## 2 Test Process and Design  
A triaxial compression test was conducted on granite samples to study the AE information, precursor characteristics, and energy distribution of the samples during loading deformation and failure under high confining pressure. The granite samples were all excavated from 1150 m underground in Sanshandao Gold Mine in Laizhou, Shandong, China. All samples were collected from the same rock mass, with uniform mass and without obvious joint layers, thereby avoiding the effect of varied samples on the test results. According to the method recommended by ISRM, the sample was prepared into a cylinder with a diameter of 50 mm and a height of 100 mm, maintaining the unevenness error of both ends of the sample less than 0.05 mm and the diameter error along the sample height less than 0.3 mm. In general, when the deformation ratio of the testing machine is inconsistent with that of the granite sample in the loading process, the friction at the sample end will limit the end deformation of the sample, thus affecting the mechanical properties, such as the stress distribution of the sample. To eliminate this sample end effect, we polished both ends of the sample with 400 grit emery paper, reducing the friction of the end faces. The in situ stress measured at \(-1150\) m is 30 MPa. To simulate the real rock occurrence environment and conduct the control test, the confining pressure in this test was preset to 10, 20, and 30 MPa. The size of each granite sample was measured and recorded as shown in Table 1.  
The test equipment includes a rock mechanics loading system and an AE system (Figure 1). The loading system adopts a TAW-2000 triaxial hydraulic servo testing machine. During the loading process, the preset confining pressure value was first applied, and then the axial pressure on the rock sample was gradually increased until its failure under the constant confining pressure. In the later stage of loading, to obtain a clear post-peak curve, the loading rate was reduced at 0.008 mm/s until the sample was destroyed. For AE signal acquisition, the PCI-II acoustic emission monitoring instrument and its software were used. Vaseline was smeared on the end of the AE monitoring probe to ensure the coupling effect between the probe and the sample. The sampling evaluation rate of the AE monitoring system was set at 1 MSPS, the acquisition threshold was 40 dB, and the front gain was 40 dB to eliminate the effect of external noise.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Specimen** & **Height/mm** & **Diameter/mm** & **Weight/g** & **Density/g/cm\({}^{3}\)** & **Confining Pressures/MPa** \\ \hline A1 & 100.81 & 50.04 & 518 & 2.613 & \\ A2 & 100.53 & 50.05 & 509 & 2.575 & \\ A3 & 100.28 & 50.21 & 514 & 2.590 & \\ A4 & 100.96 & 50.14 & 523 & 2.625 & \\ \hline B1 & 100.30 & 50.03 & 516 & 2.617 & \\ B2 & 100.21 & 50.31 & 512 & 2.572 & \\ B3 & 100.46 & 50.07 & 521 & 2.635 & \\ B4 & 100.75 & 50.02 & 516 & 2.608 & \\ \hline C1 & 100.08 & 50.19 & 525 & 2.653 & \\ C2 & 100.65 & 50.00 & 513 & 2.597 & \\ C3 & 100.12 & 50.22 & 517 & 2.608 & \\ C4 & 100.36 & 50.06 & 515 & 2.609 & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Mechanical parameters of deep granites.  
Figure 1: Rock mechanics test system and acoustic emission test layout. (**a**) PCI-II acoustic emission system; (**b**) TAW-2000 triaxial hydraulic servo testing machine; (**c**) the layout of acoustic emission monitoring probe.