Construction Schemes for Shallow and Asymmetrically Loaded Tunnels Crossing Below a Bridge

With the development of infrastructure, new tunnel crossings below existing bridges are becoming increasingly common. This paper presents a case history of a shallow, double-line tunnel crossing below a bridge on a hillslope that was excavated using nonshield tunneling techniques. Due to poor geological conditions and unsymmetrical tunnel pressure, bridge stability is of great concern during tunneling. In addition to pregrouting the foundations of the bridge piers, selection of an appropriate construction scheme is essential. In this study, investigation of ground subsidence, bridge behavior, and an evaluation of bridge stability was carried out using the elastoplastic finite-element method (FEM) to model the excavation by three construction schemes: the ringlike drift-heading, center-diaphragm (CD), and cross-diaphragm (CRD) methods. Comparisons of ground subsidence and bridge deformation for the three construction schemes were made; monitored displacement data was also provided. Comprehensive analysis of the results indicated that the FEM gave a reasonable approximation of excavation-induced displacement and was able to discriminate the relative displacement-controlling capability of the three excavation methods. The CRD method demonstrated superior deformation-controlling capability and was ultimately adopted as the excavation method. The project was completed successfully, and no cracking was observed in either the piers or bridge deck during tunneling; the monitored displacements were larger than those calculated, which may be attributed to the excavation disturbance of the rocks. It is not easy to approximate discontinuous deformation by continuity-based FEM. This case history may serve as a reference for similar projects.