Study On Mechanical Characteristics Of Metro Tunnel Based On Soaking Of Loess Layer

Some intervals of Xi’an subway locat in large thickness collapsible loess,in early stage,influence of loess on tunnel does not appear with no soaking condition,influenced by objective factors,such as rainfall fissure infiltration,the river water seepage,tunnel excavating surface catchment effect,may lead to water rate change,resulting in the long-term operation of state changes and tunnel structure stress change,threatening the safe operation.In order to study the force and deformation of the subway tunnel structure affected by water immersion,parameters was studied caused by soaking(micro pore change,soil-water characteristics and permeability,etc.),and the model test and numerical analysis methods were used to study the different soaking conditions on the formation of macro stress of tunnel structure and the effects of deformation,putting forward the subway tunnel collapsible foundation treatment scheme.The main results are as follows:(1)Microstructure of loess in immersion and different load levels was studied based SEM and MIP test,micro pore distribution function was established,unsaturated loess moistening deformation is divided into three stages,four states,pore ratio and moisture content,the matrix suction and relative permeability were studied,taking water content and load as the two main effestions,the collapsible calculated model was established combining Fredlund theory.(2)Soil water characteristics test was carried by 15 bar pressure membrane apparatus,experimental results were fitted by classical model of soil water characteristic model;influence of void on micro pore distribution function was proposed,the introduction of V-G model considering the soil water characteristic the change of pore curve model,without considering the effect of void ratio(V-G)when the degradation model,model prediction results show that the soil water characteristic curve of porosity change is more practical.(3)Permeability coefficient model was studied based soil-water characteristic curve model,and based on the soil water characteristic test data,predict the different pore deformation condition,the permeability coefficient with suction changes the law,the definition of relative permeability coefficient,porosity on deformation of unsaturated loess permeability analysis.(4)Unsaturated loess state(unsaturated state,quasi saturation and saturation state)were predefined,assumes that soil water seepage accords with Darcy’s law,water gas shift accord with energy conservation and mass conservation law,describe the hydraulic coupling state change,considering the soil water characteristic curve mode,determine the relationship between saturation and suction,in the framework of thermodynamics and mechanics of porous media,equation of control system as an unsaturated state to saturation,formation of unsaturated loess and saturated mathematical model state transition hydraulic coupling.(5)Based on the previous theoretical study,the influence of immersion to parameters was studied,based the actual engineering,the possible water conditions was predicted,different soaking conditions was considered with plane strain assumption,the influence on metro tunnel structure stress through the measurement of inside and outside,lining strain,soil pressure and displacement,the size of the evaluation of different strata conditions on the influence of water tunnel,tunnel basement collapsible satandard was proposed.(6)Taking the metro shield section as research object,put forward the surface three axis mixing pile treating tunnel collapsible foundation scheme,treatment depth,treatment range and pile spacing was analysed,with full-range foundation immersion,the force and displacement of tunnel structure was studied,optimize parameters was proposed,pile spacing 2.1m,treatment depth 6m,treatment range 1m,under the same conditions,should follow the "prefer long not close" principle,which should increase the length of pile increase the depth of foundation treatment,rather than reduce the pile spacing increases pile body density.Water immersion in layer has obvious effect on water-sensitive loess parameters,loess parameters change have detrimental effects on stress of tunnel structure,studying based on the change of parameters of loess tunnel water stress characteristic,put forward the effective treatment of collapsible foundation a,can provide the basis for loess area iron pre design and post operation.