In practice, most of engineering projects are embedded in the undisturbed soils. Finally, the wet samples are statically or dynamically compacted to a desired dry density. Subsequently the soil powder is mixed well with required water to obtain a certain water content and let stand for a few days for moisture homogenization. For such compacted samples, the soil samples are usually air-dried, crushed, sieved through a 2 mm mesh. However, most of the previous studies were carried out on the thermal properties (e.g., thermal conductivity) of compacted or repacked samples (i.e., remolded samples). (Abuhamdeh and Reeder, 2000 Xu and Chung, 2000 Liu et al., 2007 Tang et al., 2008 Hiraiwa and Kasubuchi, 2010 Ye et al., 2010 Cui et al., 2011 Barry-Macaulay et al., 2013 Cai et al., 2015 Zhang et al., 2015 Chen et al., 2018 Chen et al., 2019 Xu et al., 2019a). In the past two decades, numerous experimental studies have been devoted to investigating the thermal properties of various soils and their affecting factors such as water content, dry density, particle size, pore water salinity, organic matter, mineralogical composition, temperature, aging time and additives, etc. Specifically, thorough study of soil thermal properties is an essential part for engineering design, such as layout of energy piles and ground heat exchanger (GHE) (Bozis et al., 2011 Raymond, 2018), thermal remediation of contaminated soils (Chang and Yen, 2006 Apul et al., 2016), disposal of high-level radioactive waste (Villar and Lloret, 2004 Chen et al., 2017 Abootalebi and Siemens, 2017 Zhang et al., 2020), heat dissipation of buried high-voltage power cables (Ocłoń et al., 2015 Ocłoń et al., 2016) and appropriate design of liner system in a landfill (Rowe, 2005 Ali et al., 2016). Soil thermal properties govern the heat transfer or storage process of underground space, where many thermal engineering projects are embedded in. The hysteresis effects on soil thermal conductivity can be explained by the ink-bottle effect, effects of pore size distributions induced by shrinkage and swelling, and differences in the volume of entrapped air between drying and wetting. Test results from the scanning electron microscope and mercury intrusion porosimetry confirmed that compared with compacted specimens, the undisturbed specimens have a relatively uniform pore system that improves the extent or quality of heat transfer paths through the contact points and water bridges, thus have higher thermal conductivity. For the same volumetric water content, thermal conductivity of specimens produced by drying was higher than that by wetting due to hysteresis effects. The specific heat capacity of the undisturbed and compacted specimens was roughly the same. Results revealed that thermal conductivity and diffusivity of undisturbed specimens were higher than that of compacted specimens. To investigate the influences of microstructure alteration on soil thermal properties, thermal conductivity, thermal diffusivity and specific heat capacity were measured for both undisturbed and compacted lateritic clay specimens subjected to drying and wetting with the thermal probe method. Soil thermal properties are essential for investigating the thermal performance of many kinds of underground thermal engineering structure.
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