## 4 Discussion  
When preparing pre-peak confining pressure unloading specimens, triaxial loading of the specified confining pressure and axial pressure should be performed first. If the set axial pressure is significantly high, it will cause loading damage to the specimen, interfering with the subsequent confining pressure unloading damage process. Therefore, appropriate selection of the specified axial pressure value is particularly important to ensure that the damage to the specimen is solely caused by confining pressure unloading. Table 4 presents a comparison of the number of cracks that occurred in the specimens under different unloading pressure conditions. No cracks were observed on loading to 55% of the triaxial compressive strength. When the confining pressure unloading was 15 MPa, the number of cracks was 381, indicating an apparent increase of 34.6 and 6.7 times occurring under confining pressure unloadings of 5 MPa (11) and 10 MPa (57), respectively. These results indicate that under a triaxial loading process with a set confining pressure and axial compression, the loading did not cause damage; instead, the damage was induced by the confining pressure loading. The confining pressure unloading is closely related to the final deformation and failure of the model. To describe and analyze the relationship between them as quantitatively as possible, we used simulated acoustic emission for the analysis. The moment magnitude of the acoustic emission event caused by the breaking of a bond can be calculated by monitoring the changes in the contact force around the fault source particles. The moment magnitude reflects the failure intensity of the fracture event, characterizing the dislocation of the fault surface caused by each event and the energy released as a result. The moment magnitude calculation formula is expressed as follows [36]:  
\[M=\frac{2}{3}\text{lg}M_{0}-6 \tag{3}\]  
where \(M_{0}\) denotes the scalar moment, \(M_{0}=\left(\frac{1}{2}\sum\limits_{j=1}^{3}m_{j}^{2}\right)^{1/2}\), and \(m_{j}\) denotes the characteristic value of the moment tensor matrix. It is evident from the distribution of moment magnitudes in Figure 15 that increasing the confining pressure unloading causes the moment magnitude distribution to change from a random scattered distribution to a regular accumulation. In this process, the degree of damage is continuously strengthened in a manner closely related to the final fracture shape. In the case of bond fracture failure, the particles on both sides of the fracture produce a larger energy release--the damage is intensified as the moment magnitude decreases--which leads to surrounding dislocation deformation and causes the adjacent bonds to break, resulting in the initiation of microcracks, with a number of similar microcracks accelerating and converging to form crack clusters. When the number of microcracks increased to a certain level, the adjacent microcrack clusters propagated and connected to form dominant fractures, and the moment magnitude decreased gradually. For instance, the dominant fracture moment magnitude shown in Figure 15c is approximately \(-3.8\), whereas the potential fracture moment magnitude shown in Figure 15e is approximately \(-4.2\), with the moment magnitude concentration in the potential fracture area being more evident and the total microcrack initiation increasing rapidly. In the subsequent triaxial \begin{table}
\begin{tabular}{c c c c c} \hline \hline \begin{tabular}{c} **Confining Pressure** \\ **Unloading Conditions** \\ \end{tabular} & \begin{tabular}{c} **Loaded to 55\% of** \\ **Triaxial Compressive** \\ **Strength** \\ \end{tabular} & \begin{tabular}{c} **Confining Pressure** \\ **Unloading** \\ **5 MPa** \\ \end{tabular} & \begin{tabular}{c} **Confining Pressure** \\ **Unloading** \\ **10 MPa** \\ \end{tabular} & \
\begin{tabular}{c} **Confining Pressure** \\ **Unloading** \\ **15 MPa** \\ \end{tabular} \\ \hline \hline
\begin{tabular}{c} Number of cracks/nos \\ \end{tabular} & 0 & 11 & 57 & 381 \\ \hline \hline \end{tabular}
\end{table}