Experimental Investigation of Thermal Volumetric Changes in Clays: Unveiling Hidden Controls

Understanding the thermally induced volumetric behavior of fine-grained soils is central to designing resilient geo-energy infrastructures, including geothermal systems, underground storage facilities, and deep repositories for radioactive waste. In this study, we investigate the coupled thermal-mechanical response of silty clays from Budapest, Hungary, characterized by contrasting plasticity and stress histories. Drained heating and heating–cooling cycle tests were performed in a custom-built, temperature controlled oedometer on both low-plasticity (LP) and high-plasticity (HP) samples. Temperatures ranged from 20°C to 90°C under variable over-consolidation ratios (OCR = 1 to 22). Results confirm that normally consolidated clays exhibit consistent plastic contraction during thermal loading, whereas highly over-consolidated clays reveal a more nuanced response, largely governed by recent stress history. Significantly, reloading events led to contraction in samples where a small elastic expansion might otherwise be expected. High-plasticity samples consistently showed greater thermal strain magnitudes, underscoring the importance of the plasticity index in controlling volume change. Repeated heating–cooling cycles induced mostly plastic strain during the first cycle, transitioning toward elastic behavior and stabilizing volumetric changes by the fourth. These findings offer deeper insight into the mechanics of thermally affected clay deposits, with direct relevance to geological risk assessments for geothermal fields and to the engineered barriers of nuclear waste repositories. By highlighting the pivotal roles of OCR, plasticity index, and stress history, this research contributes to more accurate predictive models and more robust designs for thermally active subsurface infrastructures.