Hydrology of swelling soils: a review

A generally accepted theory of liquid flow in rigid systems has been used in soil science for more than 50 years. Liquid flow in systems that change volume with liquid content is not so well described and remains a major challenge to soil scientists, although its application in chemical and mining engineering and soil mechanics is increasingly accepted. Theory of water flow in swelling soils must satisfy material continuity. It must also account for changes in the gravitational potential energy of the system during swelling and for anisotropic stresses that constrain the soil laterally but permit vertical movement. A macroscopic and phenomenological analysis based on material balance and Darcy's law is the most useful first approach to water flow and volume change in such soils. Use of a material coordinate based on the solid distribution results in a flow equation analogous to that L. A. Richards enunciated for non-swelling soils. This framework is strain-independent and solutions to the flow equation exist for a wide range of practically important conditions. The approach has been well tested in clay suspensions and saturated systems such as mine tailings and sediments. It is also applied in soil mechanics. This paper reviews central elements in application of the analysis to swelling soils. It argues that, as with use of the Richards' equation in rigid soils, complexities are evident, but the approach remains the most coherent and profitable to support current need and future research. The use of material coordinates, to ensure material balance is assessed correctly, is simple.