Deformation and Protection of Existing Tunnels at an Oblique Intersection Angle to an Excavation

Abstract

The response of existing tunnels to adjacent excavation at an oblique intersection angle is still unclear. This study investigated the deformation of an existing tunnel when new excavation was conducted obliquely, based on a case history from Tianjin, China. In this project, a cutoff wall was installed to protect the tunnel from excessive horizontal movement. Finite element analyses were then performed to investigate the optimization of the cutoff wall design. The monitoring results show that due to the oblique intersection, the tunnel rings near the narrow and wide boundaries of the excavation ultimately exhibited heave and subsidence after excavation. The tunnel also underwent significant torsional deformation and horizontal displacement toward the excavation. The maximum horizontal displacement occurred at the tunnel ring closest to the midpoint of the wide boundary of the excavation. The numerical analyses show that as the intersection angle between the cutoff wall and the excavation increased, the efficiency of the cutoff wall first decreased and then increased. The effectiveness of the cutoff wall parallel to the tunnel was up to 122% greater than that of the wall parallel to the excavation, justifying the installation of cutoff walls parallel to tunnels.