One Phase vs Two-Phase Modelling of Infiltration Processes

Because of global warming, the frequency and harmfulness of climate extreme meteorological events have increased and are expected to increase. Events such as droughts, heatwaves, storms, floodings strongly impact the functioning of critical infrastructure, as well as the stability and degradation of man-made earthworks and natural slopes. The shallower portions of natural soils and earthworks are thus exposed to increasingly severe dry and hot seasons and to intense rainfalls, which affect the depth of the water table and the hydraulic circulation, triggering material degradation and instabilities. Robust modelling of the soil-atmosphere interaction, correctly accounting for unsaturated flow and boundary conditions, is a requirement of modern geotechnical engineering. The assumptions made when modelling infiltration, in terms of infiltration rate, pore and air pressure distribution and evolution, have in fact a relevant impact on model predictions. As a fraction of the pore space of unsaturated soils is occupied by the gas phase, composed of water vapour and dry air, a complete model of infiltration requires accounting for the inflow of the liquid phase and the outflow of the gas phase. Contrarily, infiltration is very often modelled accounting only for the inflow of liquid water. This work explores the consequences and limitations of such simplification by comparing the predictions obtained by adopting both a two-phase and a one-phase model for the simulation of the infiltration processes.