Numerical Investigation on the Responses of Existing Water Diversion Tunnel induced by Under-Crossing Twin Tunnels Considering the Lagging Distance

With the increasing density of urban underground construction, the necessity of constructing new tunnels near existing ones has grown, raising concerns about the potential impacts on the integrity of adjacent structures. The purpose of this study is to evaluate the effects of lagging distance during twin tunnel construction on the responses of the adjacent existing water diversion tunnel. Through three-dimensional finite element analysis, the responses of the existing tunnel induced by under-crossing twin tunnels, including tunnel settlement, horizontal displacement, cross-section deformation and lining stress, are investigated. Key findings indicate that the lagging distance significantly impacts the settlement velocity of the existing tunnel, with smaller lagging distances accelerating the attainment of the final settlement. The horizontal displacement and its velocity are complexly influenced by the lagging distance, particularly within 1.5[Formula: see text]times the tunnel diameter on the outside of the twin tunnels. The lagging distance also affects the deformation process and distribution characteristics of the existing tunnel sections, though not their final deformation state. Lining stresses in the arch crown of the existing tunnel are at a higher risk of cracking due to tension, and the sidewalls exhibit greater stress concentrations. A reasonable lagging distance of about three times the vertical clear distance between the newly constructed and existing tunnels is crucial for balancing geotechnical effects and ensuring tunnel stability, offering valuable guidance for the safe construction of tunnels in proximity to existing structures.