Numerical Simulation And Experimental Research On Time-Varying Canopy Effect Of Subgrade

The canopy effect refers to the effect that road surfaces or other surface coverings have when they block the evaporation of water from subgrade soil,causing moisture to accumulate underneath the road surface under the action of temperature gradient.When the temperature at the top of the soil is below the frozen temperature,the accumulated moisture will freeze,which further strengthens the canopy effect.For railway subgrades,evaporation is not completely blocked by the stone ballast layer.However,before winter,due to weak coverage of the stone ballast layer and the temperature gradient in soil,moisture will also accumulate in the soil beneath the ballast layer.In winter,moisture in the shallow soil gradually freezes,making the top cover effect become stronger and stronger.After the ice lens is formed,the water evaperation will be compeletely hindered by the ice lens.Strong coverage and complete coverage will further enhance the water migration and frost heave development in the soil.The phenomenon that the top cover boundary conditions changes with time,which leads to frost heave hazard,is called here the time-varying canopy effect.The main work of this thesis is as follows:(1)Through field measurements in the Wuwei section of the Lanxin Railway,the existence of the time-varying canopy effect is demonstrated,and the development mechanism of the time-varying canopy effect is analysed.(2)Considering shortcomings of the existing freeze-thaw cycle test instruments,the time-varying canopy effect laboratory test equipment is designed and manufactured,and some experimental studies are carried out.(3)Based on the existing unsaturated water-vapor migration model,the existing frost heave model and the analyses of field measurements,a time-varying canopy effect model considering the influence of the icewater-air multiphase zone and the ballast layer has been established.The deficiency of the existing unsaturated water-vapor migration model for numerical modeling of timevarying canopy effect is improved.And it is clarified that there should be ice-water-air multiphase zones in the soil during the development of the time-varying canopy effect.It is of much signficance to consider the interaction and development process of each phase in this multiphase zone for accurately analyzing the time-varying canopy effect.Besides,based on the bare soil evaporation equation and the water vapor diffusion law,the soil evaporation equation under the influence of the ballast layer is established.Based on the derivation of the analytical solution of the one-dimensional transient heat transfer problem,the calculation formula of the soil heat transfer boundary under the influence of the ballast layer is established.(4)Based on the established numerical model,the influencing factors of canopy effect and time-varying canopy effect are analyzed,and the canopy effect and time-varying canopy effect are compared.Simulated results show that the time-varying canopy effect is most likely to occur in silt.The greater the temperature difference between the top and bottom of the soil,the greater the amount of water migration at the bottom,and the greater the amount of frost heave under the time-varying canopy effect.The higher the initial volumetric water content,the greater the bottom moisture migration after freezing,and the greater the frost heave under the time-varying canopy effect.Compared with the canopy effect,the frost heave caused by the time-varying coverage is generally smaller,but it can also cause enough frost heave to hinder the normal operation of the train under the cold conditions in the Northwest,resulting in engineering hazards.