SEISMIC ANALYSIS OF TUNNELS

Abstract

Underground sub-structures such as tunnels, subways, etc., are an essential part of the transportation system. Tunnels connect point A to point B through the shortest route possible. Tunnels are also constructed for the purpose of connecting the two places, which are on the different sides of the hill or mountain. Also, in crowded cities, subways are necessary to facilitate the needs. Hence, the seismic design of tunnels becomes critical mainly in the earthquake-prone regions. The geometrical features of the tunnels make them respond very differently to seismic loading compared to super-structures. The aim of the project is to study the behaviour of tunnels under seismic loading. In this, we have investigated the effect of earthquake sequences over the tunnel by using Finite Element modelling in Plaxis software. A circular tunnel was modelled which was confined in the sand. Aluminium tunnel linings were then provided to the tunnel. The seismic force was then applied to the tunnel's base. The time history record for seismic loading was taken from Port Island station 32m below ground surface in a North-South direction. Apart from this time history, two other cyclic loadings were also applied each with different peak accelerations. The analysis took around 10 hours each. It was observed that changes in the lining forces were observed in the initial stages and in the later stage the values tend to reach a particular equilibrium value. The values of the different lining forces depend upon the magnitude of the peak acceleration. Also, the dynamic bending moments were found to be largest near the crown points, but their magnitudes were not very high. The values of the normal forces were found to be maximum near the right and left extreme points of the cross section of the tunnel. The values of the shear forces were found to be maximum at the shoulder points and the values were large enough, so the tunnels must be designed for shear while designing tunnels for seismic loading. It was also observed that the seismic behaviour of tunnels is mainly dependent on the intensity of the earthquake but not on the duration and the frequency of the earthquake, as they do not affect the values to that extent.