Transport characteristics of soil-rock agglomerates bed load in slurry shield discharge pipelines and countermeasures for blockage prevention

Large soil-rock agglomerates are easily generated during slurry shield tunnelling through interbedded clay-gravel strata, significantly affecting excavation efficiency and operational safety. To investigate their transport and blockage mechanisms, a solid-liquid two-phase flow coupled model was developed based on the Wuhu Yangtze River Tunnel. Results indicate that agglomerates migrate in the form of bed load transport with distinct velocity stratification. When the agglomerate size increases from 3.0 to 4.5 cm, the bed velocity decreases linearly while local slurry pressure increases by 35.5%. Enhancing slurry density (from 1.05 to 1.35 g/cm³) and flow velocity (from 2.3 to 3.3 m/s) significantly increases the bed velocity by 26.2% and 136.8%, respectively, indicating that increasing slurry velocity is an effective measure for suppressing agglomerate deposition and blockage. Field applications confirmed that installing high-pressure flushing pipes at the chamber bottom effectively improved mucking efficiency, increasing the actual discharge volume from 210 m³ to 345~350 m³ per ring and substantially mitigating deposition and blockage risks.