The effect of entrapped air on the quasi-saturated soil hydraulic conductivity and comparison with the unsaturated hydraulic conductivity

Entrapped air can greatly affect the hydraulic conductivity at or near saturation. In this study, we measured the hydraulic conductivity and volume of entrapped air in a quasi-saturated soil. Two soils, a Masa sandy loam soil from weathered granite rock and a TUAT light clay andisol from volcanic ash, were used. The soils, with three different dry bulk densities, were packed into a steel cylinder. To attain complete saturation, the packed soil samples were immersed in a 0.02 mol L-1 gypsum solution under vacuum conditions. The soil samples were then left on a sintered porous plate with suction of -17.0 kPa for different periods of time to allow drainage and air intrusion. After this drainage process, the samples were again immersed in water to permit air entrapment. The hydraulic conductivity was measured using the falling head method, and the amount of entrapped air was determined gravimetrically. The quasi-saturated hydraulic conductivity was found to decrease with increasing entrapped air content until the soil the maximum fraction of entrapped air, approximately 10% of the bulk soil volume. A comparison of the quasi-saturated and unsaturated hydraulic conductivities of the soil samples at or near saturation, when the suction of soil water was greater than the air-entry value, showed that the quasi-saturated hydraulic conductivity was smaller than the unsaturated hydraulic conductivity.