Inspection of excess pressure propagation in the zone of gas entrapment below the capillary fringe

Field and theoretical efforts indicate that the presence of entrapped gas bubbles below the water table may influence the propagation dynamics of excess pore pressure caused by load changes. The process of pore pressure propagation depends on the volume of the occluded gas phase and on the relation between hydraulic conductivity of the dynamics of (hydraulic or mechanical) loading changes at the boundary. In the context of safety considerations the estimation of excess pressure dissipation dynamics may be essential. Based on Boyle-Mariotte and Henry principles predictions on water saturation versus pore pressure were established and the soil-water characteristic curve was extended from the unsaturated zone throughout the zone of air occlusion. Analogous to unsaturated zone concepts a pore pressure dependent water-capacity relationship was derived for the zone of the residual gas phase and set in 2D FE-model based on Richards equation. The extended model accounting for gas entrapment was evaluated against an explicit 2-phase model, laboratory experiments and field observations. The approach presented enables the inspection of the effect of gas entrapment on pressure propagation considering the transition from saturated to unsaturated state.