Evaluation of soil thermal resistance around underground power transmission line using geophysical exploration methods

Abstract

Design of underground power transmission lines requires simple and effective techniques for estimating soil thermal resistance which is one of the most important engineering properties to determine the transmission capacity of power lines. Thermal resistance, electrical resistivity and seismic velocity vary with water saturation or porosity, respectively. So there is a possibility to estimate thermal resistance by geophysical exploration methods. We have conducted laboratory and field experiments of the thermal and geophysical properties to evaluate the effectiveness of the estimation method of the thermal resistance by geophysical exploration methods. The laboratory experimental results demonstrated that there was a positive correlation between thermal resistance and electrical resistivity. We developed an empirical formula based on soil types linking between the thermal resistance and electrical resistivity. There was a relation between S-wave velocity and soil granular size, which is related to soil types. The field experiments results demonstrated that the distribution of the artificially created soil structure was evaluated by the S-wave velocity distribution. The thermal resistance distribution estimated by the electrical resistivity distribution was consistent with that measured by thermal resistance sensors. We also found that the thermal resistance decreased when water saturation increased. Hereby, it can be concluded that soil thermal resistance distribution can be estimated by the combination survey of the electrical method and the S-wave seismic method.