Mechanisms of rock muck transport in a slurry shield chamber with series screw conveyor: Insights from visualized model tests

To clarify the transport mechanisms of rock muck in a slurry shield chamber equipped with a screw conveyor and to identify the operational conditions under which screw-assisted discharge is advantageous, a visualized model-test platform of muck transport was developed in this paper, and a transparent slurry prepared from sodium carboxymethyl cellulose and polyacrylamide was used to enable direct observation of muck migration under cutterhead-induced agitation. A series of model tests was conducted to investigate the effects of muck particle-size distribution, muck content, slurry circulation and discharge mode, chamber flushing flow rate, cutterhead rotational speed, and screw conveyor rotational speed on muck discharge performance, slurry pressure fluctuation, cutterhead torque, and screw conveyor torque. The results show that muck transport is controlled by the coupled effects of slurry entrainment, cutterhead disturbance, particle settling, and screw conveying. The applicability of the screw conveyor strongly depends on muck content and particle-size distribution. When the accumulated muck volume ratio in the chamber exceeds 2.76% or the particle size-to-screw pitch ratio satisfies ( d p / P > 0.0625), the screw conveyor markedly improves discharge performance by disrupting deposited muck and actively conveying it into the discharge pipeline, where it is subsequently removed by pump suction. Conversely, when the accumulated muck volume ratio is below 1.3% or ( d p / P < 0.0625), muck particles are readily suspended and entrained by slurry flow, and the pump-only mode provides higher discharge efficiency. Increasing the slurry inflow rate enhances muck suspension, improves discharge efficiency, and reduces cutterhead torque. Under a constant total inflow rate, the middle-lower inflow configuration produces the most favorable flow field, leading to optimal discharge performance and torque stability. Increasing cutterhead speed promotes muck dispersion, whereas excessive rotation may induce over-agitation and reduce discharge efficiency. The screw conveyor speed exhibits a nonlinear effect, with discharge performance increasing rapidly at low speeds and gradually stabilizing when the conveying rate approaches the muck supply rate. These findings reveal the coupled mechanism of passive pump suction and active screw conveying and provide experimental guidance for optimizing muck discharge systems and operating parameters in slurry shield tunnelling.