Pore Water Pressures in Unsaturated Soils

A specially designed pressure plate apparatus was used to measure the residual negative pore water pressures in compacted samples of five different clays. Samples were prepared using both static and kneading compaction. Pore water pressures at the optimum water contents varied from –4 psi to –80 psi depending on the specific surface of the soil. Pore water pressures as negative as -250 psi were measured in samples compacted only 6% dry of the optimum water content. Theoretical analysis indicated that the actual pore water pressures in unsaturated soils are unlikely to become more negative than approximately –14 psi although theoretical studies in terms of vapor pressures indicate pore water pressures as negative as –140,000 psi. It was demonstrated that the actual pore water pressures in homogeneous clays at equilibrium vary between wide limits and cannot be measured with any known technique. The requirement for equilibrium of the pore water is constant head, not constant pressure; the measured pore water pressures are those in the measuring system that are applied to reduce the head in the measuring system to the head of the pore water in the soil.