Equations for Predicting the Volumetric Water Content affecting the Resistivity of Soil in the Vicinity of a Driven Earth Electrode in Variably Saturated Homogeneous Soil

Abstract

Using previous measurements as a reference, correction factors are used to analytically estimate seasonal changes in the soil resistivity and seasonal changes in the resistance of a driven-rod earth electrode. These correction factors are based on weather or seasonal data obtained from meteorological stations in the form of rainfall and soil frost depth. When applying this weather data, it is necessary to add their effect on different soil compositions, textures, and site topographies and consider the water infiltration, water runoff, or water retention for each earth electrode site. Site topography influences water runoff and infiltration, while soil texture and structure influence its retention or release. The water content and the corresponding effect on soil resistivity can be determined using the hydraulic properties of soil (water permeability and water retention characteristics). The resistivity of a finite volume of soil around the earth electrode and the changes within this volume dominate the resistance of the earth electrode. This volume is called the Region of Influence (ROI). As soil suction or soil water pressure head changes, the saturated and unsaturated portions within the ROI change, suggesting changes to the volumetric water content and the corresponding soil resistivity. The cumulative effect of weather on soil can be expressed in terms of soil suction. The unique Soil Water Characteristic Curve (SWCC) for each soil texture may be used to evaluate the degree of saturation or water content within the ROI for any soil suction. Based on the preceding deductions, this paper presents analytical formulas for estimating the resistance of driven-rod earth electrodes in variably saturated homogeneous soil. Although not illustrated in this paper, when compared with the simulation result for a driven rod in silty soil modelled in COMSOL multi-physics, the values predicted with these formulas were found to be within 10% for volumetric water content, soil resistivity, and resistance of the earth electrodes over a range of soil suctions. This approach can potentially improve the prediction capability of existing weather and seasonal-based techniques.