Abstract:
Expansive (or swelling) clays are the most prolific problem soil across Southern Africa and many
parts of the world. Large volume changes due to seasonal wetting and drying cause millions of dollars’
worth of damage to engineering infrastructure yearly. Soil-water retention behaviour is dependent on
density, and determining the correct retention curve may be key for reliable design of infrastructure. Soilwater
retention curves and shrinkage curves were measured for recompacted samples of a highly expansive
bentonite clay from South Africa. Four samples were prepared at varying initial dry densities between
approximately 1200 and 1500 kg/m3 and subjected to total suction testing in a dewpoint hygrometer. The
water content of each sample was varied through wetting and drying. The sample volume was measured
after each suction reading, allowing relationships between suction and void ratio, degree of saturation and
water content to be determined. The relationship between gravimetric water content and suction was not
significantly influenced by initial density. Degree of saturation at a given suction varied significantly
according to initial density. Volume reductions of 25% to 36% from saturated to residual conditions were
recorded, and samples tended to a residual void ratio of 0.35 to 0.4. The swelling clay showed propensity
for maintaining high suctions (over 300 MPa at residual conditions) and hysteretic response between
primary drying and wetting.
