Thermal effect on consolidation and strain–stress behaviour of bentonite–sand mixtures with different initial states and clay contents

Abstract

Abstract Understanding the thermo-mechanical behaviour of clay–sand mixtures is critical for the design and long-term performance evaluation of geotechnical and geoenvironmental infrastructures exposed to elevated temperatures. This study systematically investigates the mechanical and microstructural responses of binary mixtures with varying bentonite contents (25%, 50%, 75%, and 100%) and different initial states under isothermal conditions (20, 40, and 60 °C) by temperature-controlled multi-stage oedometer tests. Upon initial heating, non-preconsolidated mixtures exhibit pronounced thermal contraction, with the magnitude of volumetric strain increasing with bentonite content, while preconsolidated specimens display thermal expansion. For preconsolidated specimens, the normal compression lines (NCLs) at different temperatures remain parallel, consistent with previous findings. Interestingly, NCLs of non-preconsolidated mixtures at different temperatures are non-parallel and intersect at a transitional stress, resulting in a temperature-dependent increase in compressibility, particularly for mixtures with higher bentonite fractions. The proposed equations incorporating the combined effects of temperature and bentonite content can accurately predict compression and swelling indices of bentonite–sand mixtures. The creep coefficient of binary mixtures is positively correlated with the bentonite content. Meanwhile, three-dimensional C αe surface with the consideration of bentonite content and vertical stress is plotted and shifts up with the increasing temperature. Scanning electron microscope (SEM) observations corroborate the macro-scale findings, showing that elevated temperatures densify the microstructure of non-preconsolidated mixtures but disrupt the structure of preconsolidated specimens. These findings highlight the pivotal role of initial state and bentonite content in governing the thermo-mechanical and time-dependent behaviour of bentonite–sand mixtures.