Model test and simulation analysis of starting characteristics of muck in slurry discharge pipeline of slurry shield in sandy pebble stratum

During the excavations process of slurry shield tunneling in sandy pebble stratum, the discharge pipeline transport is particularly complex due to a large number of irregular pebbles in the slurry discharge pipeline. In this study, a circulating current test device was designed, and a three-dimensional transient numerical model was established using the Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) coupling method. The start-up characteristics of the pebble under different shapes, particle sizes, pipeline inclination angles, elbow,and pipeline diameters were investigated, respectively. In the model experiment, the Sodium carboxymethyl cellulose (CMC) slurry was used as the carrier liquid, and the transparent acrylic tube was used as the carrier; In the CFD-DEM coupling model, the rheological properties of slurry and the shape of irregular pebbles were considered through rheological testing and three-dimensional scanning technology, respectively. The results indicated that: (1) under the same irregular pebble particle size and pipeline inclination angle, the starting velocity of the pebbles follows an order of ellipsoidal shape>flat shape>nearly spherical shape; (2) Under the same shape of irregular pebbles and pipeline inclination angle, the starting velocity of pebbles first increases, then decreases, and then increases with the increase of equal volume particle size; (3) For spherical pebbles, in a horizontal pipeline, the starting velocity of the pebbles first increases, then decreases, and then increases with the increase of particle size. In inclined and vertical pipelines, the starting velocity of the pebbles decreases with the increase of particle size; (4) The starting velocity of pebbles increases with the increase of pipe diameter; (5) The positions with higher starting velocity of pebbles mainly appear at elbow positions with larger angles (such as 60°and 90°). This is because when the pipeline inclination angle is≥60°, a vortex zone will appear at the elbow position. The velocity in the vortex zone is opposite to the velocity in the mainstream zone, which hinders the movement of pebbles. Therefore, when laying pipelines, it is necessary to minimize the laying of large angle inclined and vertical pipelines as much as possible, it is recommended to use more horizontal pipelines or pipelines with small inclination angle (≤45°).