Disorder pressure in dense and flowing granular materials

Dense granular materials with a volume fraction between the random loose and random close packings display several specific features that do not exist at smaller or higher concentrations. When flowing they act as weakly compressible liquids. Their compressibility stems from the many independent configurations the grains can explore when flowing, and is represented by a compaction-dependent ``disorder'' pressure which has a direct influence on many quasi-static flows. As an example, disorder pressure is a necessary ingredient to understand the layer thickness that remains on a rough incline just after flow has stopped.