Adsorption mechanism

At very low values of water content, the pore water is primarily in the form of thin films on the particle surfaces. The dominant mechanisms contributing to negative pore water pressure (suction) is the electostatic forces at the clay particle surface. Water molecules are strongly held on the particle surfaces due to these forces. They are also retained by hydration of exchangeable cations.

Consequently, capillary mechanisms play a role at relatively high degree of saturation, where as osmotioc and electrostatic mechanisms become relevant at medium to low degrees of saturation. All these mechanisms are invoked due to the interaction of air and water phase with solid phase; and are, thus, associated with the soil “matrix”. This is the reason why suction generated by these mechanisms is referred to as “ matrix suction ” or “ matric suction ” (Laloui, 2010). The water retention curve (WRC) describes the corresponding constitutive relationship between soil matric suction and water content. The general shape of the SWCC of a given soil reflects the influence of several material properties, such as pore size distribution, grain size distribution, density, organic material content, clay content, and mineralogy on the pore water retention behavior (Lu and Likos, 2004). Genral shape of WRC is given in Fig. 3.9.

Fig. 3.9. Soil water renetion curve (after, Lu and Likos, 2004)