## 2 Physical and Mechanical Properties of Rock Mass Under THMC Coupling
### THMC Coupling Test of Deep Rocks
experimentally investigated the triaxial compressive rheological properties of greenschist at the foundation of a hydropower station dam, revealing the relationship between axial and lateral strain under varying confining pressures [161]. Two primary approaches for studying crack initiation include experimental and theoretical methods. Extensive experiments have investigated cracking in pre-cracked rock samples under various load conditions. [PERSON] and [PERSON] [162], along with [PERSON] and [PERSON] [163], performed uniaxial compression tests to explore crack initiation and propagation in rock and rock-like specimens, revealing that cracking types largely depend on geometric  
Figure 6: Common fields and coupling relationships of THMC-coupled rock mass [135]. shape and fracture traits. [PERSON] et al. [164] used uniaxial and triaxial compression tests to assess how temperature variations affect the deformation and fracture coalescence in pre-cracked rock samples. Studies by [PERSON] et al. [165; 166] and [PERSON] et al. [167] on crack initiation and propagation in fractured rock under HMC coupling conditions observed that initial cracks develop and extend in the direction of uniaxial stress, with chemical effects influenced by pH and ionic concentration. When considering the interaction of water flow and stress in fractured rock masses, particularly with the added complexity of damage evolution, the research is relatively recent. Initial studies primarily examined the permeability of rocks under stress, focusing on the coupling relationship between rock stress and water flow. These studies employed laboratory and field tests to observe how the mechanical characteristics of rocks change with permeability under stress conditions. In the elastic stage, internal micro-fractures in the rock tend to close under pressure, reducing permeability. As the rock enters the elastoplastic stage, new microcracks emerge, causing permeability to first increase gradually, then rapidly peak just before or after rock failure. In the residual flow stage, permeability varies depending on pore development within the rock. It is clear that the changes in permeability closely align with the deformation and failure processes of rocks, with circumferential and axial strain playing a key role in this evolution. Since the 1980s, researchers have increasingly focused on understanding the mechanical behavior of underground rock masses under multi-field coupling. In particular, the need to study multi-field coupling for applications such as nuclear waste disposal and geothermal energy development has become urgent. The study of THM coupling in rocks, which involves heat, water flow, and mechanical stress, builds on prior research in TM and HM coupling models. However, THM coupling is more complex than these earlier models, requiring a systematic approach to establish reasonable coupling modes and clarify the interactions between the different fields. [PERSON] et al. [143] conducted a large-scale in situ heating test designed to replicate conditions in a deep geological repository for high-level radioactive waste in granite. [PERSON] focuses on THM responses within the bentonite barrier and surrounding granite, emphasizing hydration progression, heating impacts, vapor transport, and swelling pressures. Their numerical analysis effectively mirrored experimental progress, enhancing understanding of THM complexities and validating the predictive reliability of the numerical model. These models are crucial for addressing the challenges in deep underground engineering, where the influence of these fields becomes increasingly pronounced. As scientific research continues to advance, understanding the coupling effects in these environments is becoming a critical component of engineering design and safety. Parametric analyses examine how hydraulic boundary conditions (B.C.) and post-failure behavior models influence the evolution of THM behavior. Variations in drainage conditions affect thermally induced pore pressures, impacting the onset of softening [121].