Experimental Study On Water Evaporation Of Unsaturated Remolded Loess

In arid and semi-arid northwest regions,shallow loess is widely exposed to the atmospheric environment.Under the action of evaporation,soil moisture and surface temperature will change dynamically,resulting in redistribution of water content and temperature inside the soil,which will have an important impact on the physical and mechanical properties of loess,causing changes in soil structure and stress state under load conditions.Studying the soil moisture movement process under evaporation can provide theoretical support for mechanism analysis of engineering diseases caused by evaporation in loess regions.In this paper,the remolded unsaturated loess in Lanzhou area is taken as the research object,and the moisture migration law of unsaturated loess samples under evaporation conditions is studied through indoor experiments.The moisture evaporation experiments of unsaturated loess under different environmental temperatures,different wind speeds,different initial moisture contents and different compaction degrees are carried out by using the control variable method,and the variation of soil moisture content and the influence law of various factors on soil evaporation rate and cumulative evaporation amount during evaporation are discussed.On the basis of laboratory tests,the HYDRUS-1D software is used to establish a one-dimensional vertical soil water-heat coupled transport mathematical model.the test data are used to calibrate the model parameters and analyze the sensitivity of the parameters.the simulation results are compared with the measured results to ensure the rationality and effectiveness of the model parameters.the numerical model can better predict the unsaturated loess water transport phenomenon under evaporation conditions.then,the numerical simulation analysis of soil water transport under the influence of temperature,wind speed,initial water content and temperature gradient is carried out.The main research results are as follows:(1)The influence of environmental temperature on soil moisture content is related to soil depth,evaporation period and reference temperature.During most of the evaporation test period,the bottom moisture content of the soil sample is not obviously affected by the temperature as in the surface layer,and the law of the bottom moisture content affected by the temperature in different periods is basically similar to the surface layer of the soil sample,except that the time transition point is relatively lagging behind the surface layer.(2)No matter what kind of factors influence,the cumulative evaporation of soil increases with the extension of evaporation time.(3)The evaporation rate of unsaturated soil is determined by the amount of water in the soil and other soil characteristics from the beginning of the test.Under the influence of various factors,the evaporation process of unsaturated soil is mainly divided into two stages: deceleration evaporation stage and residual evaporation stage.(4)When the external atmospheric conditions are constant,the soil with large dry density and initial moisture content has large thermal conductivity,good thermal conductivity,and large temperature variation range of the soil.With the extension of evaporation time,the thermal conductivity effect of the soil is superimposed,which makes the temperature of the soil increase faster,but the temperature variation of the soil under the influence of dry density is smaller than that under the influence of initial moisture content.(5)Under the influence of temperature,wind speed and initial moisture content,the calculated results of soil moisture field are in good agreement with the experimental results,indicating the reliability of numerical calculation in predicting the dynamic changes of unsaturated loess moisture under the influence of various factors.(6)Under the action of temperature gradient,the internal temperature of the soil undergoes three stages: rapid temperature rise,slow temperature rise and relative stability;For soil moisture content,when the initial moisture content is constant,under the action of temperature gradient,the moisture content of soil at the high temperature end decreases and that at the low temperature end increases with time.(7)Based on the calculation results,the empirical formula of soil moisture evaporation and the relationship between evaporation depth,evaporation time and initial moisture content at a certain time under the condition of continuous evaporation are analyzed and obtained,and the relationship between the average moisture content gradient of soil under the condition of comprehensive consideration of temperature gradient and initial moisture content is established,which can serve as a reference for engineering construction in Lanzhou loess area.