Dewatering and the hydraulic properties of soft, sulfidic, coastal clay soils

Dewatering and consolidation of saturated swelling soils are governed by pressure-dependent soil hydraulic properties. Existing measurement techniques are difficult and slow. We illustrate a simple, rapid desorption technique, developed for industrial slurries, to measure hydraulic properties of a gel-like sulfidic, estuarine soil (∼70% water content). Measured hydraulic conductivities, K(ψ), were very small, ∼10^-10 m/s, giving a representative capillary fringe thickness of ∼7 m and characteristic gravity drainage times around 40 years. Capillarity therefore dominates flow in these soils. Estimated times for dewatering this soil under surface loading with closely spaced, vertical wick drains, are 2 to 70 years, consistent with experience. A Netherlands marine clay soil, saturated with seawater, is unexpectedly wetter than the brackish estuarine soil here at the same matric potential, ψ. However, K(ψ) for both soils overlap, suggesting the engineering approximation, K(ψ) ∝ |ψ|^-1, for marine-deposited clays. The functional dependencies of hydraulic properties surprisingly are not consistent with similar-media or double-layer theories.