Failure Mechanism of an Underground Metro Tunnel Intersecting Steep Ground Fissure at Low Angle

Abstract

Failures of metro tunnels that pass obliquely through ground fissures at low angles are a common problem; however, their mechanism is unknown. In the city of Xi’an, China, Line No. 3 of the Xi’an Metro intersects ground fissures at a low angle (<25°) in certain zones. In this study, particle flow numerical analysis and physical model tests were used to investigate ground-surface deformation, structural distortions, and failure characteristics caused by ground fissures. Tests revealed an integrated bend-shear-torsion failure mode of any tunnel that entered a ground fissure obliquely at an angle of 20° or less, under the action of activities associated with a ground fissure. The tunnel structure indicated damage characterized by long longitudinal cracks and multiple groups of subparallel vertical cracks. The range in failure of the tunnel was up to 3.5 times the tunnel diameter on the hanging-wall side and up to 4.5 times the tunnel diameter on the foot wall. When metro tunnels intersect ground fissures at low angles, traditional tunnel designs do not meet minimum safety standards. It is therefore necessary to explore alternative metro tunnel designs in fissure-prone areas.