Error analysis of the thermal cell for soil thermal conductivity measurement

Soil thermal conductivity is an important factor in the design of energy foundations and other ground heat exchanger systems. Laboratory tests in a thermal cell are often used to determine the thermal conductivity of soil specimens. Two interpretation methods have been suggested. Analysis can be based on the assumption of one-directional heat flow and the thermal conductivity calculated using Fourier's law. Alternatively the lumped capacitance method can be employed, using results generated as a specimen cools. In this study, six samples of London Clay were tested using a thermal cell. A finite-element model of the tests was then used to determine the validity of the assumptions made in analysis. The model showed substantial heat loss through the sides of the specimens, which would have a significant impact on the calculated thermal conductivity. The conditions required for the lumped capacitance method to be valid were also found not to be met. Consequently neither analysis method is recommended. A better approach would be to pursue apparatus with fewer heat losses or transient testing techniques.