A hill material is defined as a granular assembly in which the hill contact model exists at all grain-grain contacts, and this material behaves like an unsaturated granular material.
This implementation of the hill contact model follows that of Tan and co-workers (Tan et al., 2014), who refer to the synthetic material as a computational framework that “. . . has the potential to model a wide range of granular bases explicitly taking into account the coarse particle size distribution as well as the effects of moisture and fine particles.7 The coarse particle size distribution is represented explicitly through the [discrete element] DEM particles. The fine particle size distribution and moisture are represented implicitly via the force model between the coarser DEM particles.” Tan and co-workers suggest that this DEM approach “. . . offers a viable alternative to continuum models in which the form of the bulk moduli must be empirically determined. In contrast,the calibrated DEM model represents coarse particle movement distinctly and captures . . . complex trends associated with particle size distribution and moisture content measured in experiments.”
The hill contact model may exist only at a grain-grain contact. The grain-grain system behaves like two locally elastic spheres that may have a liquid bridge (see Figure below). The liquid bridge is present if the moisture state is wet, and absent if the moisture state is dry. The hill contact model provides the behavior of an infinitesimal, nonlinear elastic (no tension) and frictional interface that carries a compressive surface-interaction force and may carry a tensile moisture force. The contact force is the sum of the surface-interaction and moisture force, and the contact moment is zero.
A pavement-design package that supports creation and triaxial testing of the hill material containing geogrid is also available.
Potyondy, D. (2019) "Pavement-Design Package for PFC3D [pdPkg6.1]," Itasca Consulting Group, Inc., Technical Memorandum ICG16-8528-15TM (April 17, 2019), Minneapolis, Minnesota.