Pore pressure parameters for unsaturated soils

Analytical expressions are presented for the determination of the pore air and pore water pressure parameters in unsaturated soils under undrained conditions. The kinematic constraints governing the volumetric alterations of the soil skeleton and fluid phases, in conjunction with the effective stress principle, are utilised to derive the pore pressure parameters. The effect of hydraulic hysteresis on the effective stress parameter and the soil water characteristic curve is considered in the solutions. The expressions derived are validated against the experimental data available in the existing literature. It is demonstrated that: − with an increase in the degree of saturation of the soil, both pore pressure parameters increase and approach unity; − the change in the pore water pressure in an unsaturated soil under undrained compression is greater than the change in the pore air pressure; − the pore pressure parameters are strongly dependent on the compressibility of the soil skeleton; − the hydraulic hysteresis has a substantial influence on the pore water pressure parameter; − the pore air pressure parameter is slightly affected by the hydraulic hysteresis and it is mostly influenced by the compressibility of the air phase; − an increase in the degree of saturation leads to a reduction in the instantaneous settlement; and − the hydraulic state has a significant impact on the instantaneous settlement of unsaturated soil.