Study on the geological adaptability of the arch cover method for shallow-buried large-span metro stations

As a newly proposed construction method, the arch cover method (ACM) has been applied in the construction of shallow-buried large-span metro stations, but the geological adaptability of this method remains unclear. To address this issue, the main construction process and support scheme were first described based on an actual project. The potential failure mechanism was analyzed, and a numerical model was established to examine construction safety under different conditions. Geological adaptability zones for the ACM were further proposed according to various indices of safe construction. To expand the application scenarios of this method, countermeasures were established and verified by engineering practice. Four potential risks were found during ACM employment: tunnel face instability, excessive stress and cracks in the arch cover, large deformation of the arch cover, and arch cover instability or collapse. The geological adaptability zones could be divided into applicable area, cautious area and forbidden area considering the consequences of different construction risks. Without advanced reinforcement measures, this method is not recommended when the rock mass is of Grade V or lower and should be carefully employed when the rock mass is of Grade IV or the rock mass exhibits the upper soft and lower hard state. Regarding Grade V rock masses, pipe roof should be installed to provide advanced support at the tunnel face, and the arch foot should be reinforced with steel pipe piles and prestressed anchor cables. Single-layer primary support could be used to avoid project progress delay and material waste when the rock mass is of Grade III or higher.