Mesoscopic Mechanism And Application Research Of Foam Soil Conditioning Of EPB Shield In Sandy Strata

With the acceleration of urbanization,urban rail transit construction has entered a peak period,and Earth Pressure Balance(EPB)shield has become the mainstream method of urban tunnel construction.However,shield tunneling through sandy strata is prone to engineering problems such as gushing and poor excavation.In severe cases,it may even induce accidents such as surface subsidence and collapse,affecting the construction progress and causing great harm to the safety of construction personnel.In order to avoid the above problems,the soil must be conditioned.At present,the conditioning of EPB shield soil is mainly based on foaming agent.The conditioned soil presents a three-phase mixed state of soil particles(solid)-pore water(liquid)-foam(gas).The conditioned soil is granular particle material,and its meso-relationship determines the macro-mechanical properties.Revealing the meso-mechanism of foam-soil is helpful to realize the fine conditioning of shield soil,and then condition the conditioning effect of soil,and significantly condition the safety and construction efficiency of EPB crossing sandy strata.In this paper,a shield tunnel construction project in Jinan is taken as the background.In view of the two major problems of unclear meso-mechanism of foam-conditioned soil of EPB shield in sandy stratum and blindness of foam injection parameters in the project site,through meso-test,theoretical analysis,numerical analysis and laboratory test,the meso-mechanism and application of foam-conditioned soil of EPB shield in sandy stratum are carried out by using high-power microscope observation and image processing technology,soil mechanics and fluid mechanics laboratory test technology,slag soil conditioning test technology and other test and measurement methods,and the foam-conditioned soil is comprehensively studied at meso-scales.The basic principle of foam-soil interaction and the engineering characteristics of foam conditioned soil are revealed.The mesoscopic mechanism and macroscopic mechanical properties of conditioned soil under different foam parameters and external pressure control are systematically analyzed.Based on the liquid bridge model and pore network model,the numerical analysis method of slump and permeability of foam conditioned soil is established.Combined with the properties of sandy strata in the engineering site,the optimal parameters of foam conditioning are proposed and applied and further verified in the field construction.The main research contents are as follows:(1)Meso-stability analysis of foam conditioned soil of EPB shieldTaking the sandy stratum of EPB shield as the main research object,the self-developed foaming device,foam conditioned soil image acquisition device and mesoscopic test system were used to carry out the mesoscopic test of pure foam decay evolution through high power electron microscope and image processing technology.The foam fusion and dissipation process under the influence of different foam expansion rate(FER),foam injection rate(FIR)and external pressure(P)were explored,and the foam decay function with FER,P and time as parameters under pure foam condition was constructed.The stability analysis of foam in conditioned soil under the influence of different FER,FIR and P was carried out.The stability of foam conditioned soil was explored from the perspective of bubble instability.Based on the contact theory of soil particles and foam,the mechanical relationship and stability mechanism of their interaction were analyzed,and the contact theory of soil particles and foam interaction is clarified.(2)Study on flow plasticity of foam conditioned soil EPB shieldSlump test,direct shear test and consolidation compression test of foam conditioned soil were systematically carried out.The effects of different FER,FIR and P on the mechanical properties of conditioned soil,such as slump,shear strength and compressibility,were revealed.Combined with the macroscopic mechanical behavior of foam conditioned soil,the optimal parameters of flow plasticity of residue conditioned soil were proposed.Considering the low friction contact effect caused by foam and the influence of matrix suction,a foam-modified soil particle contact liquid bridge model was established.The calculation model of slump of foam conditioned soil was established,and the numerical simulation of slump was carried out.The variation law of friction angle and cohesion was explored,and the evolution characteristics of plastic mechanical properties of conditioned soil flow under different conditions were further clarified.(3)Study on permeability of foam conditioned soil EPB shieldThe permeability test of foam conditioned soil was studied to reveal the variation of permeability with different FER,FIR and P.The seepage characteristics of pore water-foam in the pore structure of soil are revealed,and the mesoscopic seepage model of foam-conditioned soil pore network based on Stokes flow is established.The permeability calculation model of foam conditioned soil was established,and the permeability numerical simulation was carried out to further clarify the permeability evolution characteristics of conditioned soil under different conditions,and the optimal permeability parameters of slag soil improvement were proposed.On the basis of the above,relying on a shield section of Jinan Metro Line 6,combined with the properties of sandy strata in the project site,the corresponding optimization parameters of soil improvement foam injection are proposed.