Ground-Surface Settlement Control and Prediction for Twin Tunnels Excavated Above an Existing Tunnel: Effects of Existing-Tunnel Constraint

As urban rail transit networks become denser, twin tunnels are increasingly constructed in close proximity above existing operational tunnels, making ground surface settlement control and existing tunnel protection critical challenges. Previous studies have mainly focused on the adverse effects of new tunnel excavation on existing tunnels, whereas the reverse mechanism—namely, how an existing tunnel, as a stiff continuous structure, constrains surrounding ground deformation and thereby affects surface settlement—remains insufficiently understood. Clarifying this mechanism helps explain differences in settlement between cases with and without an underlying existing tunnel and provides a basis for optimizing the overcrossing geometry, predicting settlement, and managing construction risk. This study combines numerical simulation with theoretical analysis to investigate the effects of vertical clearance, center-to-center spacing of the new twin tunnels, staggering distance between excavation faces, and crossing angle on soil deformation within the construction-affected area. The settlement control mechanism induced by the constraint of the existing tunnel is identified, and a method for calculating surface settlement is proposed. The numerical results indicate that the existing tunnel reduces surface settlement. This response is interpreted as a combination of local constraints on surrounding ground deformation and global beam-like restraint, thereby reducing ground surface settlement. Vertical clearance, the horizontal spacing of new twin tunnels, and crossing angle govern the settlement response by altering the constraint intensity, superposition of settlement troughs, and restraint provided by the embedded tunnel lining/segments, respectively, whereas the staggering distance between excavation faces mainly affects settlement evolution with excavation advance. Because the settlement profiles obtained with and without an existing tunnel are approximately linearly related, the proposed calculation method does not explicitly account for the settlement reduction associated with the existing tunnel. Verification using an engineering case study indicates that the method is suitable for preliminary settlement prediction and risk assessment in similar projects.