Excavation-induced arching effect below base level and responses of long-collinear underlying existing tunnel

Abstract

Deep excavation overlying existing tunnel becomes a common recurring problem in congested urban areas. The mechanism behind the tunnel responses to overlying excavation is not yet fully understood. This study investigates the performances of a long and deep excavation and underlying collinear existing tunnel through well-documented field observation and elaborate two-dimensional numerical analysis. Focus was placed on the ground stress transfer and displacement profile, and corresponding tunnel responses. It indicates that two types of concave arches could occur, including “frictional arch” and “end-bearing arch”. This pattern of stress transfer phenomenon was termed as “negative arching effect”, differing from the arching effect above trapdoor and tunnel in light of formation mechanism. In comparison with the end-bearing arch, the frictional arch was not stable and could exhibit appreciable overall heave as the excavation depth increased. The area below the excavation base could be split into three different zones in terms of mechanical responses: loosened zone, stress transfer zone, and stable zone, respectively. Moreover, variations of the tunnel responses with relative tunnel positions with respect to the loosened zone were investigated. It exhibits that the tunnel heave and vertical elongation in tunnel diameter experienced significant increases when the tunnel crown entered into the loosened zone.