Thermal properties of boring core samples from the Kanto area, Japan: Development of predictive models for thermal conductivity and diffusivity

The subsurface of the Earth is facing evermore thermal impact due to global warming, urban heat islands, and the widespread use of ground source heat pump (GSHP) systems. This potentially causes changes in its physical, mechanical, microbiological, and chemical properties, and in the subsurface water quality. To predict and evaluate this thermal impact (or thermal pollution), a better understanding and improved models for the thermal properties governing heat transport in subsurface sediments are needed. Also, data acquisition in high spatial resolution for the thermal properties and basic physical properties of the subsurface sediments are essential. In this study, the main thermal properties (the thermal conductivity, heat capacity, and thermal diffusivity) together with the basic physical properties (the soil texture, water content, and dry bulk density) were measured on boring core samples representing depths from 0 to 50 or 80 m, at three study sites in the Kanto area of Japan. Based on the measured data, models for thermal conductivity as functions of gravimetric water content, dry bulk density, and volumetric sand content were developed. The new models performed markedly better than presently available models from the literature and, in combination with a modified de Vries type model for heat capacity, the resulting model for thermal diffusivity was capable of describing the measured data well. The usefulness of the newly developed models were validated and illustrated by using data from a two-day thermal response test (TRT) performed at one of the three study sites. The new predictive models for the thermal properties used in a numerical heat transport simulation accurately predicted subsurface (5–50 m) temperature changes during the TRT.