# Study on Experimental and Constitutive Model of Granite Gneiss under Hydro-Mechanical Properties
## 3 Results and Discussion
### Permeability Evolution Law of Granite Gneiss  
In view of the granitic gneiss triaxial test results on the affection of hydro-mechanical properties, the granitic gneiss generally experiences three stages before the peak failure, namely the early stage of microcrack compaction, linear elastic deformation, and internal damage expansion and deformation failure. Volume strain is a parameter of the rock's overall deformation characteristics in triaxial tests that can be used to represent the rock samples' overall deformation characteristics under different stresses. The calculation formula for volumetric strain is shown in Equation (2). \[\epsilon_{v}=-\Delta V/V_{0}=\epsilon_{1}+2\epsilon_{3}, \tag{2}\]  
In the equation, \(\epsilon_{1}\), \(\epsilon_{3}\), \(\epsilon_{v}\), respectively are axial strain, circumferential strain, and volumetric strain, where \(\epsilon_{3}\) has negative values; \(\Delta V\) is the increase in sample volume; and \(V_{0}\) is the sample original volume. Figure 3 shows permeability and stress-volumetric strain curves in the coupling effect of hydro-mechanical properties. The permeability evolution laws of rocks under different stress are different, and there is currently no unified model that can indicate the relevance between rocks' permeability and total stress-strain in diverse stress states. As seen in Figure 3, there is a good correlation between volume strain and permeability of granite gneiss in testing. Therefore, the granite gneiss hydro-mechanical properties model can be further established by discussing the relevance between its permeability and volume strain. The curve in the figure shows that the volumetric strain from point O to point A is the microcrack compaction initial stage. Permeability declines in the volumetric strain. Point A to point B is part of the linear elastic deformation stage, and the permeability appears to have a weak decreasing trend with the volume strain increasing or fluctuating around a fixed value; point B to point C is theinternal damage expansion and deformation failures stage, during which the permeability appears a major increasing trend with the volume strain changing. Point B is the boundary point that transitions from elastic deformation to plastic deformation, and it is also the peak point of volumetric strain, namely the volumetric expansion point. The permeability of rock samples R1 and R2 increase significantly after point B, while those of rock samples R3 and R4 do not increase as much, mainly because the confining pressure effectively enhances these rock samples ductility characteristics, as well as effectively restricts their lateral deformation and microcrack expansion. The higher the confining pressure, the lower the permeability of the rock samples. It is in accordance with the conclusions obtained in analogous research [29]. It is also the critical point for the expansion and enhancement of cracks in the rock and the transition of permeability from stable fluctuations to stable growth. Therefore, the granite gneiss permeability volume strain curve can be divided into two stages by taking point B as the dividing point. One stage is the gradual decrease in permeability, followed by a slight rate of decrease or fluctuations around a certain value. Another stage is when the rock specimen undergoes plastic deformation, and the permeability little by little increases with the volumetric strain changing, finally reaching its peak. In order to further analyze the permeability characteristics in the mechanical characteristics test in the coupling effect of hydro-mechanical properties, a function fitting method was used to study the two stages bounded by point B.