Shaking table model tests on tunnels at different depths

There are now increasing numbers of damage reports on mountain tunnels under strong earthquake including deep buried tunnels with high overburdens. Failure mechanisms for deep buried tunnels and shallow buried tunnels are different. But the effects of high overburden during earthquake on tunnel have not been understood thoroughly. Few model tests have been applied on such conditions as the overburden is hard to be applied. A set of flexible rubber airbag loading equipment have been invented and used to apply overburden on top of surrounding rock in a rigid testing box to perform model tests on shaking table. Then, a series of model tests have been performed to study the dynamic response of tunnels at different overburden depths with the above test system. The rock condition around tunnel is designed as class V according to the China National Design Code for Mountainous Tunnels. A new similarity material with a main component of fly ash has been successfully created to simulate the surrounding rock to meet the requirement of the shaking table test. The results showed that this testing system is feasible. Tests after prototypes of 25 m, 175m and 275m overburden depths are performed. The results show that, the acceleration amplifier factor increase from the bottom to top of the rock. This trend is amplified with the increasing overburden depth. Maximum structure moments and displacements are found at the shoulder and foot of the tunnel which result in serious cracks in such areas. But failure modes of tunnels at different depths are similar. It is also found that, the additional moments from vibration decreases with the increase of overburden depth. It could be concluded that, shallow buried tunnels are much more vulnerable than deep buried tunnels, while the vulnerability of deep tunnels are similar at different overburden depths.