dilatancy

Dilatancy is the volume change that occurs in a granular material, such as soil or sand, when it is sheared. The term, dating to Osborne Reynolds (Reynolds' dilatancy), describes how dense assemblies must expand in volume to allow particles to rearrange and slide past one another.

Dense granular packings tend to dilate under shear: particles must open up additional void space as they move, causing a measurable increase in total volume. In looser packings, or under certain loading rates, the material may contract as particles rearrange into more compact configurations. The magnitude and sign of dilatancy depend on factors such as packing density, grain shape and size, moisture content, and the prevailing stress state.

In geotechnical engineering, dilatancy has practical consequences for shear strength and pore-pressure response. For saturated soils, dilation during undrained shear can raise pore water pressure, reduce effective stress, and increase the risk of liquefaction during earthquakes. Under drained conditions, dilation can modify shear resistance as pore pressures are able to dissipate. Dilatancy effects are particularly pronounced in sands and gravels and are less evident in many clays, where fabric and bonding control deformation differently.

In constitutive modeling of soils and granular materials, dilatancy is represented by concepts such as the dilatancy angle or a plastic potential, linking volume change to shear deformation. It is a key ingredient of critical-state soil mechanics, where the material may reach a constant volume for a given density and moisture content under continued shear.