Coupled effects of temperature and suction on the shear behaviour of saturated and unsaturated clayey sand–structure interfaces

The thermo-mechanical behaviour of saturated and unsaturated soil–structure interfaces plays a key role in analysing the performance of energy piles. Previous studies focused on saturated interfaces and did not investigate the coupled effects of temperature and suction on interface behaviour. In this study, a clayey sand–structure interface with a normalised roughness of one was tested through a new temperature- and suction-controlled direct shear apparatus. A variety of temperatures (8, 20 and 42°C), net normal stresses (25, 50, 100, 150, 225 and 300 kPa) and suctions (0, 50 and 200 kPa) were considered. The results show that temperature can have a minor impact on the friction angle, whose value at 42°C is smaller by about 2·2° than that at 8°C, probably because heating can reduce the shearing-induced contraction in the shear zone. More importantly, the interface strength increases non-linearly with increasing suction, and the incremental rate is temperature dependent. Heating the interface at a net normal stress of 50 kPa reduces this incremental rate due to reduction in surface tension and thermally induced changes in soil fabric. In contrast, this incremental rate increases at a net normal stress of 150 kPa with the same temperature increment, probably because the heated specimen has more small pores due to thermal contraction and more menisci water lenses, whose influence outweighs the effects of surface tension.