Collapse Mechanism of Tunnel below Underground Structure
The collapse mechanism of a tunnel in free field sandy soils with a high water table is generally known as cone- or chimney-shaped. However, when a tunnel is constructed below an underground structure with a much larger width than the tunnel diameter, the stiffness of the structure significantly affects the settlement profile and even the collapse mechanism. The recently reported tunnel collapse at site A showed a bottom-enlarged cylindrical shape. This study investigates the settlement profile and the collapse mechanism of the ground below an existing underground structure, caused by tunneling. A pre-study, using numerical and empirical methods, was conducted to evaluate the ground deformation profile, and physical model tests were performed to investigate the collapse mechanism. It is concluded that the factors causing the failure mechanism are the structural confinement and continuous inflow of groundwater into the tunnel.