Bentonite is a moisture retaining clay used as an earth electrode back-fill to help lower soil resistivity. The Bentonite clay is a sodium activated montmorillonite which when mixed with water swells to many times its original volume. If underground power cables are required to run at maximum direct burial rating when installed in cable duct, then the cable duct should be filled with Bentonite mixture (100 parts of 10:1 Bentonite / water mix to 20 parts sand and 8 parts cement). The Bentonite mixture should be pumped into the cable duct so totally excluding the air and on completion of the filling be sealed to prevent any escape of Bentonite mixture.