Structural Behaviors of Large-Section Mine Tunnels Undercrossing Existing Metro Lines

Abstract

To meet the operational requirements of a new metro line in Guangzhou, China, a complex tunneling project was undertaken in a business area surrounded by high-rise buildings and underground utility pipelines. Two large-section mine tunnels, with a maximum excavation span of 15.6 m, were constructed, crossing beneath the existing metro line 7 in close proximity. To ensure construction safety, extensive field monitoring was conducted, investigating the deformation and stress characteristics of the newly built tunnel lining structures and settlement of existing tunnels during various construction stages. The findings indicate that the structural integrity of the newly built tunnels may be compromised by the ground pressure resulting from deep-hole grouting, which is aimed at protecting the existing metro line above. The primary lining of the new tunnels is particularly susceptible to stress at the arch waist, where it experiences significantly higher stress compared to the arch crown and arch foot. While constructing the secondary lining before removing temporary diaphragm supports can be advantageous in controlling ground settlement, the stress levels within the secondary lining can fluctuate significantly once the temporary supports are removed. Once the tunnels are completed as closed loops, the stress conditions in both the primary and secondary lining tend to stabilize. The secondary lining primarily undergoes flexural stress, with internal force diagrams roughly symmetrical to the cross-section center line, and the most vulnerable position being at the left arch waist or invert. This investigation provides valuable insights for the design and construction of similar engineering projects in the future.