Temperature and pressure determine the state of matter. The three standard states exhibit very different geometric and density characteristics: solids have their own shape and density; liquids keep their density, but acquire the geometry of the container; and gases have neither a fixed geometry nor density, so they can fill a container of any size and shape. Soils, and all particulates in general, behave like no other material at any phase condition. A soil mass responds as a solid when confined, flows like a liquid on a ramp, and may be stable in a wide range of possible densities (Savage, 1994; Buchanan, 2003; Corwin et al., 2005; van Hecke, 2005). Therefore it is not possible to define a characteristic density for a soil (Buchanan, 2003; Umbanhowar, 2003). Volume change in soils is the result of various particle-level mechanisms. Contact deformation prevails in smallstrain deformation. This is a constant-fabric process, and the terminal density will reflect the initial fabric.