Investigation Of Unsaturated Hydraulic Conductivity And Its Application

The flow of pore water in unsaturated soil can be expressed by Darcy's law,and the ratio of flow velocity to the hydraulic gradient is called the hydraulic conductivity.The hydraulic conductivity of unsaturated soil is an important parameter for studying seepage,unsaturated soil consolidation and contaminant transport.It is mainly affected by the mineral composition of soil particles,the size and distribution of pores,the water content,temperature and differention concentrations.This paper focuses on the unsaturated hydraulic properties and includes:1.This paper introduces an improved pressure plate instrument.With this instrument,water evaporation from the unsaturated soil surface in the pressure chamber can be ignored,and the water evaporation loss from the water outflow vessel can be compensated.The error correction of water evaporation loss on hydraulic conductivity are also discussed.2.In this paper,an improved pressure plate instrument with an evaporation compensation system is developed to measure the transient water content curve of the soil in multi-step outflow experiments.The soil water retention curves of silicon micro-powder and Guangxi Guiping clay are derived from this transient curve.Therefore,a direct measurement method is presented to describe the change of hydraulic conductivity during each step.Comparison with the Gardner outflow analysis method and the van Genuchten-Mualem model indicates that our approach shows better agreement with test data for these two types of soils.3.This paper proposes a permeability function to estimate the water permeability coefficient of unsaturated soil.It is derived on the basis of the equilibrium equation of the water phase.Only one parameter is required in the proposed function,this parameter can be more easily obtained by using permeability tests of saturated soil with different porosity values.A number of test results in the literature are employed to validate the proposed permeability function,and the model prediction exhibits good agreement with the experimental data.4.The importance of the hydraulic properties of unsaturated soil for geotechnical designs has been well understood.However,most of the previous investigations of hydraulic properties are limited to a single drying and wetting cycle or are based on multiple drying-wetting cycles with limited soil experiments.In this study,the hydraulic properties of silicon micro-powder and Guangxi Guiping clay were studied for the same soil specimen under five cycles of drying and wetting in multi-step outflow experiments.Results also show that hysteresis plays an important role in the hydraulic properties for both soils.5.Based on the proposed permeability function,Comsol software was used to simulate the distribution of the range and concentration of the solute in the unsaturated soil layer.Compared with the van Genuchten-Mualem model,the practicability of the permeability function is proved.