Thermal-mechanical modelling of rock response and damage evolution during excavation in prestressed geothermal deposits

Rock damage induced by underground excavation would cause irreversible degradation in the hydro-mechanical performance of the engineering rock mass depending on the excavation method, in-situ stress, and change of ambient conditions. As exploitation of resources continues being operated at hostile depths of the Earth's lithosphere, adaptive excavation design thus becomes imperative for ensuring sustainable mining development in the increasingly challenging environment. In this study, a new thermal-mechanical framework is developed to characterize the successive blast loading, transient unloading, and air-cooling sequence for dynamic excavation in prestressed geothermal deposits. To capture the complex rock behavior across vastly different timescales, a new diagnostic tool is proposed for identifying the exact source, nature, and range of excavation damage based on the stress projection technique. By scrutinizing the rock response throughout the excavation process via loading path evolution, novel insights are obtained in this study into the underlying mechanism and relative contribution of each sub-loading process in damage generation under different geopressure, geothermal, and excavation conditions. Our calculations demonstrate quantitatively that the release of in-situ stress and air-cooling of the working environment constitute the predominant causal mechanisms of rock damage for major deep excavation projects. Furthermore, our analysis has shown explicitly that the perturbation to the redistributed stress field by tunnel ventilation could stimulate rock creeping and thus give rise to stability concern for excavations buried in highly stressed deposits of intense geothermal anomalies. The findings of our study could refine the understanding of the complex excavation response and thus contribute to improved adaptive design for excavation work in hostile geological environments.