Investigation On Water Retention Characteristics Of Unsaturated Granular Materials Based On Micro-macro Experiments And LBM Simulation

Water retention characteristic is an important hydraulic parameter in unsaturated soil,which plays an important role in investigating the hydraulic and mechanical properties such as permeability,deformation and strength of unsaturated soil.The water retention characteristic of unsaturated soil is affected by many factors(particle morphology,pore structure,fluid wetting characteristics,stress state,etc.),but due to the limitations of traditional test technology and measurement methods,there are still many deficiencies in the research of porosity and contact angle on the water retention characteristic of unsaturated soil and the microstructure of unsaturated soils.Based on this,the water retention characteristic curve and relative permeability coefficient with two porosity are investigated by pressure plate instrument using high roundness glass bead materials with a particle diameter of 0.2～0.4mm.The basic characteristics of liquid clusters and pore throat structures in the dehumidification process of unsaturated granular materials and the evolutionary characteristics of the interface of fluid-solid and gas-liquid are analyzed by using the self-developed miniaturized soil-water characteristic curve test device suitable for CT scanning.Finally,based on the microscopic analysis results of CT scan images and combined with LBM numerical simulation,the variation patterns of water retention characteristic(water retention characteristic curves),interface area of two phases and the relative permeability of unsaturated granular materials with different porosity and contact angle were simulated and analyzed.The main conclusions are as follows:(1)The water retention characteristic curves of high roundness glass beads with two different porosity(φ=0.2619 and φ=0.3053)are tested by pressure plate instrument.The results show that VG model can fit the soil-water characteristic curve of high roundness glass beads with different porosity,and the determination coefficient R2 is greater than 0.99.With the increase of porosity,the water retention characteristics of the high roundness glass bead material decreases,and the inlet value and residual saturation both gradually decrease.The inlet value of the sample at φ=0.3053 is 0.15 kPa lower than that of the specimen at φ=0.2619,and the residual saturation is reduced by 0.4%,the matrix suction for residual saturation of the sample at φ=0.2619 is about 1 kPa higher than the sample at φ=0.3053.The predicted relative hydraulic conductivity using VG model fitting parameters showed that the relative hydraulic conductivity of both liquid and gas phases gradually increased with the increase of porosity.(2)Based on the CT scan images of the self-developed miniature soil-water characteristic curve test device,the local analysis of porosity,saturation,and interfacial area from the representative volume element scale was conducted and compared with the global analysis,and the trends of the two were basically in agreement.The results of the local analysis showed that,the specific interfacial area of water-solid and the specific interfacial area of air-solid show opposite trends with the change of saturation,the specific interfacial area of water-solid decreases with the decrease of saturation,and the specific interfacial area of air-solid increases with the decrease of saturation,and the sum of the two remains constant(equal to the ratio of the surface area of solid to the total volume of the sample).The specific interfacial area of airwater shows a trend of increasing and then decreasing with the decrease of saturation,and its peak occurs in the interval of 30%～40%of saturation.With the increase of matrix suction,the desaturation process is mainly divided into two stages,the large volume pore is preferentially dehydrated,and when the saturation drops to the range of about 30%,the pore water starts to be discharged from the small volume pore;during the evolution of the test desaturation,with the discharge of pore water,the large volume liquid clusters are gradually dissipated and partially decomposed into small volume liquid clusters,leading to an increase in the number of small volume liquid clusters and a corresponding decrease in the volume of the largest volume liquid clusters within the sample.The comparison results of the coordination number of the pore at the first scan and at the fifth scan indicate that there is a small change in the pore throat structure of the sample during the dehumidification process,but the effect of this change on the overall structure of the sample can be neglected due to the presence of lateral constraints.(3)Numerical simulations of two-component,two-phase flow in porous media with different porosity and different contact angles were carried out based on lattice Boltzmann method with mesoscopic scales,the results show that the relative permeability of wetting fluids increases with the increase of porosity,but there is no uniform change pattern of the specific interfacial areas of wetting fluid-solid,nonwetting fluid-solid and nonwetting fluid-wetting fluids with the increase of porosity.With the increases of contact angle,the inlet value and residual saturation of the sample both gradually decrease,and the specific interfacial area of the wetting fluid-solid and the specific interfacial area of the wetting fluid-nonwetting fluid gradually decrease,while the specific interfacial area of the nonwetting fluid-solid gradually increases.The effect of contact angle on the relative permeability of wetting fluids is saturationdependent,and the trend of the effect of contact angle on the relative permeability of wetting fluids with different saturation.The relative permeability of wettable fluids gradually decreases with the of decrease saturation.The simulated water retention characteristic curve data were fitted using the VG model,and the relationship between the fitting parameter a and the contact angle can be described by a quadratic relationship.The results of numerical simulation and the microscopic test were compared in terms of the fluid-solid interface and the gas-liquid interface,the results showed that the characteristic interfacial area of nonwetting fluid-solid from the numerical simulation was larger than the local analysis results of the microscopic test representing the volume element scale;the characteristic interfacial area of wetting fluid-solid,the characteristic interfacial area of nonwetting fluid-wetting fluid and the ratio of wetted solid from the numerical simulation were basically consistent with the local analysis results of the microscopic test representing the RVE scale.