Soil Thermal Characteristics in Relation to Underground Power Cables AIEE Committee Report

Soil moisture exists simultaneously in a variety of states, ranging from the solid, through plastic and liquid, to the gaseous state. Its condition and characteristics in each state are determined by the interplay of the properties of the soil solids and the water molecules. The two categories of soil water of most interest to the cable engineer, because of their direct and indirect influences on soil thermal conductivity, are hygroscopic, or adsorbed, water and capillary water. Hygroscopic water is spread over the surfaces of the individual soil particles, where it is held very tightly. It increases the contact area between the individual soil particles enough to provide paths for the flow of heat, thus being very efficient in decreasing the over-all thermal resistivity of the soil below what it would be in the dry state. Hygroscopic water contributes to the swelling and shrinking of montmorillonite clay soils. A very small increase in moisture content above the hygroscopic coefficient is sufficient to provide the lubrication required for the most effective compaction of clay soils. Capillary water provides an additional increase in conductivity, but its effectiveness in this respect is comparatively less because it can only increase the area of heat-flow paths already established by the hygroscopic water.