Coupled effects of stress state and void ratio on thermal conductivity of saturated soils

Soils surrounding energy geostructures (e.g. energy piles and borehole heat exchangers) always have different overburden pressures and void ratios. Their thermal conductivities are important for analysing the performance of energy geostructures such as the heat exchange rate between soils and geostructures. So far, however, the coupled effects of stress state and void ratio on thermal conductivity have not been fully understood. In this study, six series of tests were conducted to measure the thermal conductivity of three different soils at various stress states and void ratios, including reconstituted kaolin clay, compacted silt from Hong Kong and compacted Toyoura sand. The results indicate that when the stress increases from 0 to 1200 kPa, the thermal conductivity increases by 60% for the clay, 25% for the relatively looser silt, 20% for the relatively denser silt, 10% for the relatively looser sand and 7·5% for the relatively denser sand. The increment is attributed to not only a reduction of the void ratio, but also an enhancement in the inter-particle contact. Based on the experimental results, a new equation for thermal conductivity was proposed. It is able to well capture the variation of thermal conductivity with stress and void ratio using a single set of parameters.