Zoned Excavation of an Oversized Pit Close to an Existing Metro Line in Stiff Clay: Case Study

Abstract

Because greenfields available for new developments in congested urban areas are scarce in China, more and more excavations for building basements or other underground facilities (e.g., new metro lines, underground shopping malls, parking garages) have to be carried out in the close proximity of existing metro lines. To ensure project safety, it is essential to know potential adverse effects of excavations on adjacent metro lines in service. Until now, many studies have contributed to the cases of excavations overlying existing tunnels. In contrast, only a few were known for excavations parallel to adjacent existing tunnels. With regard to the responses of existing metro stations to adjacent excavations, few case studies were reported in the literature. Through an extensive field instrumentation program in combination of numerical simulations, this study examines the performance of an oversized deep excavation in stiff clayey deposits and the corresponding responses of the adjacent metro station and twin shield tunnels in service. Taking advantage of the adopted zoned-construction procedure, both wall deflections and ground settlements of this oversized pit were not as significant as those reported in the literature, even smaller than those of long and narrow metro station pits. Consequently, both deformations and displacements of the existing metro line were within acceptable limits, and no obvious structural damage was observed. Throughout the excavation, the two-level island-type metro station exhibited a good structural integrity, which settled uniformly along the transverse direction and tilted slightly along the longitudinal direction. As to the twin shield tunnel linings, the one located within the primary ground influence zone developed considerably larger settlements and deformations than the other one within the secondary influence zone. As a product of the lateral stress relief attributable to soil removal, the tunnel lining experienced apparent elongations in the horizontal direction.