Volume changes of an unsaturated clay during heating and cooling

Thermally induced soil volume changes can have significant influence on many geotechnical structures. So far, research on thermal volume changes of unsaturated soil is very limited, particularly at temperatures lower than typical room temperature (23°C). The principal objective of this study is to investigate the volumetric behaviour of normally consolidated intact and recompacted low-plasticity clay specimens (loess soil) over a wide thermal cycle ranging from 5 to 53°C using a modified double-cell triaxial apparatus. It is found that contractive volumetric strain increases as the temperature increases. During the cooling process, soil volume keeps contracting until the temperature decreases to 5°C. Different from previous studies in the literature on saturated remoulded illite and natural silty clay, a plastic contraction at a much higher rate is observed from 13 to 5°C for both recompacted and intact loess specimens. The plastic volume changes during cooling are probably because cooling-induced contraction of soil particles leads to particle rearrangements in loess. Moreover, the cooling-induced plastic volume change, which only occurs when the cooling temperature is less than a critical value (13°C for the test conditions considered in this study), cannot be captured by the existing thermo-mechanical models, which predict elastic contraction during cooling. A new yield surface (temperature decrease) is proposed to simulate the observed elastoplastic behaviour during cooling.