Research On Water,Air And Heat Conduction Characteristics And Its Heat-Moisture Transfer Of Loess

So far,the properties of loess materials are still a hot topic in the field of loess research all over the world.The loose and porous structure also makes the water,air and thermal movement in the loess layer very active.Mass energy transfer in unsaturated loess is accompanied by water-air transport and transformation.For loess with collapsibility,water sensitivity and weak cementation,the impact of this mass-energy transfer on the structure and strength of the soil is irreversible,and it often lays hidden dangers for various loess projects.In view of this,this paper has carried out a series of experiments around the conduction characteristics of loess water,air and heat and its heat-moisture transport.The main contents of the research and the results obtained include:1)By analyzing a fresh soil section and soil samples from Yan'an New Area,northwest China,an engineering shallow loess slope was investigated.Temporal variation in the saturated horizontal hydraulic conductivity(K_h)and saturated vertical hydraulic conductivity(K_v)of loess samples were measured using variable head permeability tests.The K_h and K_v values indicate that the loess exhibited permeability anisotropy in space and time,and the K_h and K_vat different depths decreased as seepage time increased.K_h-time showed a power function decreasing trend,and K_v-time showed a linearly decreasing trend.Analysis of the initial state and micromorphological features of the loess indicate that the orientation of soil particles in the deposition process may be inherently anisotropic.The K_h and K_v values of the loess tended to decrease with depth,as described by an exponential decay model.2)This study investigated the anisotropic characteristics in vertical and horizontal directions of the preferred orientation of grains and aggregates(POGA)at different buried depths in Malan loess in Yan'an.Quantitative information of particle aggregates from Scanning Electron Microscopy(SEM)photographs of vertical and horizontal solidified slice specimens was derived by Arc Gis vectorization tool.The variation of roundness(R),anisotropy index(Ia)and entropy(Hm)of clay grain aggregates was analyzed.The results show that the azimuthal of the POGAs in the horizontal direction is between 130°and 150°(N40°W-N60°W),while the POGAs in the vertical direction is parallel to the ground plane,and the deeper the burial depth,the more significant the preferred orientation(PO)is.With the burial depth,both vertical and horizontal grain aggregates are based on rounded grains,and the Ia of soil increases.The Hm with the buried depth has little change in the horizontal direction,but decreases in the vertical,and the POGAs of the vertical direction is better than the horizontal one.Finally,the Ia and Hm of the undisturbed loess are compared with that of the remolded clay in the published literature,and the relationship between the microanisotropy and the POGAs is discussed.The results show that when the water content is lower than the threshold value,the anisotropy and order degree increase as the water content increases;when the water content is on the right side of the threshold value,the soil fabric has significant anisotropy and a directional arrangement.3)Many modified Kozeny–Carman(KC)equations have been used to predict the saturated permeability coefficient(K_s)of porous media in various fields.The effectiveness of the KC equation will be evaluated via laboratory penetration tests and previously published data,which include void ratio,specific surface area(SSA),liquid limit(LL),and permeability coefficient values.This paper demonstrates how to estimate the SSA of cohesive soil from its LL.Several estimation algorithms for determining the effective void ratio(e_e)of cohesive soil are reviewed.The obtained results show that,compared to the KC equation based on porosity and geometric mean particle size(D_g),the KC equation based on the SSA and the e_e estimation algorithm can best predict the K_s of remolded loess.Finally,issues associated with the predictive power of the KC equation are discussed.Differences between measured and the predicted K_s values may be caused by the uniformity of the reconstructed specimen or insufficient control of the test process and errors in the SSA and e_e.4)We have improved a transient vacuum decay air permeability instrument,which can be successfully used for in-situ and in-lab air permeability testing:a)The in-situ and in-lab intact air permeability test results show that the in-situ is slightly smaller than the in-lab test,but the two have a good 1:1 linear relationship;The in-situ air permeability of the three sites was smaller with the increase of depth.Xi'an loess had the best air permeability,followed by Yan'an loess,and Heifangtai the worst;The results of the in-lab undisturbed test of the two sizes show that the size effect is not significant;The air permeability of undisturbed loess gradually decreases with the increase of burial depth,decreases with the increase of initial dry density and initial water content,and increases with the increase of void ratio and aerated porosity;The air permeability of the undisturbed loess using the YO model is low;b)The results of remolded loess permeability show that:dry density?_d=1.60 g/cm~3 may be a turning point of remolded loess permeability.When?_d<1.60 g/cm~3,the three soils are near the optimal moisture content,the air permeability is large.If the moisture content exceeds the optimal moisture content,a slight increase in the moisture content will cause a rapid decrease in air permeability;when?_d>1.60 g/cm~3,the air permeability decreased with the increase of water content;In the process of wetting or drying,when the same saturation is reached,the air permeability under multi-step path is generally smaller than that of the low-step path.The air permeability has a good power function relationship with saturation and porosity,and the R~2 is above 0.90.In addition,the air permeability and saturation water conductivity can be described by a power function,and the parameters?,?and dry density?d can be expressed as quadratic polynomial functions,and the correlation degree is good,with the R~2 reaching 0.788 and 0.821,respectively.5)Experimental research on the thermal parameters of remodeled loess using the Lambda Netzsch HFM 446 heat flow meter shows that:With the increase of water content and dry density,the thermal conductivity and specific heat of remolded loess increase,and the thermal conductivity and specific heat have good curved surface fitting relationship with water content and dry density,and the correlation is good.When the moisture content is constant,the soil's dry density is greater,the structure is closer,the thermal conductivity is better,and the thermal conductivity is greater;when the dry density is constant,the soil's moisture content is greater,the heat storage is enhanced,and the specific heat is greater.Finally,the calculation models of soil thermal conductivity and specific heat are compared and evaluated.6)In order to study the law of heat and moisture transfer of loess under the action of temperature gradient,a constant temperature environment test box was carried out to conduct soil column test research.The results showed that:Under the temperature difference of 20-5°C,the temperature of the soil column at different locations showed a rapid decrease with time.The farther away from the heat source,the longer the time required to reach the temperature stable period.The temperature at each location of the soil column after 4 days changes with time basically stabilized.In the initial stage of the experiment,the water content of the soil column decreased at the top,and the water content at the bottom increases.After about 12 hr,the water content change rate slows down,becoming steady.The longer the migration time,the more sufficient the movement of water vapour in the soil and the water content gradually stabilizes.7)The effect of moisture and heat transfer on unsaturated loess slope is discussed by numerical simulation.The results show that the influence of the humidity field on the slope is more significant than that of the temperature field.However,due to the existence of a temperature gradient to a large extent determines the moisture and heat transport process in the unsaturated loess slope,and then plays a very important role in determining the humidity field of the slope.In essence,temperature conditions affect the stability of unsaturated slope by affecting the distribution of humidity field.