Abstract: Muck retention during large-diameter slurry shield tunneling in the high-viscosity strata of the Jiaozhou Bay second subsea tunnel in Qingdao, China poses a major challenge. Hence, a muck-discharge solution assisted by a short-screw conveyor is proposed and evaluated. A coupled computational fluid dynamics and discrete element method (CFD-DEM) is used to establish a 1:1 full-scale model including the cutterhead, slurry chamber, and discharge system. The Johnson-Kendall-Roberts contact model serves as an equivalent numerical tool, and its surface energy parameter is calibrated using indoor adhesion shear test data to accurately reproduce the macroscopic adhesion and agglomeration behavior of wet clay particles. By setting up an experimental group with a short-screw conveyor and a control group with the traditional structure, the differences in flow-field characteristics, particle migration, discharge efficiency, and energy consumption between the two configurations are systematically compared. The results show that: (1) The short-screw conveyor considerably improves the uniformity of the flow field in the slurry chamber, reducing the standard deviation of the velocity from 0.82 to 0.47 m/s and effectively mitigating local flow stagnation. (2) The particle retention phenomenon is fundamentally improved, with the particle retention ratio in the chamber decreasing from 30%－40% in the control group to ~10%. (3) The discharge mode changes from intermittent to continuous and stable; the discharge rate of the experimental group increases rapidly after 90 s of operation and stabilizes at approximately 250.00 kg/s, approaching the theoretical excavation rate of 258.00 kg/s, and the cumulative muck discharged within 180 s is 238% higher than that of the control group. (4) Although the initial energy consumption increases slightly, the total energy consumption decreases significantly after the system stabilizes (after 2 min), with an overall energy saving of 12.8% within a 3-min cycle.

Key words: slurry shield, mud retention, short-screw conveyor, computational fluid dynamics and discrete element method coupling analysis