A novel sequential excavation method for constructing large-cross-section tunnels in soft ground: Practice and theory

Abstract

Sequential excavation (SE) methods involve the division of a tunnel passage into drifts to limit ground settlement. However, an increase in the number of tunnel drifts may prolong the construction duration. In this case study, a 4-drift centre cross diaphragm (CRD) method with a suspension system at the ground surface, termed the suspension-type CRD (S-CRD) method, was proposed and applied to the construction of a large-cross-section tunnel in soft ground. A theoretical settlement prediction model based on Mindlin’s elastic equations was established to predict the ground settlement induced by the excavation of an S-CRD tunnel. The proposed theoretical model was validated to have satisfactory performance, as the predicted ground settlement agrees well with the field monitoring results. The effects of three crucial design parameters (the prestress Fa, spacing Sa, and diameter Da of the prestressed anchors) were analysed and compared using this theoretical model. It was found that the spacing of the prestressed anchors is more effective than the prestress and diameter in limiting the ground settlement.