Definition - What does Inert material mean?
Inert material is any material that doesn't cause or promote any change. In the trenchless world, stones and gravel are considered inert material. They play an important part in soil resistivity – how well the soil can conduct electricity – which influences the corrosive nature of the soil. As the amount of inert material in increases so does soil resistivity. As soil resistivity increases, the corrosive nature of the soil for certain types of pipe decreases.
Trenchlesspedia explains Inert material
The measure for resistivity is ohms-centimeters. Soil resistivity (SR) of 5000 ohm-cm indicates soil offers little potential for corrosion. SR in the range of 3000 or less generally requires some kind of corrosive protection for pipes, depending on how high or how low the soil pH is. Stones and gravel contribute to the granular nature of soil which means increased space between particles and a reduction in soil cohesiveness and density, breaking up the conductive path within the soil and lessening the likelihood pipes will need cathodic protection.