Mechanical behavior of unsaturated soils

The deformation and strength of unsaturated soil depend, on the one hand, on the mineralogical nature of the particles that make up the soil skeleton and, on the other, on the state of the soil (porosity, degree of saturation, water pressure, gas pressure, stresses due to gravity and external loads). The description of mechanical behavior as a relationship between loads (stresses, water and gas pressures) and soil deformation is based on tests where stress and suction can be controlled separately (see article "Water in unsaturated soils"). These testing techniques, derived from equipment used for conventional soil behavior studies (articles "Compressibility. Consolidation. Settlement" and "Shear strength"), are dealt with in the first part of this article. The main behavioral characteristics of unsaturated soils (volumetric aspects and shear strength) are then described in the light of the test results, following an analysis of the framework for interpreting the results (effective stresses or independent variables). The third part presents the analytical formulation of behavior laws adapted to current knowledge of unsaturated soil behavior.