Spewing Prevention Mechanism And Soil Migration For Shield Tunneling In Water-Rich Cohesionless Stratum

China is the country with the fastest urbanization in the world.As the "name card" of urbanization,rail transit is the k ey to solving urban congestion.There are 50 cities with 270 urban rail transit lines in China,and the length of lines under construction is more than 9000 km.Earth pressure balance(EPB)shield has become the mainstream construction method of urban rail transit construction with the advantages of mechanization.The urban construction environment is complex and changeable,and the construction risk and difficulty increase exponentially.The water-rich cohesionless stratum accounts for more than 40%of the urban soft soil stratum,which is very prone to the spewing disaster of screw conveyor,resulting in engineering accidents such as instability of excavation surface and ground collapse.The excavation efficiency is greatly restricted by spewing,and the construction safety is difficult to guarantee.Based on the two difficult problems of shield spewing in water-rich cohesionless stratum and unclear migration law of conditioned cohesionless soil,spewing prevention mechanism and soil migration for shield tunneling in water-rich cohesionless stratum were carried out through theoretical analysis,model test,rheological test,and numerical simulation.The mechanical behavior of cohesionless soil before and after conditioning is systematically revealed.The mechanism of shield spewing in water-rich cohesionless stratum is revealed.The flow plastic evaluation index of conditioned cohesionless soil is put forward,and the spewing prevention technology in water-rich cohesionless stratum is established.This paper explores the rheological mechanical behavior of conditioned soil in water-rich cohesionless stratum and reveals the interaction law between particles and fluid in the migration process of soil.The specific research contents are as follows:(1)The groundwater seepage equation in the process of soil migration under the non-full soil bin condition is deduced,and the occurrence mechanism of shield spewing in water-rich cohesionless stratum is revealed.The soil migration model test system was developed,and the model tests of soil migration under water-rich conditions were carried out.The time history distribution characteristics of water pressure in the soil migration process are revealed.The effects of water pressure,water flow rate,spiral speed,and soil discharge opening rate on the water pressure distribution are studied.The conditions for the occurrence of spewing are put forward and the critical threshold theory of spewing is modified.(2)Based on the non-ionic polymerization foam compressibility test and Young-Laplace equation,the change rule of foam volume under pressure loading-unloading conditions and the formula of foam compression coefficient are proposed.Based on the requirements of spewing prevention and TEM transmission electron microscope test,a liquid spewing-preventive agent with polymer long-chain structures was developed to block the seepage channel by adsorbing and wrapping particles with different particle sizes.Through the flow plastic tests and the migration model tests of the conditioned cohesionless soil,the fluidity,permeability,friction,and viscosity of the soil are analyzed.The evaluation indexes of flow plasticity and conditioning agents injection ratio are put forward,and the spewing prevention technology for EPB shield tunneling in cohesionless stratum is established.(3)Based on the sensitivity of foam mechanical properties to the ambient pressure,a gas-loading rotary rheometer and testing methods were developed independently.The rheological tests of the soil conditioning agents and the conditioned soil were carried out.The rheological properties of both the agents and the conditioned soil under normal pressure and gas-loading were explored.The three-phase evolution law of foam conditioned soil during the gas-loading and unloading process is analyzed.By introducing the concept of foam volume replacement in the process of soil conditioning,a formula for calculating the actual foam injection ratio(FIR)and saturation(Sr)is proposed.(4)The microscopic structure and occurrence state of the conditioned soil are studied by the developed three-dimensional digital microscope system.The distribution characteristics and time evolution rule of the foam particles are revealed.The contact relationship between conditioned soil particles and the deformation equation of the shear process is proposed,and the mesoscopic contact parameters are calibrated.Particle flow software PFC and embedded CFD module are used to realize fluid-solid two-way coupling.The interaction between particles and particle-fluid in the conditioned soil migration is simulated,and the section soil discharge efficiency and particle mechanical behavior are proposed.(5)Based on the above research and taking the shield project of Jinan Metro and Nanchang Metro as the background,the spewing prevention technology and corresponding conditioning scheme are put forward,which has successfully solved the problem of shield spewing in water-rich environment and improved the tunneling efficiency.