Transportation Performance of Large-Sized Pebbles in Slurry Circulation System: A Laboratory Study

The slurry circulation system is a key component of a slurry-pressure-balanced shield tunnel because it facilitates the effective operation of slurry shield tunnelling. However, the research on transportation performance including critical velocity and motion characteristics of large-sized pebbles is scant because it is difficult to observe the performance of pebbles. In this study, an indoor test was designed and built to observe the transportation performance of large-sized pebbles in a slurry circulation system. This test takes a lot of time because obtaining the critical velocity and observing the motion characteristics are extremely complex. Herein, we find a slurry substitute solution that conveniently allows observation of the transport characteristics of pebbles. In addition, to analyze the velocity of the transportation of large-sized pebbles more accurately, we redefined the critical velocity of pebble transportation in the slurry system and corrected the prediction formula of critical velocity proposed by Wasp. The results show that the critical velocity of pebbles at the turning joints is the highest in the whole slurry circulation system. The particle property parameter (FL) proposed by the Wasp formula was corrected to 0.536, 0.563 and 0.683 for pebbles with flat shape, ellipsoid shape and near-spherical shape, respectively. The motion of large-sized pebbles in the circulation pipeline mainly manifests in three forms: sliding, rolling and jumping. The motion characteristics of pebbles in the circulation pipeline and their impacts on the pipeline are analyzed. The results can provide guidance for designing and preparing the slurry circulation system of slurry shield tunnelling projects.