Seismic response of twin tunnels in weathered rocks
With rapid urbanization and wide use of underground space, the behaviour of underground structures under various conditions has gained significant attention. Twin tunnels are two closely constructed tunnels. Underground structures are generally supported by the surrounding medium like soil or rock. Due to this additional constraint from the ground, the underground structures are not as vulnerable as compared to the surface structures during an earthquake event. But recent earthquakes have shown that these structures are also prone to damages. Unlike surface structures, the soil-structure interaction is very important in the seismic analysis of underground structures. Moreover, in the case of structures like tunnels, rather than inertial forces the deformation behaviour is controlling the seismic behaviour. The various waves generated during an earthquake event generate additional forces and moments, which may jeopardize the stability of tunnels/underground structures. Among the different types of ground instability associated with seismic loading, the upward and downward movement of the ground due to seismic waves is studied here. In this study, a two dimensional numerical simulation using finite element method is done to understand the response of twin tunnels in weathered type of rock. The dynamic load is applied in the form of earthquake loading. The effect of relative spacing between tunnels is studied when they are subjected to earthquake loading. The results show that due to the dynamic loading the effects on ground settlement is significant especially when the relative spacing between the tunnels are less.
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