Mechanism of slag discharge failures in earth pressure balance shield screw conveyor: A theoretical model-based investigation

The screw conveyor gushing may cause a sudden drop in pressure in the earth chamber, leading to excessive settlement of the surface and nearby buildings or structures, and even catastrophic accidents such as tunnel collapse. This paper presents a comprehensive investigation into slagging failures associated with earth pressure balance shield screw conveyors, categorizing them into rheological failure and permeability failure. Further, a permeability failure theoretical model and a Bingham fluid-based rheological failure model are developed. The above models can describe the conditions and mechanism of screw conveyor spurt taking into account shield parameters, formation characteristics, chamber pressure, and conditioned soil properties. In addition, a sensitivity analysis is conducted on the critical permeability coefficient and critical shear strength of the discharged soil, with a focus on a specific case project. The results underscore the significant impact of the screw conveyor pitch, water head at the entrance, and chamber pressure on the critical permeability coefficient and shear strength. Building on these findings, this paper proposes an anti-surge control index and strategy for shield screw conveyors, taking into account the ratio of shield covering soil thickness to shield diameter. It is recommended that when the shield soil covering layer thickness exceeds twice the shield diameter, real-time modification of the soil parameters, based on the shield tunneling depth, especially the shear strength, is essential for anti-surge control. This study provides engineers with valuable insights into conditioned soil and implements effective surge management strategies for screw conveyors.