Determination Of Hydraulic Properties Of Unsaturated Soils: Theory And Methods

The hydraulic properties of unsaturated soils include the soil-water characteristic curve, a relationship between matric suction and water content, and the hydraulic conductivity function, a relationship between hydraulic conductivity and water content or matric suction. These properties play a crucial role in solving the issues related to the unsaturated soils, such as analysis of the mechanical behavior of unsaturated soils, prediction of subsurface flow and transportation, and so on.This research is concerned with the theory and methods that can be effectively applied to determine the unsaturated soil hydraulic properties, and it is summarized as follows:1) In using the pressure plate extractor to measure the soil water characteristic curve, the soil water content is usually determined by weighing the soil sample or weighing the outflow. Due to their intrinsic drawbacks, however, both methods can induce errors in the measurement. To resolve the problem, a new testing method, the adjusted method of weighing outflow, is proposed that can be used to determine the soil water characteristic curve by using the modified air pressure and the adjusted method. By comparing the method of weighing soil and the method of weighing outflow to the adjusted method of weighing outflow, the applicabilities of these three methods are discussed.2) Conventionally, the soil water characteristic curve and the hydraulic conductivity function are not concurrently measured in the same sample, and in measuring the hydraulic conductivity function, the constant head method is usually applied, which is not applicable when the conductivity is very low. To resolve this issue, a combined testing system, which can concurrently measuring the soil-water characteristic curve and hydraulic conductivity function of unsaturated soils, is developed. Comparisons between the testing results using the combined system and those using the conventional methods show that the newly developed testing system works well.3) Based on the theory of multiphase porous media, a linear visco-elastic model is proposed, which can be used to derive the static soil water characteristic curve from the non-equilibrium outflow testing data. Based on this model, a method is proposed to effectively determine the hydraulic properties of unsaturated soils.4) Based on the above model, a novel method is proposed to determine the time for the soil sample to attain the equilibrium. We discuss the effects of the adopted time intervals, the size of suction step, the water capacity and the characteristic time on the equilibrium.