A Heat-flux Boundary Model And Its Application For Embankments In Permafrost Regions

There are many embankment projects in a wide area of frozen soil in China.The phenomenon of uneven settlement,cracking and deformation often occurs in frozen soil embankments,most of which are caused by the local melting and softening of the underlying frozen soil due to thermal disturbance,resulting in the destruction of the mechanical balance.Traditional frozen soil disease treatment ideas pay more attention to the study of internal factors of frozen soil,but external factors such as meteorology are often simply treated in a black box,and the impact of solar radiation and surface convection on the temperature of frozen soil under the embankment cannot be evaluated.There is still a lack of understanding of heat absorption,heat storage and heat transfer on the upper surface of the embankment under solar radiation.The heat absorption and heat transfer on the embankment surface are asymmetric,and the phase transition of permafrost is nonlinear,so it is difficult to obtain an accurate analytical solution of temperature change in the frozen soil layer.Therefore,numerical simulation provides the possibility.Accurate simulation of the heat absorption and heat transfer process on the upper surface of the embankment is very important for the scientific evaluation of the temperature of the underlying frozen soil under the embankment.In this paper,the heat flow mechanism of permafrost embankment under solar radiation is discussed by establishing a heat flow boundary model,and the measurement method of reflectivity is optimized through experiments;Combined with the measured data and numerical model,the influencing factors of the underlying frozen soil temperature are analyzed,and the regulation of the embankment Yin and Yang slopes is discussed;Finally,according to the derivation and Simulation of empirical formulas,this paper discusses the heat distribution characteristics of the underlying frozen soil,puts forward two different concepts of cold storage and cooling,deepens the theoretical research of the underlying frozen soil of the embankment,and provides a scientific basis for the heat transfer and regulation of the upper boundary of the subgrade.The main work and findings are as follows:(1)Based on the simulation of permafrost embankment with heat flow boundary conditions,the theory of solar radiation on the embankment surface and the heat flow boundary model are proposed to accurately simulate the ground temperature of permafrost.The heat flow boundary model covers the surface solar radiation absorption,convection heat transfer,long wave radiation,heat storage and other heat transfer parameters,presenting the heat transfer process between the upper surface of the embankment and its adjacent ground,The sensitivity analysis of the temperature of the frozen soil layer under the embankment to the ground heat transfer parameters has become a reality,and the experiment shows that the reflectivity of the embankment surface can be effectively measured.By comparison,near the phase transition point,the temperature boundary condition has a temperature jump,which has a large deviation from the measured value,and the heat flow boundary model can be used to explain the absorption and release of ice water phase transition energy,so as to achieve a more accurate temperature fitting.If the parameters of sinusoidal temperature change on the upper boundary of the embankment are supported by reliable data,the forced temperature boundary model can be used,and the heat flow boundary model is recommended in other cases.(2)Through the observation data and Simulation of frozen soil subgrade for many years,the influence factors of frozen soil temperature under the embankment surface and the regulation law of Yin-Yang slopes are revealed.The main factor affecting the temperature of frozen soil under embankment by solar radiation is the reflectivity of embankment surface,and the other factors are secondary factors;The annual average temperature determines the formation temperature but cannot be controlled,and the influence of wind speed can be ignored.It is proposed that the single factor?R is used to quantitatively regulate the effect of embankment Yin-Yang slopes,?R=(1-R)(∑I_S-∑I_N)/∑I_S∑I_Nare the annual solar irradianceson the southern side slope and the northernone,respectively.R is the initial reflectivity of the slopes on both sides.The factor?R is not only related to the incident solar irradiance∑I_Sand∑I_Non both sides of the embankment,but also depends on the initial reflectivity R of the embankment slope.Increasing the reflectivity of the southern side slope of the embankment can alleviate or even eliminate the Yin-Yang slope effect of the embankment in permafrost regions.For materials with rough surfaces,it is necessary to consider the photon trapping effect of their surfaces and increase the reflectivity to an accurate and appropriate value.(3)Through formula derivation and numerical simulation analysis and comparison,this paper discusses the distribution characteristics of heat collection and distribution in the soil layer below the natural ground of the embankment,puts forward different concepts of cooling and cold storage,identifies effective ways to maintain the thermal stability of frozen soil,and discusses the application law of reducing the heat intake of frozen soil inside the embankment.The energy concentration profile?E in the embankment is the largest at 0-1m below the frozen soil upper limit,which increases with the increase of the service time of the embankment and the water content of the soil layer;The energy accumulation profile?E of the slope(shady slope)on the north side of the embankment at 0-1.0m above the original frozen soil upper limit is the smallest.If it is negative,it means that cold energy is obtained there,indicating that the frozen soil layer is in a process that can absorb and release a large amount of energy.The morphology of?E and?T near the phase change point is very different.The frozen soil layer?E is prone to step over and the temperature T does not appear,which also shows that the energy consumption of frozen soil phase change is far more than the energy consumption required by the simple temperature rise of soil particles.The energy concentration profile of the soil layer with high water content has a large?E,but the temperature changes little.For the less frozen soil layer,the ground temperature can be effectively reduced by injecting a small amount of cold energy,while for the rich frozen soil layer,injecting a large amount of cold energy may not be able to effectively reduce the formation temperature.The cooling measures for frozen soil should pay more attention to the cold storage(heat dissipation)capacity of frozen soil,that is,the rate of change of heat of frozen soil layer over time rather than the temperature rise rate of frozen soil layer.