Performance of Supporting Structures of a Single-Floor Three-Span Metro Station during Column-Drift Tunneling in Dry Sandy Ground

Although the column-drift method is commonly used for constructing a shallow underground metro station, there are few comprehensive field monitoring studies to investigate the mechanical behavior during complicated construction procedures. This study investigates the stress and deformation characteristics of a single-floor three-span metro structure built by the column-drift method in dry sandy ground based on the field case of the Great Wall Qiao Station in Shijiazhuang Metro Line 1. The excavation started at the two pilot tunnels to erect the two columns, which then served as support structures for the middle drift and then the two side drifts, which were excavated last. By comparing the results obtained from field monitoring and numerical analysis, it was found that the crown settlement and convergence of the primary and temporary supports mainly occurred during the excavation of the lower parts of the top caverns in both the pilot tunnels and side drifts. The crown settlement of the middle drift was small due to the support of the early columns and longitudinal beams in the pilot tunnel. After the primary and temporary supports were closed, the deformation of the structure did not develop, suggesting that the temporary structure should be closed as soon as possible. When the middle and side drifts were constructed, the temporary supports were dismantled, resulting in an obvious increase in the stress of the remaining primary supports. Thus, the demolition length of the temporary structure per cycle should be strictly controlled. The surrounding load was partially transferred to the secondary lining in the side drifts after its final placement, which resulted in a decrease in the stress of the remaining primary supports. Once the middle drift was excavated, the top of the monitored column moved toward the station center line. After the transversal temporary supports between the left and right longitudinal top beams were fixed with prepressure, the top of the columns turned back to the opposite direction and their movement increased after the crown supports of the middle drift finished. In the meantime, the stress difference between the two longitudinal beams became obvious. The tilting of the column roofs decreased after the two side drifts were simultaneously constructed. It was suggested that the temporary transverse supports and the symmetrical construction of the side drifts were effective measures in controlling the tilting of columns.