Longitudinal mechanical response of tunnels under active normal faulting

Abstract

This paper aims to clarify the mechanism of the longitudinal response of a tunnel under normal faulting via a comprehensive analysis of available experimental data and numerical simulations. Four 1 g condition model tests were reviewed and reanalysed to highlight the key characteristics of the tunnel response under normal faulting: S-shaped deformation and inverted S-shaped bending strain distribution in the longitudinal direction; the main affected zone of faulting is approximately six times the tunnel diameter to the fault plane. A three-dimensional finite element model was also established and verified, followed by a sensitivity analysis of key parameters, including the fault dislocation, dip angle, tunnel rigidity and relative stiffness between the hanging wall and footwall. All results reveal that the longitudinal mechanical response under normal faulting is dominated by a combination of bending, tension, and shearing. Bending and shearing are induced by the large unbalanced rock pressure at the vault in the hanging wall and the inverted arch in the footwall; the value of unbalanced rock pressure is directly proportional to the dislocation but negatively correlated with the dip angle. Although the main part of the tunnel stays in tension, axial compressive strain exists around the fault plane when the dip angle is greater than 70°, which may be related to the ovaling effect of the tunnel. Such an ovaling effect is caused by the compression at the cross-section of the tunnel and may lead to more complicated internal strain.