Tunnel and Flexible Joints

Tunnels, where they pass through soft-hard strata, are severely damaged during earthquakes. These issues have not yet been well understood. In this study, the seismic performances of a tunnel passing through soft-hard stratum with a transition tunnel and flexible joints under earthquake motion were investigated by proposed analytical solutions and scaled shaking table tests. First, a mechanical model of a tunnel passing through soft-hard stratum with flexible joints is proposed, and it is derived by the Green’s function method. Then, a parametric analysis is conducted to investigate the effects of important variables on tunnels through soft-hard stratum. Finally, shaking table tests are conducted to verify the proposed solution and further investigate the seismic behaviors of the tunnel. The results show that: (1) the analytical solutions are workable and effective; (2) the influence of the soft-hard stratum junction on the tunnel responses is remarkable—the largest bending moment is located at the side of soft rock near the sharp contact area and the maximum shear force appears at the contact; (3) the joints and the transition tunnel could mitigate the potential adverse effects of the sharp contact area—the region affected by the joint is approximately 4.5 times the tunnel diameter on both sides of the stratum interface; and (4) the influence of sharp change of ground layers is more remarkable with a larger excitation amplitude.