# Experimental Study on the Mechanical Properties of Deep Granite under Gradient-Confining Pressure
## 3 Results and Discussion  
### Rock Mechanical Properties and Failure Morphology  
Based on the test data, the various mechanical properties of deep granite under a gradient confining pressure are presented in Table 1, and the relationship curves of the elastic modulus, Poisson's ratio, and peak strength of deep granite under different confining pressures are depicted in Figure 4.  
Figure 3: The conventional triaxial test system for hard rocks included 1—oil outlet pipe; 2—oil inlet pipe; 3—high stiffness load bracket; 4—axial pressure loading system; 5—valve; 6—air outlet pipe; 7—data cable; 8—data acquisition system; 9 and 12—washers; 10—rock specimen; 11—pressure chamber; and 13—heat-shrink tube. As shown in Figure 4, with increasing confining pressure, the trends in the elastic modulus, Poisson's ratio, and peak strength of the deep granite are basically consistent, and there is a relatively large dispersion. This presents a substantial distinction from the outcomes of traditional triaxial tests on shallow granite. In traditional triaxial compression tests on granite subjected to gradient-confining pressure gradients, as the confining pressure increases, the elastic modulus tends to increase. This occurs as the increase in confining pressure leads to the closure of cracks within the rock, reducing the defects and discontinuities within, thereby enhancing the overall integrity and stiffness of the rock. Simultaneously, under the influence of confining pressure, the contact between the particles \begin{table}
\begin{tabular}{c c c c} \hline \hline \(\mathbf{\sigma_{3}}\) **(MPa)** & **E (GPa)** & \(\mathbf{\mu}\) & \(\mathbf{\sigma_{\epsilon}}\) **(MPa)** \\ \hline
8 & 90.08 & 0.22 & 89.71 \\
9 & 101.77 & 0.16 & 183.77 \\
10 & 101.79 & 0.25 & 245.59 \\
16 & 128.25 & 0.44 & 202.62 \\
18 & 105.70 & 0.22 & 187.31 \\
20 & 151.48 & 0.31 & 321.54 \\
24 & 144.09 & 0.38 & 203.23 \\
27 & 101.63 & 0.25 & 185.41 \\
30 & 109.72 & 0.32 & 274.95 \\
32 & 104.39 & 0.32 & 285.64 \\
36 & 102.91 & 0.23 & 258.18 \\
40 & 112.17 & 0.33 & 264.93 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The results of triaxial compression tests on deep granite under gradient-confining pressure. Figure 4: Fitting curve of the mechanical properties of deep granite under gradient confining pressure: (**a**) gradient-confining pressure–elastic modulus; (**b**) gradient-confining pressure–Poisson’s ratio; (c) gradient-confining pressure–peak strength. within the rock becomes closer, enhancing the bearing capacity of the rock. For deep granite situated in a high-stress environment, the rock is typically more compact, with a stronger binding between particles and a larger elastic modulus, while the sensitivity of the elastic modulus to the influence of confining pressure is not high. Along with the increase in confining pressure, the inhibitory effect on the radial strain of the rock also correspondingly intensifies, resulting in a possible decrease in the Poisson's ratio. However, due to the greater stiffness and more compact internal structure of the deep granite, this leads to a reduction in the inhibitory effect of confining pressure on the radial strain, consequently exhibiting a significant degree of dispersion. In most cases, as the confining pressure increases, the peak strength of the rock likewise increases. This is attributed to the fact that the confining pressure can suppress the expansion of cracks within the rock, facilitating a closer arrangement between the particles, resulting in a better overall integrity of the rock and an enhanced ability to withstand the load. However, considering that the internal structure of deep granite itself is highly compact and possesses a greater strength, coupled with the more complex geological conditions in deep rock compared to shallow rock, significant differences in the composition of the rock are observed. Consequently, there is also a degree of dispersion in the peak strength.