# Study on Experimental and Constitutive Model of Granite Gneiss under Hydro-Mechanical Properties
## 5 Conclusions  
In this research, the granite gneiss mechanical properties under hydro-mechanical properties were studied experimentally. A fully automatic triaxial servo system for rock was used to conduct triaxial mechanical tests under hydro-mechanical properties. There are several conclusions as follows:  
1. The results divide the stress-strain process into three stages to characterize rock materials' mechanical behavior characteristics under triaxial compression conditions. (i) The microcrack compaction stage is a relatively small nonlinear deformation stage on the rock's internal microcracks and pores under stress. (ii) The linear elastic deformation stage is mainly affected by the pressure of the rock skeleton. Its linear slope is the rock elastic modulus, which is a vital parameter for characterizing mechanical properties. (iii) The damage and failure stage is a nonlinear plastic deformation stage where cracks expand rapidly.
2. The boundary point between the first and second stages generally distributes around the first intersection of the stress-strain curve and the permeability-strain curve. The volumetric expansion point is the peak value of volumetric strain, which is the dividing point in the second and third stages, identified from the volume stress-strain curve. The granite gneiss hydro-mechanical properties model can be established by the piecewise fitting curve.
3. In view of the experimental data, a hydro-mechanical properties constitutive model that is suitable for isotropic porous media was built to model the granite gneiss mechanical behavior under different seepage pressure conditions. To test the model's reliability, compare the numerical and experimental results. The simulated peak strengths of rock samples R1 to R4 were determined to be 237.56 MPa, 214.48 MPa, 248.91 MPa, and 235.82 MPa, respectively. Compared with the experimental data, all of these were less than 1%. The axial peak strains were 6.38, 6.05, 6.55, and 6.93, respectively. Compared with the experimental data, all of these were less than 5%. The two datasets match well, which proves this model can offer a good representation of the rock behavior prominent distinctions, that is, the plastic deformation, material damage, and hydro-mechanical properties, in different seepage pressure conditions.  
The constitutive model could describe the mechanical behavior characteristics of deep-buried hard rock under the coupling effect and has important engineering value in describing rock mechanics and for underground structural safety research. In future studies, further tests on various rocks should be conducted with more loading conditions for the purpose of verifying the preliminary conclusions drawn in this research. Important value could be provided for the major national infrastructure projects' long-term stability and safety, for instance, large-scale water conservancy and hydropower projects, as well as underground energy storage and transportation projects in complex environments.  
Conceptualization, L.L.; methodology, L.L.; software, B.W.; validation, L.L. and B.W.; formal analysis, L.L.; investigation, L.L. and B.W.; resources, L.L.; data curation, L.L.; writing--original draft preparation, L.L.; writing--review and editing, B.W.; visualization, L.L.; supervision, B.W.; project administration, L.L. and B.W.; funding acquisition, L.L. and B.W. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Natural Science Foundation of Jiangsu Province, grant number BK20191302, and the Natural Science Foundation of Jiangsu Higher Education Institutions of China, grant number 22 KJB560026.  
Not applicable.  
Data Availability Statement: Data are contained within the article.  
The authors declare no conflict of interest.