Numerical Simulation of Large-Span Bifurcated Tunnels with Large Cross-Sections in Urban Underground Interchanges

The stress distribution after excavation becomes highly complex in large-span bifurcated tunnel sections commonly found in urban underground interchanges. This study investigates the stress evolution induced by the excavation of large-span and bifurcated tunnel, focusing on the 32.17 m maximum-span section of the Shenzhen Baopeng–Shahe Underground Interchange. The results show that stress concentration near the tunnel walls of large-span sections is greater than that in sections with bifurcated tunnels. Adjusting the burial depth of the large-span tunnel, the influence of stiff layer thickness on the redistribution of surrounding rock stress was analyzed. When the tunnel is buried at a shallow depth and the stiff layer thickness is small, the maximum tangential stress of the surrounding rock occurs at the stiff layer boundary, and the surrounding rock remains entirely elastic. In large-span tunnels, as the thickness of the stiff layer increases from 5 m to 20 m, the stress relaxation zone grows from 0 m to 8 m, and the stress-bearing zone expands from 10 m to 27 m. As the burial depth increases and the stiff layer thickness grows, the maximum tangential stress shifts to within the stiff layer. In this case, the tangential stress distribution at the stiff layer boundary becomes non-smooth. Therefore, an appropriate stiff layer thickness must be selected to prevent the surrounding rock from entering a plastic state. The findings provide theoretical guidance and technical support for the design of large-scale underground interchange bifurcated tunnels, advancing the intelligent and scientific development of urban underground transportation facilities and offering significant practical and social benefits.