Experimental Research On The Hydraulic Conductivity And The Water-air Flow In A Sand Column

Because the phreatic water surface is connected with atmosphere and its re-charge mainly comes from atmospheric precipitation,its movement is close impacted by atmosphere. With the drawdown of the phreatic water surface, the unsaturated zone appears upside the surface. Air naturally enters the unsaturated zone to form water-air flow at the same time,because the pressure in the zone is negative. When the phreatic water surface falls caused by those reasons such as pumping water from well, air diffuses to the surface immediately to fill up the spaces left by water, but air inflow is maybe different at different depth. Therefore, a sand column model is em-ployed to study the relation between water outflow and air inflow. A high 240cm transparent organic glass column, whose radius is 10cm, is used to simulate water-air flow and measure hydraulic conductivity. The experiment to hydraulic conductivity of the same kind of porous medium indicates that the hydraulic conductivity changes with depth, and it increases with the depth. The main influencing factor, which brings to instability of hydraulic conduc-tivity, is the change of sand height; the higher is the sand column, the more stable is hydraulic conductivity. The relation between fluctuating of hydraulic conductivity and sand height is in exponential relation. Air inflow is not equal to outflow, but less than water outflow and the volume of air is also less than water, during the saturated state changed to unsaturated in the sand column, but the relationship between air and water is linear. The reasons of air inflow less than water outflow are hysteresis phenomenon exiting in unsaturated flow, and air expands and diffuses as soon as it entering sand. When the upper ori-fices participate into air entering, the good relation between air inflow and water out-flow is gained, on the country, only lower orifices enter air, the relation is poor. When the upper orifices enter air, the outflow of water also exceeds that when the lower en-ter, but the trends of water outflow and air inflow with time are attenuation curve. After analyzed the air inflow obtained from experiment at different depth, the flow from upper orifices is exceed that from the lower.