Abstract
The process of water infiltration into initially dry sand was studied in horizontal sand columns under various airtight conditions. To investigate the interrelations among water inflow behavior, air pressure, air confinement effect, and vent effectiveness in unsaturated porous media experiencing dynamic infiltration, a total of five dynamic infiltration experiments with fixed inlet water pressure were performed with different air vents open or closed along the column length. Visualizations of the infiltration process were accompanied by measurements of water saturation, air pressure, and accumulated water inflow. In a column system with an open end, the absence of air pressure buildup reveals that the vent at the column end can significantly reduce the internal air pressure effects during infiltration, and the air phase can be ignored for this case. However, in columns with a tight end, the coupled air and water flow processes can be divided into two completely different periods. Before the water front passed by the most distant open vent, the internal air pressure effects on retarding dynamic infiltration are negligible, similar to the open end case. After this period, the open vents can certainly influence the inflow behavior by functioning as air outlets while they cannot equilibrate pore air pressure with the atmospheric pressure. The remaining air ahead of the front will be gradually confined and compressed, and the significant increase in air pressure highlights the great role of air pressure buildup in reducing the water infiltration rate. The closer the last open vent was to the water inlet, the higher was the increase in air pressure and the greater was the delaying effect on water infiltration. This work may extend the experimental study of water infiltration into the unsaturated soils with different airtight conditions and provide experimental evidence on these coupled mechanisms among the water and air phases in soils.
Highlights
The influence of air entrapment on water infiltration into unsaturated porous media has been studied for a long time [1,2,3,4,5,6,7]
We performed five dynamic infiltration experiments in horizontal sand columns considering various airtight conditions by the use of a single or multiple air vents, with a focus on the interactions among internal air pressure, inflow behavior, vent performance, and air entrapment effect. The results of this experimental study suggest that the infiltration behavior in unsaturated porous media with different airtight conditions can be dramatically differently influenced by internal air pressures and this air pressure effect is strongly controlled by the distance to the farthest air vent from which air can escape
In column systems where the end is open, the air pressure effect would not be expected to influence water inflow behavior because the buildup in pressure is low enough to be ignored during the entire infiltration process
Summary
The influence of air entrapment on water infiltration into unsaturated porous media has been studied for a long time [1,2,3,4,5,6,7]. In cases in which the gas phase displaced by the invading liquid can freely escape from the system, the traditional assumptions that soil air pressure remains constant and in equilibrium with atmospheric pressure are valid [8,9,10,11]. In many cases, the movement of air may be impeded by impervious layers or obstructions, leading to a buildup in air pressure and a reduction in the rate of water infiltration due to the entrapped and compressed air phase [2, 12,13,14]. As reported in Slater and Byers [15], for a finegrained soil underlain by an impervious rock or clay, the rainfall might seal the surface soil and further resist the air escape. Weeks [16] and Guo et al [5] showed that heavy
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