Study On Mud Pumping Mechanism And Long-term Dynamic Characteristics Of Ballasted Track Subgrade

Although ballastless track accounts for an increasing proportion in the development of high-speed railway,ballasted track is still one of the main forms of railway track structure at this stage,especially in the existing railways.Due to the low control standard and the lack of relevant construction and maintenance experience of the existing ballasted railway built in the early stage,typical subgrade diseases such as subgrade mud pumping,uneven settlement and collapse occur frequently.On the one hand,under the action of long-term train traffic load,rainfall wetting and other complex environments,due to poor subgrade drainage and poor permeability of subgrade filler,large local excess pore water pressure is generated in the subgrade,which leads to the fine particles migration in the subgrade and turn out the subgrade system to form mud pumping.On the other hand,the subgrade filler in practical engineering is usually in an unsaturated state,and its hydraulic mechanical properties is more complex than that of saturated soil.Under the action of long-term train load and complex external environment,the unsaturated subgrade filler will produce cumulative deformation,which will lead to uneven settlement and even collapse of the subgrade.Therefore,in view of the above typical subgrade diseases,it is urgent to carry out comprehensive and in-depth experimental and theoretical research on the mud pumping particle migration characteristics of ballasted track subgrade and longterm dynamic characteristics of unsaturated subgrade filler under the coupling effect of long-term traffic load and complex environment,so as to provide a valuable reference for ensuring the safe operation of railway engineering,optimizing the design method of railway subgrade and improving its long-term service performance.Taking the typical mud pumping disease of Laiwu east station of Xintai railway as the engineering background,the model test research on the of mud pumping particle migration characteristics of ballasted track subgrade and the dynamic triaxial test research on the long-term dynamic characteristics of unsaturated subgrade fillers are carried out by using the representative soil specimens taken from the site,the hydromechanical coupling bounding surface model and the fractional order bounding surface model which can accurately describe the dynamic stress-strain relationship of unsaturated subgrade filler are establised.The main work and research results of this paper are as follows:(1)A model test system for mud pumping in ballasted track subgrade is developed.Based on the macro and micro test technology,a mud pumping model test system is developed.The system has the following advantages: it can realize the coupling effect of cyclic load and wetting,and simulate a variety of complex working conditions;the particle migration characteristics in the process of mud pumping can be captured in real time through high-definition images.Only one set of sensors can measure the matric suction and saturated pore water pressure of the model specimen at the same time.The developed test system was used to successfully carry out the model test research under five working conditions: long-term train load wetting coupling(LCL-W),train load bidirectional wetting coupling(C-BW),intermittent train load wetting coupling(IC-W),multistage train load wetting coupling(MSC-W)and multifrequency train load wetting coupling(MFC-W).(2)Through the model test research,the mechanism of mud pumping and the dynamic characteristics evolutions of ballasted track subgrade under complex working conditions are revealed.The experimental research shows that: the particle migration phenomenon of model specimens under different working conditions in unsaturated state is not significant,and the cumulative deformation is mainly caused by the intercalation between ballast layer and fine particle layer and the compression of fine particle layer itself.In the saturated or near saturated state,driven by the dynamic pore water pressure,significant mud pumping occurs,and the cumulative deformation is mainly caused by particle migration.The fine particle layer density of model specimens under different working conditions is significantly different,which leads to different mud pumping degree.The mud pumping degree is the most serious under the C-BW working condition.This condition poses a great threat to the safe operation of ballasted railway and should be paid enough attention to.(3)Through laboratory test,the dynamic characteristics evolutions of unsaturated subgrade filler under various influencing factors is revealed.The dynamic triaxial test considering the effects of initial water content,initial dry density,dynamic stress amplitude,net confining pressure and loading frequency and multistage loading dynamic triaxial test are carried out for typical unsaturated subgrade filler by using GDS dynamic triaxial apparatus.The cumulative axial strain,hysteresis curve,resilient modulus and damping ratio evloutions of the specimens are obtained.The test results show that the cumulative axial strain,resilient modulus,damping ratio and hysteresis loop area of the specimens increase with the increase of initial water content,loading amplitude and loading frequency,and decrease with the increase of initial dry density and net confining pressure.The slope of hysteresis loop shows the opposite evolutions.Under multistage loading,the slope of hysteresis loop decreases with the increase of dynamic stress amplitude.When the dynamic stress amplitude increases step by step,there will be a short-term significant decrease in resilience modulus and a short-term significant increase in damping ratio.(4)A hydromechanical coupling bounding surface model for unsaturated unsaturated subgrade filler considering the microscopic pore structure and interparticle bonding effect is established.By introducing micro degree of saturation,a new effective stress and bonding variable are established as the constitutive variables.The equations of normal consolidation line and yield curve are derived.Based on the plastic theory of bounding surface,the deformation calculation is carried out,and the hydromechanical coupling is realized combined with the soil-water characteristic curve model considering deformation and hysteresis effect.The proposed model is verified by a series of constant suction isotropic compression tests,triaxial shear tests and constant net stress drying/wetting cycle tests of unsaturated soil.The results show that the proposed model can effectively predict the hydromechanical coupling characteristics of unsaturated soil.(5)Based on the bounding surface plasticity theory and BBM theoretical framework,a fractional bounding surface model suitable for the cyclic dynamic analysis of unsaturated subgrade filler under constant matric suction is established.The fractional cumulative strain rate is introduced to consider the influence of cyclic loading history on the dynamic stress-strain relationship.The movable mapping center is used to describe the hysteresis characteristics of dynamic stress-strain curve.The shape function which can independently describe the initial loading,unloading and reloading process is used to accurately describe the shape of the hysteresis curve.Based on this,a fractional bounding surface model which can accurately predict the dynamic stress-strain relationship of unsaturated soil is established.The dynamic triaxial test data of unsaturated silty sand sand and clay under constant matric suction are used to verify the proposed model.The comparison between the model prediction results and the test results shows that the proposed model can reasonably predict the dynamic stress-strain relationship of unsaturated soil under constant matric suction.