Angle of Dilation
The angle of dilation controls an amount of plastic volumetric strain developed during plastic shearing and is assumed constant during plastic yielding. The value of ψ = 0 corresponds to the volume preserving deformation while in shear.
Clays (regardless of overconsolidated layers) are characterized by a very low amount of dilation (ψ ≈ 0). As for sands, the angle of dilation depends on the angle of internal friction. For non-cohesive soils (sand, gravel) with the angle of internal friction φ > 30° the value of dilation angle can be estimated as ψ = φ - 30°. A negative value of dilation angle is acceptable only for rather loose sands. In most cases, however, the assumption of ψ = 0 can be adopted.