Analytical and numerical modelling of air trapping during wetting of unsaturated soils

At high values of degree of saturation, the apparent soil water retention curve (SWRC) measured in a wetting test in the laboratory may differ from the true SWRC, because of the occurrence of air trapping, meaning that gas pressure in the trapped air is greater than the externally applied gas pressure. Physical arguments indicate that the true SWRC will reach full saturation at a positive value of suction. Analytical and numerical modelling of the phenomenon of gas trapping during wetting shows that, once air trapping occurs, the apparent SWRC depends upon many aspects of the wetting test conditions and is not a fundamental representation of the soil behaviour. The only correct way to represent the occurrence and influence of air trapping during wetting within numerical modelling of boundary value problems is to use the true SWRC in combination with a gas conductivity expression that goes to zero at the air-discontinuity point.