4. Water transfer in unsaturated soils

4.1 Transfer equations in liquid phase

The equations governing water transfer in an unsaturated medium were initially defined in the case of undeformable unsaturated soils, with continuity of the gaseous phase, allowing us to consider that at all times the air pressure is equal to atmospheric pressure (u _a = 0). These assumptions are valid in sands, for which the air inlet pressure corresponds to low suctions. The first calculations are due to Richards.

In the case of a non-deformable unsaturated soil, the approach is analogous to that for saturated soils:

• the expression for the hydraulic head is deduced from the water potential and is written as :