## 3 Constitutive Model of Rock Mass Under THMC Coupling
### THM Coupling Models
#### 3.2.3 Influence of Multi-Field Coupling on Rock Damage  
The mechanical damage that occurs in rocks under the influence of multi-field coupling cannot be neglected. Initial rock damage often takes the form of fractures. As these fractures evolve under the coupled influences of temperature, water flow, and stress, they can lead to significant structural changes and eventual failure. In establishing mathematical models of multi-field coupling, it is crucial to account for the impact of mesoscopic damage on coupling effects. For instance, rock stress impacts permeability and thermal conductivity, and the heat and fluid within them significantly influence the mechanical properties of rocks. Challenges and Future Directions  
To construct a comprehensive mathematical model for multi-field coupling in rock mechanics, several assumptions must be made: (1) rock material heterogeneity must be acknowledged, with mesoscopic damage distributions derived from digital image analysis; (2) heat conduction within the rock must follow Fourier's law; (3) the water flow process should be based on [PERSON]'s consolidation theory and the modified Terzaghi effective stress principle; and (4) mesoscopic units of the rock are elastic and brittle. Various strength criteria, such as the Mohr-Coulomb criterion and maximum tensile strain criterion, along with elastic damage theory, are applied to describe the mechanical behavior of rocks. In coupled THM interactions within rock masses, damage plays a crucial role. Initial fracture distribution influences mechanical properties, while fracture evolution under coupled fields leads to progressive damage until failure. Modeling THM coupling must thus account for mesoscopic damage, impacting stress, permeability, thermal conductivity, and mechanical properties. Notably, post-failure, thermal conductivity and permeability shift significantly, necessitating an evolution equation to describe interactions between the temperature and water flow fields during rock failure. These simplifications can limit the models' applicability in complex field scenarios, where real rock structures may display unpredictable permeability, porosity, and mechanical responses. Advanced models that combine multiple scales (micro, meso, and macro) are needed to accurately simulate the complex evolution of fractured rock properties under multi-field influences. Overall, studies in THMC coupling provide critical insights into the behavior of rocks under diverse field conditions, highlighting the intricate interactions between THMC processes. By integrating laboratory experiments, mathematical modeling, and numerical simulations, researchers continue to refine these models to improve predictive accuracy, aiding engineering applications where rock stability is paramount. Multi-Field Coupling for Geomechanical Applications  
Researchers applied fractal theory to construct models of fracture structural planes. Their approach calculated the average fracture opening of a single fracture under high-pressure water, using a mathematical model to characterize permeability changes under the coupled effects of water flow, stress, and chemistry. This model illustrated how water flow channels form during rock dissolution under pressure. They developed a comprehensive coupling model of dissolution-solid-water flow-mass transfer, integrating three-dimensional solid deformation with water flow and diffusion to reveal interactions and transformations in rock variables. They developed a rock model that considered a rock matrix, fractures, and spherical pores, with evenly distributed pores, deriving an effective porosity expression and establishing a mesoscopic statistical water flow model. This model, which treats permeability during crack evolution as a nonlinear dynamic process, considers the combined effects of crack development and water flow. The time-dependent coupling of water flow and stress fields in fractured rock masses has valuable implications for engineering stability.