To model the influence of an additional substance surrounding the conductors of a buried earthing system on grid resistance, touch and step voltages.
Treat the actual buried conductor and surrounding substance combined as a buried conductor of an equivalent (calculated) radius.
Approximate analytical equation
Model in SafeGrid under Design Settings
 Kurtovic, M. and Vujevic, S. “Earthing grid parameters with conductor surrounded by an additional substance.”, IEEE Transactions, 2000