A hypoplastic model for sand
The implemented model has been published by von Wolffersdorff (1996). The implementation also contains Niemunis and Herle (1997) extension for predicting small strain stiffness effects.
Niemunis A and Herle I. (1997). Hypoplastic model for cohesionless soils with elastic strain range. Mechanics of Cohesive-Frictional Materials 2, 279-299.
von Wolffersdorff, P. A. (1996). A hypoplastic relation for granular materials with a predefined limit state surface. Mechanics of Cohesive-Frictional Materials 1, No. 3, 251-271.