Model tests and simulation analyses of starting characteristics of muck in slurry-discharge pipelines of slurry shield in sandy pebble stratum

During the excavation 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 pipelines. In this study, a circulating current test device is designed, and a three-dimensional transient numerical model is established using the computational fluid dynamics-discrete element method (CFD-DEM) coupling method. The start-up characteristics of the pebbles under different shapes, particle sizes, inclination angles of pipelines, elbow and pipeline diameters are investigated, respectively. In the model tests, the sodium carboxymethyl cellulose (CMC) slurry is used as the carrier liquid, and the transparent acrylic tube is used as the carrier. In the CFD-DEM coupling model, the rheological properties of slurry and the shape of irregular pebbles are considered through the rheological testing and three-dimensional scanning technology, respectively. The results indicated that: (1) Under the same particle size of irregular pebbles and inclination angle of pipelines, the starting velocity of the pebbles follows an order of ellipsoidal shape > flat shape > nearly spherical shapes; (2) Under the same shape of irregular pebbles and inclination angle of pipelines, the starting velocity of pebbles first increases, then decreases, and then increases with the increase of the isometric particle size. (3) For the spherical pebbles, in a horizontal pipeline, the starting velocity of the pebbles first increases, then decreases, and then increases with the increase of the particle size. In the inclined and vertical pipelines, the starting velocity of the pebbles decreases with the increase of the particle size. (4) The starting velocity of the pebbles increases with the increase of the pipe line diameter. (5) The positions with higher starting velocity of pebbles mainly appear at the elbow positions with larger angles (such as 60°and 90°). This is because when the inclination angle of pipelines 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 the pebbles. Therefore, when laying pipelines, it is necessary to minimize the laying of large-angle-inclined and vertical pipelines as much as possible, and it is recommended to use more horizontal pipelines or pipelines with small inclination angles (≤45°).