Abstract: In order to explore the effect of suction between soil and water on the mechanical response of unsaturated soil, the water in soil is divided from its existing environment into the free water which is store in capillary tubes of soil predominantly by the sucking action of liquid surface tension and the adsorbed water which is bound surrounding soil particles predominantly by the absorptive action of matrix electrostatic charge. After the soil particle matrix together with the absorbed water is thought of as soil skeleton phase, the free water is thought of as fluid phase and the gas in soil is thought of as gas phase, respectively, the work expression is obtained for unsaturated soil considering absorbed water. Based on the thermodynamical dissipative theory，entropic flow and entropic product expressions for unsaturated soil in the process of adiabatic deformation are derived from the work expression and local thermodynamic equilibrium assumption. The adsorption equation of adsorbed water in soil is achieved by the thermodynamical adsorption equilibrium theory; and a practical soil-water characteristic curve for adsorbed water is further proposed. Employing the thermodynamical conjugates of entropic product expression together with Drucker’s plastic postulate in dissipative space, the soil-water fully coupling elastoplastic theoretical framework of unsaturated soil in consideration of absorbed water is established by virtue of a couple of stress variables named as pure simplified effective stress and effective suction. The flexibility matrix with respect to net stress and suction is obtained from this theoretical framework, which can be used to compare directly theoretical model with experimental results in the application of the above theoretical framework on establishing the practical model.

Key words: adsorbed water, unsaturated soil, work expression, dissipative space, elastic-plastic flexibility matrix