Transient Plane and Line Source Methods for Soil Thermal Conductivity

Experiments were conducted to compare two sensing techniques for measuring thermal conductivity of unsaturated soils: (i) a modified transient plane source (MTPS) method for non-destructive measurements using a planar, interfacial heat reflectance sensor, and (ii) a transient line source (TLS-SP) method utilizing an embedded single-probe heat source. Measurement protocols for coarse-grained and fine-grained soils were developed. Thermal conductivity dry-out curves (TCDCs) were measured for five soil types, including poorly graded sand, well-graded sand with silt, silty sand, silt, and clay. The MTPS sensor consistently produced higher thermal conductivity for degrees of saturation greater than about 50 % but lower thermal conductivity for saturations less than 50 %. Saturated thermal conductivity measured using the MTPS sensor ranged from 8 % to 26 % greater than values measured using the TLS-SP sensor. Dry thermal conductivity measurements were comparable (<5 % difference) for fine-grained soils but were consistently and appreciably greater using the TLS-SP for coarse-grained soils. Mechanisms responsible for these differences include thermally induced water migration, latent heat transfer, sensor-soil contact resistance, gravity-induced water migration, and specimen heterogeneity. Secondary experiments indicated that the effects of gravity-induced water migration were insignificant within the short (<5 min) time frame elapsed between sample preparation and measurement.