Verma-Pruess equation [43].
k = k_0 \left( \frac{\phi - \phi_c}{\phi_0 - \phi_c} \right)^n,
where k is the permeability, k_0 is the initial permeability, \phi is the porosity, \phi_0 is the initial porosity, \phi_c is the critical porosity, and n is the exponent.
No additional info.
Used in no end-to-end test cases.