Partially saturated soils are three-phase media consisting of a deformable soil skeleton and the two
fluid phases water and air. These two fluids, filling the entire void space, are separated by interfaces
that play an important role in the mechanical behavior of the soil skeleton. The curved shape of
these water–air interfaces and the presence of surface tension cause a difference between the
pressures in the water and the air phase, called capillary pressure or matric suction. Experimental
evidence shows that an increase in matric suction produces an increase in stiffness and shear