[1] Infiltration is often assumed to occur with little or no impedance from the air within the vadose zone. If this assumption is not valid air counterflow may occur, while the infiltration rate and degree of saturation within the transmission zone may be significantly reduced. Accurate predictions of infiltration rates are important for applications such as moisture balance calculations and predictions of pore water pressures in landslide triggering. Existing results for confined infiltration show contradictory evidence for either air pressure remaining at a threshold or continual increase of air pressure. In this paper, the effect of air entrapment is investigated in the laboratory using recently developed techniques of unsaturated transparent porous media and digital photograph interpretation. These techniques enable the full saturation profile to be quantified every 5 s. The experimental data are used to quantify the decrease in infiltration rate and degree of saturation within the transmission zone in the confined infiltration, to accurately locate the wetting front, and to assess the stability of the wetting front. The results confirm previous observations in which infiltration in an open system was observed to occur significantly faster than in a closed system. However, in this study, the air pressure ahead of the wetting front was observed to reach a threshold value, which was a function of the ponding height and suction at the wetting front. A Green-Ampt infiltration model based on this observation of air confinement was observed to provide a better fit to the experimental data than the one based on the continual increase in air pressure assumption.

Experimental characterization of unsaturated soils is of primary importance to further understanding of fundamental behavior, as well as allow for accurate modeling and predictions, of constitutive and field behavior. In the laboratory, the most common research methodology used to investigate the hydraulic behavior of unsaturated soils involves placing the unsaturated soil in a column apparatus with measurements of pore pressure and moisture content being made at discrete locations distributed along the elevation of column. These types of tests have provided many valuable insights into unsaturated flow phenomena; however, there are some limitations with this methodology including the discrete nature of the measurement points. In this paper, an alternative method is proposed which aims to combine the use of digital image analysis with a transparent soil to avoid the ambiguity of traditional boundary image measurements of moisture content in column experiments. At 100% saturation, the transparent soil particles appear invisible and allows for the ability to see through the soil mass. Any air bubbles will be visible within the soil voids and as a result, at varying degrees of saturation less than 100%, the soil will become progressively non-transparent. The relationship between pixel intensity of the unsaturated soil and degree of saturation is defined and validated. This relationship allows definition of the degree of saturation throughout the column profile thus giving the opportunity to verify and further develop constitutive models for unsaturated hydraulic behavior.

Infiltration is a vadose zone process of interest to a wide range of research communities including agriculture, soil physics, and geotechnical engineering. In geotechnical engineering, transient infiltration is important to moisture balance problems such as cover systems, capillary breaks, and landslide triggering. Design of cover systems, capillary breaks, and landslide analysis applications depend on accurate models for the transient pore pressure and moisture migration response under a wide range of environmental conditions. Infiltration is typically modeled using Richards’ equation, which assumes no impedance from the pore-air phase. However, if this assumption is invalid, the ground response during infiltration is significantly affected. An optically matched pore fluid – transparent soil, which allows for high temporal and resolution measurements of degree of saturation, was used to examine the effect of air entrapment on infiltration. Homogeneous and layered profiles were subjected to closed and open infiltration conditions. Following the completion of the experimental program, the results were simulated using a finite element program that allows for consideration of the air phase during infiltration. The results show the impact of ignoring the effect of air entrapment is to significantly underpredict the time to saturation and overpredict the pore pressure response.

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