Expansive clays, collapsing sands, unbound pavement materials and unsaturated soil theory

The importance of soil suction on the behaviour of unsaturated geomaterials is widely recognized. Unsaturated soil theory relies heavily on Soil Water Characteristic Curves (SWCC), which can be established experimentally. The primary use of an SWCC is to convert moisture content, which is easily determined, to matric suction. Theoretical prediction of volume change of expansive soils remains difficult, however. Expansive clays cause damage to light structures. Movement can be estimated if suction-depth profiles are established and the soil profile is understood. Simple tests for soil reactivity have proven useful for calculating ground movements. The zone of influence of tree root systems on clay soils has been inferred from total suction-depth profiles and subsequently used to prepare guidelines for footing design for drying caused by trees. Suction can be an important bond, helping to hold collapsing sandy soils in a loose state. However, wetting reduces suction and other bonding agents, and so collapse follows, usually with the assistance of external loading. The suction of compacted unbound granular material is quite low relative to clays, yet can be important and has been shown to significantly influence properties important to pavement design. In this paper the author's general experience with soil suction and SWCCs is related.