Characterization of thermal damage evolution of rock specimens and its effect on rock brittleness

Abstract

Understanding thermal damage in rock is crucial for safe deep underground engineering, as elevated temperatures at depth significantly alter the physical and mechanical properties of rock masses. Thermal expansion leads to the initiation and propagation of micro-cracks and mineralogical changes, which in turn reduce compressive strength, tensile strength, and elastic modulus. These effects are critical in applications such as deep mining, geothermal energy, underground coal gasification, and nuclear waste storage. Although the transition from brittle to ductile behavior under thermal loading has been widely studied, the combined effects of high temperatures and pressures (HTPs) on rock brittleness remain insufficiently understood. This knowledge gap potentially limits our understanding of the thermal damage evolution of rock under HTP conditions. In addition, despite the existence of many thermal damage constitutive models, their validity in different contexts (e.g., triaxial tests with multiple temperatures and pressures) has not been carried out yet. This study aims to characterize the damage evolution in rock specimens subjected to heat treatment at different confining pressures and to derive empirical thermal damage constitutive equations. To this end, a critical and comprehensive literature review was conducted (Chapter 2). Next, selected statistical thermo-mechanical damage constitutive models were assessed (Chapter 3). Crack development and mineral transitions of rocks at high temperatures were then examined (Chapter 4), followed by the determination of the thermal damage of rock specimens subjected to heat treatment (Chapter 5). In Chapter 6, a suitable brittleness index of rocks at HTPs was chosen. Finally, the implications of these results for brittle failure of rock masses in deep mines were discussed in Chapter 7. The results indicated that (1) statistical thermo-mechanical constitutive models based on the Weibull distribution and multiple rock failure criteria yield stress-strain predictions closely matching experimental data, (2) P-wave velocity, porosity, uniaxial compressive strength, and tensile strength are key parameters influencing thermal damage, and (3) the best brittleness index capturing thermo-mechanical changes under HTPs was chosen. The effect of thermal damage evolution on brittleness is demonstrated by a brittleness index based on stress-strain curves. The original contribution of this thesis is that the selected index is sensitive to thermal and mechanical degradation mechanisms, including the development of microcracks and mineralogical changes, which directly influence the post-peak behavior of rocks. However, further research could investigate (1) direct measurement of residual stress and strain that could improve the proposed brittleness index under HTPs and (2) the application of different failure criteria can be used to develop statistical thermo-mechanical damage models for rocks. Overall, this work combines published experimental data with microstructural evidence and analytical study to determine the effect of HTPs on the thermal damage and brittleness of rock masses. The findings could advance theoretical understanding of thermal damage evolution and provide a predictive framework with direct applications in deep mining, geothermal extraction, and nuclear waste disposal.