| The canopy effect refers to the phenomenon where liquid water and vapor in the subgrade migrates upwards due to factors such as temperature gradients,and accumulates below the impermeable surface.When the temperature drops below freezing point and the accumulated water freezes into ice,the canopy effect will be further strengthened and cause engineering disasters such as frost heave.However,there are subgrades without impermeable surface originally,for example,the subgrades of ballasted railways.Moisture at the top of these subgrades can freeze into ice when the surface temperature drops below freezing point in winter,which hinders vapor evaporation and changes the coverage conditions,resulting in the accumulation of moisture and frost heave due to moisture migration.This phenomenon can be referred to as the time-varying canopy effect.Prediction of the frost heave deformation and force caused by canopy effect and time-varying canopy effect have an important significance in the design of engineering in cold regions and the prevention of frost damage.This thesis explores the thermos-hydro-mechanical(THM)coupling characteristics of unsaturated freezing soil from three aspects: thermodynamic theory,indoor experiments,and numerical simulation.The main research results are as follows:(1)The thermodynamic equilibrium relationship between liquid water,vapor and ice in soil pores was derived to describe the driving force of liquid water and vapor in frozen and non-frozen regions,as well as the relationship between liquid water and vapor content,temperature,and pressure in equilibrium state.Besides,A physical empirical model was established to estimate SWCC and SFCC based on grain-size distribution.(2)Based on PINNs,a method for obtaining soil moisture and thermal parameters through indoor experimental data was presented.The existing experimental instruments for canopy effect have been further improved to explore the influencing factors of frost heave amount and frost heave force through indoor experiments.It is pointed out that the moisture supply conditions,temperature boundary conditions,and initial moisture content can affect the frost heave of unsaturated soil.(3)The numerical techniques to deal with the discontinuities at the moving interface between frozen and non-frozen regions were proposed.A finite element model describing the liquid water,vapor and heat transfer in unsaturated freezing soils has been established,which can be applied to explore the mechanism of water and heat transfer in unsaturated freezing.The model was verified by the existing freezing tests.The model can reproduce the canopy effect and time-varying canopy effect,and the significance of vapor transfer was stressed.(4)An unsaturated-saturated freezing soil model was established to describe the variation in saturation condition of freezing soil and the interaction between THM coupling processes as well as ice segregation.The model can calculate the frost heave amount and frost heave force of unsaturated freezing soils and fully-saturated freezing soils.The model was verified by the results of indoor freezing experiments and field measured data of the subgrades with frost heaving problem in Lanzhou-Xinjiang Railway.Besides,the main factors affecting frost heave of subgrades were analyzed,indicating that the frost heave force and frost heave amount of the freezing subgrades are related to the soil type,temperature boundary conditions,water supply conditions,evaporation boundary conditions,and mechanical boundary conditions.The calculation results can guide the prevention and control of frost heave disasters. |
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