Soil Discharge Model Test and Spewing Prevention of Shield Tunneling in Water-Rich Sandy Stratum

ABSTRACT Given the prevalent issue of earth pressure balance shield tunneling encountering spewing in water-rich sandy strata, a dedicated soil discharge model test system was devised. Through comprehensive tests on both unconditioned and conditioned soil under submerged conditions, this system unveiled the temporal distribution patterns of water pressure throughout the soil discharge and discharge phases. The test results show that in the case of unconditioned soil, once the water pressure within the soil chamber (Pt) surpasses 100 kPa, significant fluctuations occur upon opening the soil outlet, leading to a heightened risk of spewing. The severity of spewing can be gauged by analyzing both the average water pressure at the measurement point and the coefficient of variation in water pressure. The water pressure and water flow in the soil chamber have a great influence on the water pressure distribution in the soil discharge process. Changing the spiral speed n and soil discharge opening ratio ξ cannot restrain the spewing. For conditioned soil, individual agents like foaming agent and bentonite are prone to being washed away under high water pressure. Bentonite combined with a high molecular polymer can effectively seal the dominant flow channels within the sandy soil gap. Through the use of a shield tunnel within the Nanchang Metro, the soil conditioning system underwent renovation, introducing a spewing prevention technology employing multiple pipeline quantitative injections. This conditioning approach effectively mitigated spewing issues and enhanced soil discharge efficiency, offering valuable insights for similar engineering endeavors.