Model tests on progressive collapse mechanism of a shallow subway tunnel in soft upper and hard lower composite strata

Numerous subway tunnels have been built in soft upper and hard lower composite strata. However, the stability and bearing capacity of composite strata are difficult to predict compared to homogeneous strata, so collapse always occurs in composite strata. The evolution of progressive failure leading to the collapse of shallow subway tunnels in soft upper and hard lower composite strata is complicated and merits further research. Based on the Qingdao subway tunnel, scaled model tests have been conducted to explore the progressive collapse mechanism of shallow subway tunnels in soft upper and hard lower composite strata. The results demonstrate that the stress distribution and collapse process of composite strata are significantly impacted by hard lower layer thickness above the tunnel roof. A stable collapse arch cannot be formed when only the soft layer is above the tunnel roof, and collapse quickly extends to the ground surface once failure occurs in the soft upper layer. As the thickness of the hard lower layer increases, a temporary stable collapse arch cannot only be formed in the hard lower layer but also in the soft upper layer during collapse; thus, a slow and progressive collapse process of the composite strata is observed under an increasingly imposed load. The greater the thickness of the hard lower layer above the tunnel roof, the higher the stability and bearing capacity of the composite strata. Additionally, the collapse mechanism of composite strata is further investigated using the discrete element method, and the minimum rockspan ratio is suggested for various buried depth tunnels based on whether a closed load-bearing arch can be formed in the hard lower layer. Finally, the proposed rock-span ratio is successfully applied in Qingdao subway tunnels, allowing for a suitable reference for constructing similar engineering.