An Investigation into the Seismic Resilience of Shallow Cut-and-Cover Tunnel in Urban Environments

Abstract

Understanding the dynamic behaviour of shallow-buried tunnels in liquefiable soils is critical for the seismic design of underground infrastructure, particularly in congested urban areas, where interactions with adjacent surface structures further complicate this dynamic soil-structure interaction (SSI). This dissertation presents a comprehensive investigation combining dynamic centrifuge tests conducted at the Schofield Centre of the University of Cambridge, and corresponding 2D finite element (FEM) simulations to evaluate the seismic response of a rectangular cut-and-cover tunnel under varying conditions. The study begins with the evaluation of an isolated tunnel embedded in both dry and saturated sands. Results reveal that while dry sand conditions result in negligible tunnel movement, liquefied soils lead to significant tunnel uplift and lateral displacement, governed primarily by excess pore pressure generation and reduction in soil shear stiffness. A detailed force mechanism in both vertical and horizontal directions is then further developed to explain the resulting tunnel uplift and lateral movements in liquefied soils, supported by centrifuge data and particle image velocimetry (PIV) analysis. Key force components such as the excess pore pressure-induced uplift force, sidewall friction, and slip surface resistance are identified. These mechanisms are extended to explore the dynamic interaction between the tunnel and an adjacent surface building. Centrifuge tests reveal that structural loads alter excess pore pressure distribution and soil deformation patterns, affecting both tunnel flotation and building settlement. Reduced tunnel-building separation leads to reduce tunnel uplift and decreased building rotation due to non-uniform settlement buffering. The 2D dynamic numerical simulations show good agreement with the centrifuge observations and confirm that the soil column directly beneath the building foundation does not reach full liquefaction, primarily due to the self-weight of the structure. These findings highlight that a tunnel's longitudinal alignment passing beneath or near buildings with various separation distance may experience significant localized dynamic movements during seismic events. Finally, a mitigation method using tension piles commonly used in practice is evaluated through centrifuge tests and numerical modelling. Tension piles were found effective in restraining tunnel uplift, though they eliminated beneficial suction beneath the tunnel. It was also observed that neither the tunnel sidewall friction nor the friction along the soil slip surface can be fully mobilized in the absence of dynamic tunnel movement. Limitations of the 2D modelling in capturing full 3D pile-soil interaction are acknowledged, suggesting directions for future refinement.