Study On Permeability Mechanism And Control Technology Of Coarse-Grained Soil Conditioned With Foam And Bentonite Slurry For EPB Shield

During earth pressure balance(EPB)shield tunnelling in coarsegrained ground under high water pressure or restarting after a long-time standstill,with the coarsening and dissipation of foam,the screw conveyor is prone to the risk of water spewing,and even cause the instability of excavation face due to high permeability of muck.Therefore,combined conditioning should be considered.Foam-bentonite slurry-conditioned coarse-grained soil was used as the main research material,and the permeability mechanism and control technology of coarse-grained soil were studied in-depth by means of laboratory test,theoretical analysis and field verification,providing theoretical basis and technical support for permeability conditioning of coarse-grained soil.The main study contents and conclusions are as follows:(1)Large-scale permeability tests were carried out and the permeability characteristics of foam-bentonite slurry-conditioned soil were proved.It is found that with an increase of hydraulic gradient,the timedependent curve of permeability coefficient of conditioned soil passes through the initial stable period and the fast growth period quickly,and enters the slow growth period earlier.When the hydraulic gradient is constant,the time duration of the initial stable period extends with an increase of water pressure at the top and bottom of the specimen,and this phenomenon is more obvious with an increase of hydraulic gradient.(2)Combined with the interaction principle of foam-soil particles,the conditioning mechanism of foam-bentonite slurry-conditined soil was revealed.Foam bubbles fill in the soil pores and block the permeation channel.Further,the structure of foam bubbles will be more stable due to the three-phase system(solid-liquid-air)in the conditioned soil.On one hand,bentonite can supply fine particles and enhance the grain gradation of the conditioned soil.On the other hand,fine bentonite particles can stabilize foam bubbles,slowing down the foam coarsening and dissipation,and maintain the stability of the water-bolcking structure.(3)Based on the mechanism of fine particles stabilizing foam,the calculationmodel of the initial permeability coefficient of foam-bentonite slurry-conditioned soil was developed.Based on the theory of permeability of pure foam and Kozeny-Carman equation,the system of foam and bentonite was established by introducing the monolayer of bridging particles stabilizing foam,and the theory of effective permeation channel for foam-conditioned soil under hydraulic gradient was modified by using compression coefficient of pore fluid.Compared with the results of permeability test,the model was generally feasible.(4)Based on the research results of foam conditioning and foambentonite slurry combined conditioning,a soil conditioning method of coarse-grained soil in shield tunnelling was proposed.The process mainly includes evaluation of the possibility of water spewing,selection of the conditioning agents,and determination of the suitable conditioning parameters.The application of the proposed method of soil conditioning was carried out based on two practical engineering cases,achieving quite good results.This article includes 92 figures,17 tables,and 112 references.