A generally accepted theory of liquid flow in rigid systems has been used i
n soil science for more than 50 years. Liquid flow in systems that change v
olume with liquid content is not so well described and remains a major chal
lenge to soil scientists, although its application in chemical and mining e
ngineering and soil mechanics is increasingly accepted. Theory of water flo
w in swelling soils must satisfy material continuity. It must also account
for changes in the gravitational potential energy of the system during swel
ling and for anisotropic stresses that constrain the soil laterally but per
mit vertical movement. A macroscopic and phenomenological analysis based on
material balance and Darcy's law is the most useful first approach to wate
r 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-inde
pendent and solutions to the flow equation exist for a wide range of practi
cally important conditions. The approach has been well tested in clay suspe
nsions and saturated systems such as mine tailings and sediments. It is als
o applied in soil mechanics. This paper reviews central elements in applica
tion of the analysis to swelling soils. It argues that, as with use of the
Richards' equation in rigid soils, complexities are evident, but the approa
ch remains the most coherent and profitable to support current need and fut
ure research. The use of material coordinates, to ensure material balance i
s assessed correctly, is simple.