​Soils are particulate materials. Grain size determines the balance between particle-level forces (skeletal, weight, van der Walls attraction, double layer repulsion, capillary and viscous drag). Grain size distribution establishes the range of possible particle packing configurations. And, particle shape (i.e., sphericity, angularity and roughness) defines particle interactions, such as slippage vs. rotation, and the development of both inherent and stress-induced anisotropy. Therefore, macro-scale behavior is determined by particle-level characteristics.