Relating Plastic Potential Function to Experimentally Obtained Dilatancy Observations for Geomaterials with a Confinement-Dependent Dilation Angle

Dilatancy (volumetric expansion) during yielding is an important aspect of geomaterial behavior that can be mathematically defined using a dilation model. A common way of modeling dilation has been to use a dilation angle, ψ, which is derived from the Mohr-Coulomb potential function assuming ψ is a constant. Theoretically, the constant ψ value is directly related to a ratio of plastic strain increments using an equation. Several researchers have found, however, that the ratio of plastic strain increments as measured in laboratory tests both changes during the deformation process and depends on the confining stress within a material, meaning that in general, ψ cannot be constant. In this study, we demonstrate the origin of this inconsistency and derive the relation between the dilation angle and the ratio of plastic strain increments acknowledging confinement dependency of ψ. Then, we show how an iterative scheme can be used to calculate the confinement-dependent dilation angle values from a series of triaxial tests. The dilation angle values calculated using the novel approach are found to be consistently higher than those obtained using the conventional approach that assumes a constant ψ as part of the calculation.