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.

Automated summary

At high saturation levels, the measured soil water retention curve in wetting tests can differ from the true curve due to air trapping. The true curve only reaches full saturation at a positive suction value. The most accurate way to represent air trapping is to use the true curve combined with a gas conductivity expression.