# Experimental Study on the Pre-Peak Mechanical and Seepage Characteristics of Granite
## 3 Analysis of Test Results
### Analysis of Seepage Evolution  
Rock permeability is essential in determining the occurrence of water inrush in seabed mining engineering. Many factors affect permeability, including stress, seepage pressure, and confining pressure. Evidently, these factors affect the permeability characteristics of a rock by altering its cracks. In this section, the results of the mechanical characteristics are combined during the progressive failure of the granite in the previous section to reveal the pre-peak evolution of the permeability of the granite. \begin{table}
\begin{tabular}{c c c c c c} \hline \(\sigma_{3}\) (MPa) & \(\sigma_{ci}\) (MPa) & \(\sigma_{ci}\)/\(\sigma_{p}\) (\%) & \(\sigma_{cd}\) (MPa) & \(\sigma_{cd}\)/\(\sigma_{p}\) (\%) & \(\sigma_{p}\) (MPa) \\ \hline
5 & 74.26 & 37.16 & 90.58 & 45.33 & 199.85 \\
10 & 85.87 & 35.23 & 109.28 & 44.84 & 243.74 \\
20 & 110.27 & 34.68 & 120.58 & 37.92 & 317.97 \\ \hline \end{tabular}
\end{table} 
Table 2: Stresses and stress ratios of granite under three confining pressures. \begin{table}
\begin{tabular}{c c c c c} \hline \(\sigma_{3}\) (MPa) & \(\sigma_{cc}\) & \(\sigma_{ci}\) & \(\sigma_{cd}\) & \(\sigma_{p}\) \\ \hline
5 & a & b & c & d \\
10 & a\({}^{\prime}\) & b\({}^{\prime}\) & c\({}^{\prime}\) & d\({}^{\prime}\) \\
20 & a\({}^{\prime\prime}\) & b\({}^{\prime\prime}\) & c\({}^{\prime\prime}\) & d\({}^{\prime\prime}\) \\ \hline \end{tabular}
\end{table} 
Table 3: Volumetric strain curve of each point interpretation summary table. The Pre-Peak Evolution Characteristics of Permeability  
The permeability of the granite was calculated according to Equation (1), and the permeability pre-peak evolution curves under different confining pressures were plotted, as shown in Figure 9.  
Figure 8: The volumetric curves of rock under different confining pressures. Figure 9: Evolution process of rock permeability in pre-peak process. It should be noted that the four parts, I, II, III, and IV, divided in Figure 10 correspond to the crack closure stage (I), linear elasticity stage (II), stable crack expansion stage (III), and unstable crack expansion stage (IV), respectively. No stress point is set in this part, considering that the stable crack expansion stage (III) is shorter in the whole progressive failure process. Figure 10: Permeability and effective stress curves. According to Figure 9, The initial permeability of the granite at confining pressures of 5 MPa, 10 MPa, and 20 MPa were 3.55 \(\times\) 10\({}^{-8}\) Darcy, 2.57 \(\times\) 10\({}^{-8}\) Darcy, and 2.07 \(\times\) 10\({}^{-9}\) Darcy respectively. The initial permeability of the granite under the different confining pressures was the maximum throughout the loading process. It was found that the initial permeability of the granite gradually decreased with the increase in the confining pressure. Previous researchers have also concluded the same negative correlation between the permeability of rock samples and the confining pressure [37]. It is noteworthy that although the difference between the three levels of confining pressures was two times, the permeability values of the rock samples at 5 MPa and 10 MPa were in the same order of magnitude. In contrast, the difference between the rock samples at 20 MPa and those at 10 MPa was by one order of magnitude. The final permeability of the granite at the confining pressures of 5 MPa, 10 MPa, and 20 MPa were 5.80 \(\times\) 10\({}^{-9}\) Darcy, 1.08 \(\times\) 10\({}^{-9}\) Darcy, and 1.28 \(\times\) 10\({}^{-9}\) Darcy, respectively. The final permeability of the granite under different confining pressures was the minimum throughout the loading process. Interestingly, the final permeability values of the granite at 5 MPa, 10 MPa, and 20 MPa were in the same order of magnitude. Furthermore, comparing the initial permeability and final permeability of the granite under different confining pressures, it was found that both were one order of magnitude different. Therefore, the axial pressure and confining pressure were key to the decrease in the permeability.