Experimental And Numerical Study On Macro-meso Mechanical Characteristics Of Epb Shield Foam Conditioned Soil

Shield tunneling technology has been the mainstream construction method of urban rail transit construction with its characteristics of high mechanization,safety and reliability and superior construction efficiency.Over 90%of tunnel-boring machines produced for excavations in soft ground worldwide are constructed as earth pressure balance(EPB)shields.The soil cut by cutterhead needs to be conditioned to improve its flow plasticity and ensure efficient excavation of the EPB shield in complex stratums.Foam agent is the most widely used soil conditioner.There are soil particles,foam particles and water in the foam conditioned soil.The soil particles and foam particles are granular materials,discontinuous and uneven.The pore space variation characteristics and particle contact force transFER mechanism directly determine the plastic characteristics of the foam conditioned soil.The existing experimental detection methods are difficult to describe the mesoscopic mechanical properties of particles in foam conditioned soil,and it is impossible to reveal the condition mechanism of foam to the soil from the particle level,and the related research progress is slow.In view of the unclear mechanism of foam agent condition and the confusion of the technical scheme of soil condition,the foam distribution mesoscopic measurement system was developed independently.The size distribution and evolution law of foam conditioned soil were analyzed to establish the meso distribution evolution model of bubbles in foam condition soil.A nonlinear contact model of foam conditioned soil considering foam deformation is proposed.The direct shear test system of foam conditioned soil and compression-penetration test system of foam conditioned soil have been developed independently.The laboratory tests and numerical simulation study on the shear resistance,compressibility and permeability of the foam conditioned soil have been carried out.The influence of the foam expansion ratio(FER)and foam injection ratio(FIR)on the flow plasticity of conditioned soil and the porosity variation law of foam conditioned soil and the mechanism of particle contact force transFER were analyzed,and the mechanism of the foam in the condition of the soil was revealed.The macro and mesoscopic evaluation index of foam conditioned soil flow plasticity was proposed.The research on the stratum adaptability of the soil improvement scheme was carried out,and the dynamic condition technology system for the whole stratum adaptability was proposed and applied in engineering.The research contents are as follows:(1)A non-ionic polymeric foaming agent was developed.the experimental research on the size distribution and evolution law of foam particles was carried out by the self-developed foam distribution mesoscopic measurement system,and the mesoscopic distribution model of foam size in the foam conditioned soil was established.The change rule of actual FIR after condition was explored,and the volume ratio of soil particles and foam particles in the foam conditioned soil is obtained.Finally,the evolution model of the mesoscopic distribution of foam conditioned soil was constructed.(2)Based on the Young-Laplace law and the principle of "force deformation and constant volume of foam",three types of foam deformation in the foam conditioned soil were defined,and the control equations for the contact deformation of foam-foam,foam-boundary and foamsoil particles were derived.A nonlinear elastic normal contact model of foam conditioned soil was formed,and the traditional tangential contact model is used to describe the shear deformation characteristics of the foam condition soil.A nonlinear contact model of the foam conditioned soil was built.Based on the Fish language,the contact model is implemented into PFC3D and compared with the analytical solution of the foam contact model to verify the accuracy of the model.(3)Aiming at the shortcomings of general direct shear equipment,such as "poor sealing and inability to strictly control drainage conditions",a direct foam shear test system was developed.The influence of FIR on the shear resistance of foam conditioned soil was analyzed.Numerical simulation of direct shear test of foam conditioned soil was carried out to analyze the mechanical transFER characteristics of particles in the foam conditioned soil under difFERent pressure and FIR.Based on phase transformation theory,a criterion for evaluating flow plasticity of mesoscale scale soil under shear disturbance was proposed.Combined with the macroscopic mechanical behavior of foam conditioned soil,the control standard of soil condition is proposed.(4)Aiming at the shortcomings of general compression and penetration test equipment,such as "unable to achieve automatic and precise acquisition of test results",a foam penetration test system for compression and permeability characteristics of foam conditioned soil has been developed independently.The experimental study on the compression and permeability characteristics of foam conditioned soil was carried out.The influence rule of FER and FIR on the compressibility and permeability of the foam conditioned soil is revealed.Numerical simulation of the permeability test of foam conditioned soil is carried out,and the influence rule of FIR on porosity and effective stress transformation rate of foam conditioned soil is analyzed.The evaluation index of soil flow plasticity is established based on the compression control standard and effective stress conversion rate of foam conditioned soil.