Laboratory Investigations on Factors Affecting Soil Electrical Resistivity and the Measurement

Soil electrical resistivity is an important parameter in grounding system design. In this paper, laboratory measurements of soil electrical resistivity are carried out using a soil box. Two measurement methods are used and compared, namely, a two-electrode method and a four-electrode method. The measurements indicate that the results reported by the two-electrode method are affected by the contact resistance between the electrodes and the soil, which increases as the soil water content increases. Moreover, the soil electrical resistivity was observed to decrease as the test signal frequency increases. Using the four-electrode method and a 50-Hz ac signal, the effects of soil water content, soil porosity, pore fluid composition, and temperature on the soil electrical resistivity are then investigated. The results show that increasing the soil saturation level results in a power function decrease in the soil electrical resistivity. For a given gravimetric water content, as the soil porosity decreases, the resistivity decreases. In addition, owing to the mobility of the ions, different electrolytes in the pore fluid contribute differently to the soil electrical resistivity. Finally, the dependence of soil electrical resistivity on soil temperature is found to be divided into three stages: above 0 °C, around 0 °C, and below 0 °C. An abrupt change in the soil electrical resistivity is observed at around 0 °C.