Effective stress in saturated soil: a granular solid hydrodynamics approach

Effective stress is one of the most fundamental concepts in soil mechanics. A comprehensive interpretation of its physical essence is important for a better understanding and prediction of the mechanical behaviour of the soil. The granular solid hydrodynamics approach is used in this study to establish a more generalised effective stress principle for saturated soils. In this approach, the stress formulation is theoretically derived from the constraints between conservation laws and thermodynamic principles, in which the energies in soils are quantitatively described by the classical non-equilibrium thermodynamics. Five physical mechanisms of the soil deformation, including the elastic potential, the elastic relaxation, the granular fluctuation, the viscosity and the effect of bound water, are taken into account in the derivation of the stress formulation. Consequently, the effective stress derived in this study is classified into four categories. The first is elastic effective stress that is determined by the elastic potential energy and is limited by a stress state boundary line. The second is granular effective stress that is relevant to the granular fluctuation and is very important to the unrecoverable soil deformation. The third is bound effective stress which is a result of defining the bound water as an independent phase of the soil system. It is an important driving force in the non-isothermal unrecoverable soil deformation induced by the conversion of bound water to free water. The last-viscous stress-is another important part of effective stress.