Advanced numerical analysis of tunnel behaviour in structured clayey soils under seismic loading

Abstract

Tunnels are crucial components of transportation networks and considered as “lifeline” utilities as their continued operation is of vital importance during and in the immediate aftermath of an earthquake. It is, therefore, imperative to assess the engineering performance of such important geotechnical structures to ensure their resilience during and after seismic events. To achieve an accurate prediction of the tunnel behaviour during earthquake, a better approach should be implemented that can capture the multi-directional propagation of the seismic waves and the realistic soil response to seismic loads. Seismic wave propagation has an arbitrary direction with respect to the axis of the structure that causes multi-directional loading for the soil deposit and tunnel lining. Two-dimensional (2D) simplifications of these three-dimensional (3D) effects can impact the seismic response of tunnels and underestimate the lining forces. Furthermore, most natural soils particularly natural clays are characterised by high stiffness and peak strength due to initial structure. Extreme events such as an earthquake can induce sufficient stiffness degradation in the soil associated to strainsoftening processes. Under such condition, the initial structure and its progressive destructuration may significantly alter the soil behaviour and its interaction with the tunnels. This dissertation investigates and presents novel results from advanced numerical simulations of the behaviour of shallow circular tunnels in natural clays accounting for soil structure degradation induced by earthquake loading. Moreover, it adopts 3D space model applying multi-directional seismic input motions. Notably, the results show that the soil destructuration facilitates the transmission of higher loads in the longitudinal lining forces while reducing the transverse lining forces. All these studies highlight for the first time the importance of the initial structure and its degradation and the benefits of the 3D approach in controlling the magnitude of the tunnel lining forces and, consequently, the overall seismic tunnel design.