The physicochemical interactions between different phases of the soil have an important impact on the geotechnical engineering related to environment and energy. The principle of effective stress is always the most important and effective theory to solve the relevant problems. However, the change of inter-particle stress caused by physicochemical effects cannot be accurately described by current effective stress equation. In response to the above problems, a mean intergranular stress expression that uniformly describe capillary, adsorption and osmosis effects between soil particles has recently been proposed. The purpose of this research is to realize the quantitative calculation of the mean intergranular stress and verify its stability and effectiveness. Firstly, the physical meaning of each part in the expression of mean intergranular stress is analyzed. Through analyzing the parameter in the expression, the variation rule of each part of mean intergranular stress with the water content and the concentration of pore water are obtained. Then, the formula characterizing the solid-liquid interface interaction for the surface force potential is derived, which can be used for the quantitative calculation of the mean intergranular stress equation. Finally, the mean intergranular stress of unsaturated soil in the critical state is calculated, and applied to simulate the coupled chemical-mechanical loading test of unsaturated soil. The calculated results show that there is a unique relationship between mean intergranular stress and shear strength in the critical state, and the chemical-mechanical calculation results give good agreement with the experimental results, verifying the stability and effectiveness of the mean intergranular stress.