Damping mechanism of shallow cylindrical parallel tunnel with grouting reinforcement zone

The parallel small clear-distance tunnels have obvious dynamic amplification effect under earthquake loading. Small neighbourhood shield tunnels in the strong earthquake region were selected to investigate the dynamic response of shallow-buried parallel small clear-distance tunnels. A series of dynamic stress solutions was deducted for shallow-buried composite lined tunnels subjected to vertically propagating incident P waves using the Fourier-Bessel expansion method. Moreover, we discussed the influence of the distance between two tunnels and the grouting reinforcement zone on dynamic stress of tunnels. The results show that the distance between tunnels plays a vital role on the dynamic stress response of tunnels, and dynamic stress concentration factors of linings increase obviously as decreasing the distance of tunnels. The critical distance between the centres of two circular tunnels is proposed as 2.5 times diameter. The aseismic and damping design should also be suggested when the distance is below the critical value. The dynamic stress of linings decreases with grouting reinforcement ring zone set outside around tunnels, and the dynamic stress of linings decreases with increasing the stiffness of the grouting zone. The optimal ratio of shear wave velocity of the grouting zone to that of surrounding rock is suggested as 1.2. The recommended ratio of the thickness of the grouting zone to the tunnel radius is 0.5.