The slow dynamics of granular flow is studied as an extension of static granular problems, which, as a consequence of shaking or related regimes, can be studied by the methods of statistical mechanics. For packed (i.e. ‘jammed’), hard and rough objects, kinetic energy is a minor and ignorable quantity, as is strain. Hence, in the static case, the stress equations need supplementing by ‘missing equations’ depending solely on configurations. These are in the literature; this paper extends the equilibrium studies to slow dynamics, claiming that the strain rate (which is a consequence of flow, not of elastic strain) takes the place of stress, and as before, the analogue of Stokes's equation has to be supplemented by new ‘missing equations’ which are derived and which depend only on configurations.
Brujic, J., Edwards, S. F., & Grinev, D. V. (2003). Jammed systems in slow flow need a new statistical mechanics. Philosophical Transactions of the Royal Society A, 361(1805), 741-751. https://doi.org/10.1098/rsta.2002.1162