Experimental Study On Permeability Of Unsaturated Soil And Mechanism Of Saturation Infiltration

It is important for the unsaturated infiltration of the determination of unsaturated permeability coefficient and water-air flow in soil. The testing technique and experiment methods of unsaturated soils are the important means and tools to investigate the mechanic behavior of unsaturated soil. Due to it's difficult to measure the unsaturated permeability coefficient, many researchers put forward various forms of permeability functions. However, neither any apparatus or function forms are popularized nor any of them can satisfy the request of the theory of unsaturated soil and the engineering. The air pressure changes and its influence under ponding infiltration are important to guide water saving irrigation and soil water conservation.This paper conducts comprehensive and in-depth research on the determination of unsaturated permeability coefficient and water-air flow during ponding infiltration by experiment research and theory research. The research is reduced as the following:(1) In this paper, in terms of the shortage of the current apparatus of water permeability for unsaturated soils, the new unsaturated steady seepage apparatus for measurements of unsaturated permeability coefficient has been developed based on improvement and amendment on original apparatus. Water pressure controller is used to control water pressure and measure the volume of water inflow. The efficient operating specifications were summarized according to the working theory of the apparatus. The relationship between the unsaturated permeability coefficient and the degree of saturation by experiment and regression estimation was founded.(2) The interaction principle of equilibrium analysis of multiphase media was putted forward by Shao Longtan. The kinematic equations for saturated and unsaturated soil are deduced again based on the principle and the conception of laminar flow velocity in hydraulics. In this way, the formula expressing of saturated and unsaturated permeability coefficient is also derived. The physical meaning of the interaction coefficient between soil and water for saturated soil is explained. Some permeability testing results in steady hydraulic gradient are presented to analysis the problem in application of the formula.(3) The critical equilibrium condition for the soil of infiltration zone with the operation of air pressure is founded by equilibrium anlysis.The experiment results indicate that the poresity and the antecedent water content affect the moving state of air at earlier infitration, and then affect the change of soil structure and the infiltraion procedure. The greater soil porosity and the smaller antecedent water content, the close role of water to air is greater at initial infiltration , the gas escape is easier, the distubance of soil is greater in extent and range, the fluctuations and frequency of gas pressure is greater, and led to a ladder change of infiltration; Instead, the surface water have a stronger close role to air at initial infiltration, the gas escape more difficult, the saturated layer of soil near the surface lift and form a crack at the wetting front for the effect of air pressure, soil above crack is distrubbed, soil under crack is almost free from soil disturbance, the fluctuations and frequency of air pressure is smaller, the soil infiltration id significantly reduced, and lead to stop the infiltration process.(4) Soil tests found there are two clear interfaces in infiltration soil, and defined another interface besides wetting front as saturation front. The definition is poved reasonable through the equilibrium analysis of soil above crack. The test results indicate that the position of saturation front is related to infiltration procedure. For more loose structure of the soil, the position of saturation front increases with the increase of soil disturbance depth. For more solid structure of the soil, the position of saturation front increase with the increase of cracks depth. The destrution procedure is studied from the move of saturation front. The results show that the destruction procedure occure in three phases, and corresponding to three different statuses: critical state, the transitional state and steady equilibrium state.(5) Different types of soil, the cracks in different shapes. For the caly (siliceous powder), the destruction of soil is more irregular because of viscosity of among soil particles. For the sandy soil (fly ash, fine sand), the crack is more regular. The gaps in the soil layer have defferent development with different specific gravity of soil grain. For the soil with small specific gravity, the gaps disappear quickly with the soil disturbance by air break. For the soil with great specific gravity, the gaps can not disappear. The gaps will high frequency with the fluctuation of air pressure.(6) Experiment results indicate that the infiltration procedure of unsaturated soil actually is the procedure of the increasing of saturation area. After infiltraion into the soil of water, water move toward downward and side in the soil. With infiltration, the soil saturation and scope for surface soil gradually increase.