THE EFFECT OF FAR-FIELD EARTHQUAKES ON THE BEHAVIOUR OF SHALLOW TUNNELS IN STRUCTURED CLAYS

Far-field earthquakes are typically characterised by their low peak ground acceleration, peak ground velocity and Arias intensity, having far lower damage potential than near-field signals. In places like Bangkok and Singapore, where known active seismic faults lie at considerable distance, the effects of far-field earthquakes on tunnels are frequently ignored. However, far-field motions have relatively longer duration and a greater probability of containing long-period waves which can result in a higher response in the low-frequency region of the acceleration spectrum. When thick layers of structured clay deposits, such as those found in Bangkok, are subjected to far-field earthquakes, they can significantly amplify a long-period ground motion. This can result in large ground deformations and generate high shear strains, which in turn induce large forces in geotechnical structures such as tunnel linings. In this paper, a set of two-dimensional finite element simulations have been conducted to predict the behaviour of a shallow tunnel built in a thick Bangkok soft clay deposit and subjected to both far-field and near-field earthquake motions. A kinematic hardening model developed for natural clays has been used to capture the essential features of the dynamic soil behaviour. The results clearly demonstrate that, despite having less intensity, a far-field, long-period, long-duration earthquake can induce equally destructive forces in the tunnel lining as those generated by a strong, near-field, short-duration motion. This highlights the importance of considering the effects of far-field earthquakes in designing seismic-resistant shallow tunnels in soft clays and reduce the risk of geotechnical infrastructure systems.