Compressibility of pore fluid of unsaturated soils and pore pressure parameter

In engineering, the pore pressure parameter is a useful factor. However, the expression of the compressibility coefficient of pore fluid, which is needed in calculating the parameter, is still not acceptable. Based on the pore water pressure, the definition of compressibility coefficient is proposed. The difference between pore air pressure and pore water pressure is considered according to the effect of surface tension. In order to simply the expression, introduced are two parameters: parameter T_R which deals with the influence of surface tension; parameter Φ which is used to reflect the status of pore air. The expression of compressibility coefficient derived can be extended to the application on unsaturated soils with lower degree of saturation. Then, the pore pressure parameters for K₀ loading and isotropic loading are presented. Finally, the formula is validated with the experimental results from field and laboratory tests reported in the literatures, and the agreement is good. The pore pressure parameter is not constant during loading, as should be indicated.