Measuring Soil Thermal Stability with a Line Heat Source.

Underground heat sources, such as buried power cables, cause drying of the soil around the source.  Water evaporates from the warmer soil and condenses in cooler locations.  The drying of the soil around the heat source results in a matric potential gradient toward the cable, and liquid water moves back to the cable due to this gradient.  If the hydraulic conductivity of the soil is large enough so that the liquid return flow matches the vapor distillation from the heat source the soil is said to be thermally stable.  If the liquid return flow is too slow, the soil will dry out around the heat source.  Such a soil is said to be thermally unstable, since decreasing water potential leads to decreasing thermal conductivity, which results in increasing temperature gradients and increased vapor flow.  A thermal runaway condition ensues where the soil around the heat source dries completely.  A field assessment of the thermal stability of a soil would be useful for obvious reasons.  We constructed a needle probe which is heated at rates comparable to heat dissipation rates of buried cables.  Its behavior under heating in soil indicates the thermal stability of the soil.  A plot of temperature vs. log time is a straight line in a stable soil.  In an unstable soil the slope of the line increases as the soil dries.  The time for drying can be determined from the plot and used to predict the behavior of the soil with a buried cable.  The thermal conductivity of the soil, both dry and wet, can be determined from the slopes of the lines.  The apparatus will be demonstrated and results of the measurements shown.