Mechanical Response of Supporting Structure of Closely Spaced Super Large Span Twin Tunnels

This paper presents a study on the mechanical response characteristics of supporting structure of closely spaced super large span twin tunnels based on the field monitoring. The pressure on the primary supporting and the axial force of rock bolt were taken into account, and three sections of the twin tunnels were selected to conduct a continuous monitoring which lasted for 328 days. It was found that the steel arch and the shotcrete were the main bearing structures during the excavation of a single super large span tunnel, and the bolts play a certain suspension role only after the excavation of the lower half of the section. During the excavation of the lagging tunnel, obvious asymmetry was found between the loading statuses of the leading tunnel and the lagging tunnel. The bolts of the leading tunnel approximately failed after the excavation of the lagging tunnel, the steel arch and shotcrete bore larger pressure. Obvious asymmetry was also found between the pressure on the two sides of the leading tunnel, the primary supporting on the side away from the rock pillar borne larger pressure. According to the monitoring results, it is recommended to adopt unequal supporting parameters in the design of closely spaced super large span twin tunnels to improve the supporting strength of dangerous parts. This paper might serve as a reference for the design and construction of similar tunnels.