Pore pressure coefficient in frozen soils

The Skempton pore pressure coefficient, B, is defined as the variation in pore pressure with the unit change in confining pressure under undrained conditions. The B parameter is an essential parameter to consider the coupled effects of solid–fluid compressibility and skeleton compressibility in a porous system. It is a key factor in exploring a possible definition of effective stress in frozen soil. However, limited experimental and theoretical research is available in the literature to give insight to the problem. Therefore, a series of B tests on frozen clay was conducted in this study. Results from these tests, along with tests on Ottawa sand, as available in the literature, are analysed considering the effect of the ice crystallisation mode on the skeleton stiffness. The measured B values were lower than expected compared with B values using models that consider single grain bulk stiffness. However, when the difference in bulk stiffness of ice and that of soil grains is considered, even an increase in pore volume, for an increase in fluid pressure, at constant Terzaghi effective stress is possible. The ‘pore stiffness’, which is different from the solid phase stiffness, can take a negative value and can be used to explain the low measured B values.