The soil is considered as a colloidal clay system of visco-elastic body. Force applied to the system causes the change of chemical potential of water in soil, and when the free energy overcomes the barrier, the visco-elastic body becomes a Newtonian liquid. pF of the system is equal to the magnitude of barrier (the yield value) required to break the particle links.
The hydrophilic and the hydrophobic colloid correspond to the visco-elastic gel and the viscoelastic sol respectively. This concept can be refered to as the flow condition of a colloidal system. The results are summarized as follows.
1) The yield value corresponds to the transition point from solid to liquid. Under the stress smaller than the yield value, the soil behaves as a Voigt body, and under the larger stress, as a Maxwell liquid or a Newtonian liquid.
2) The behaviour of hydrophoilic olloid mainly depends on the entropy elasticity, and that of hydrophobic colloid on the energy elasticity.
3) The negative shear rate thixotropy flow is a phenomenon of rate process. When the rate of hydration is greater than that of dehydration in course of the decrease in stress. The hydrophobic property changes to the hydrophilic one, and the system behaves as a visco elastic body. The entropy production accelerates the rate of structure-building in the colloidal clay system.