The effect of multi-directional seismic loading on the behaviour of tunnels in structured clays

Current research extensively relies on two-dimensional (2D) modelling to predict the seismic behaviour of tunnels. However, seismic wave propagation can occur in arbitrary direction with respect to the axis of the structure, leading to multi-directional loading of the soil deposit and the tunnel lining. Using 2D simplifications to represent these three-dimensional (3D) effects can significantly impact the prediction of the tunnel's seismic response. Furthermore, most natural soils typically exhibit high stiffness and peak strength due to their initial structure. During strong earthquakes, such soils may experience significant stiffness degradation which may alter the response of the soil-tunnel system. This behaviour cannot be captured by simple elasto-plastic constitutive models, requiring the need to use advanced constitutive laws which incorporate soil initial structure degradation during dynamic loading. This paper presents novel results from advanced 3D numerical simulations of shallow circular tunnels in natural clays subjected to multi-directional seismic motions while considering soil structure degradation. Notably, the results indicate that soil destructuration facilitates the transmission of higher longitudinal loads in the lining while reducing the transverse forces. Therefore, the work highlights the significance of soil destructuration in accurately predicting the magnitude of tunnel lining forces under arbitrarily-directed seismic loading.