Frequency Dependent Resistivity of Soil, Bentonite and Soil-Bentonite Mixes: With Special Attention to Electrical Grounding Systems

Frequency dependent resistivity of a selected type of soil, bentonite and their mixes by weight have been presented. The main objective is to provide the much-needed information for the industrial usage of bentonite-based backfilling materials in conjunction with electrical grounding systems. As expected the resistivity decreases with both the moisture content and frequency. However, for all materials, the decrement in resistivity with moisture content (by weight) at a given frequency is more prominent than that with frequency at a given moisture level. At a given frequency, the percentage drop-in resistivity of sand between 0 % (dry material) and 10% of moisture content is above 99%. At a given moisture content, the resistivity of soil shows a marked reduction from 0 Hz (steady state) to about 0.6 kHz. The resistivity remains at steady value from 0.6 kHz to about 1 kHz and then varies in a slow downward ramp. It attains rather insignificant variation after about 10 kHz and level of around 100 kHz. Bentonite and soil-bentonite mixes also show similar variation of resistivity. However, the absolute resistivity of bentonite is much less than that of soil at all frequencies and moisture contents. Mixing of bentonite, even in relatively small quantities, such as 25%, could achieve appreciable reduction in resistivity of backfilling materials in practical electrical earthing systems.