Thermally induced volumetric response of natural clays: effects of OCR, plasticity, and recent stress history

Abstract

Abstract Understanding the thermally induced volumetric response of fine-grained soils is important for geo-energy systems, underground storage facilities, and radioactive waste repositories. This study examines the thermo-mechanical response of two natural silty clays from Budapest, Hungary, with different plasticity levels and stress histories. Slow-heating tests under drainage-promoting boundary conditions and heating–cooling cycle tests were performed using a temperature-controlled oedometer on low-plasticity (LP) and high-plasticity (HP) samples. The specimens were heated from $$25~^\circ \textrm{C}$$ to $$85~^\circ \textrm{C}$$ under different over-consolidation ratios, ranging from OCR = 1 to 22. The results show that normally consolidated samples contract during slow heating. Overconsolidated samples show a more variable response, indicating that OCR alone is not sufficient to describe the thermal volumetric behaviour. The HP samples generally developed larger thermal volumetric strains than the LP samples, showing the influence of plasticity on the magnitude of thermal deformation. The low-stress naturally overconsolidated specimens, LP12 and HP22, showed mainly contractive behaviour, although their response should be interpreted with caution because their OCR values were inferred from apparent preconsolidation pressures obtained from separate room-temperature oedometer tests. During repeated heating–cooling cycles, most irreversible volumetric strain developed during the first cycle, while later cycles showed smaller strain increments and a trend towards more recoverable behaviour. The tests were not independently verified as fully drained by pore-pressure measurements; therefore, the results are interpreted as laboratory-scale evidence under drainage-promoting conditions rather than as proof of a unique drained mechanism.