Sedimentation characteristics of gravel in discharge pipe of slurry shield tunneling and its impact on slurry

The slurry shield tunneling in weathered rock layers is prone to gravel accumulation within the slurry pipe. In order to study the transportation characteristics of gravel in the slurry pipe, a model of gravel transportation in the slurry pipe of the shield tunneling was established based on the computational fluid dynamics-discrete element (CFD-DEM) coupling method. The rheological characteristics of the slurry were considered through the rheological tests, and the accuracy of the model is verified through the gravel sedimentation tests. The velocity distribution, force characteristics and motion forms of gravel during transportation as well as the influences of gravel bed deposition on the slurry were analyzed. The effects of gravel size, gravel shape, slurry flow rate and slurry density on the sedimentation characteristics of gravel are explored. This study indicates that gravel particles move intermittently in a sand dune-like bed. The gravel velocity is layered along the height, and the surface gravel velocity is fast, with collision and rolling as the main movements. The velocity of gravel in the near wall layer is slow, and its movement is mainly dominated by sliding. There is a transition layer between the near wall layer and the surface gravel layer. The gravel bed layer locally increases the flow velocity and pressure of the slurry in the pipeline. The velocity of the gravel bed increases linearly with the increase of the slurry flow rate. The height of the gravel bed increases with the size of the gravel. The higher the density of the slurry, the greater the drag force on the gravel. The smaller the gravel shape factor, the greater the difficulty of transportation. Finally, the risk assessment indices are established for gravel sedimentation, and the targeted control methods are proposed to provide reference for preventing and treating sedimentation blockage of gravel.