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Dilatant is a term used in geology to describe a type of soil or sediment that exhibits a characteristic property when subjected to stress, such as wetting or drying. Specifically, it refers to the increase in the resistance to plastic deformation of a material as the rate of strain increases, rather than decreasing as would be expected in normal, viscous materials.

This phenomenon was first observed by Hughes in 1927, who noticed that certain soils exhibited an increase in resistance to deformation when wetted, rather than a decrease. Further research has shown that this is due to the interaction of particles in the material, where the presence of water causes an increase in the electrostatic forces between particles, leading to a more rigid and less deformable state.

Dilatant behavior is observed in some clay-rich soils, such as those found in the coastal regions of Australia. In these areas, the soil can exhibit a dramatic increase in strength when wetted, making it highly resistant to deformation. This can have significant implications for engineering and geotechnical applications, particularly in the design of foundation structures and the assessment of soil mechanics.

Studies have also shown that dilatant behavior can be influenced by factors such as particle shape, size, and chemistry, as well as the presence of other materials, such as organic matter or salts. Further research is needed to fully understand the mechanisms behind dilatant behavior and its implications for a range of engineering and geotechnical applications.