## 3 Test Results and Analysis
### Analysis of Acoustic Emission Energy  
Acoustic emission (AE for short) energy represents the energy released from crack rupture, extension, and penetration during the deformation and damage process of the rock body, and its change characteristics are closely related to the internal damage evolution process of the rock body [15]. The AE energy generated in each deformation stage can be calculated to obtain the cumulative AE energy of the entire rock body deformation and damage process. Due to space constraints, several sets of typical AE energy relationship curves were selected for analysis, as shown in Figure 11. The AE energy parameters of the intact specimen and the grouting specimens are statistically presented in Table 4. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Type of Specim** & **Cumulative AE Energy** & **Peak Axial Stress** & **Peak AE Energy** \\ \hline the intact specimen & 2,448,528 mV\(\cdot\)ms & 26.481 MPa & 21,430 mV\(\cdot\)ms \\ \hline the ultra-fine cement & 9,877,722 mV\(\cdot\)ms & 24.103 MPa & 106,431 mV\(\cdot\)ms \\ \hline the C-S grouting specimen & 1,893,329 mV\(\cdot\)ms & 22.487 MPa & 27,316 mV\(\cdot\)ms \\ \hline the MARITHAN® & 1,268,946 mV\(\cdot\)ms & 25.197 MPa & 19,864 mV\(\cdot\)ms \\ \hline \hline \end{tabular}
\end{table} 
Table 4: AE energy parameters of different grouted specimens. Figure 11: _Cont._  
Figure 10: Crack extension of specimens. (**a**) The ultra-fine cement grouting specimen; (**b**) The C-S grouting specimen; (**c**) The MARITHAN® grouting specimen. Note: the blue color represents the original rupture surface, the green color represents the nascent rupture surface, and the red color represents that the original rupture surface coincides with the nascent rupture surface. It can be seen from Figure 11 that the cumulative AE energy curves of the fissure specimens with different grouting materials generally show a downward concave step-type trend of change: the slope of the curve is small at low-stress levels; the growth rate of the curve increases gradually with the increase in axial stress; and the curve generally exhibits a slow then suddenly increasing trend as loading proceeds. According to the previous related literature [16; 17; 18], the curve is roughly divided into five stages: crack closure stage (I), elastic deformation stage (II), stable crack expansion stage (III), unstable crack expansion stage (IV), and post-peak damage stage (V). The AE characteristics of each stage are as follows:  
Stage I: With the increase in load, the microcracks inside the specimen are gradually compacted and closed. Compared with the intact specimen, the cracks in the grouted specimen are compacted and closed for a longer period of time. The AE energy counts are increased at the end of this stage, and the slope of the cumulative energy curve is increased. Stage II: As the load continues to increase, the specimen undergoes elastic deformation, the AE energy counts are distributed more uniformly, and there is no significant change in the cumulative AE energy curve. Stage III: When the axial load reaches cracking stress, the cracks start to sprout and expand stably, and compared with the previous two stages, the slope of the cumulative AE energy curve at this time increases, and the curve shows an upward trend. Compared with the intact specimen, the slope of the cumulative AE energy curve of the grouted specimen increases significantly, and the AE energy counts of the ultra-fine cement grouting specimen and the C-S grouting specimen reach larger values. Stage IV: At this time, the internal microcracks of the specimen develop rapidly, and the cracks interconnect with each other, forming obvious macroscopic cracks and generating a large number of AE signals. The AE energy counts increase rapidly, and the slope of the cumulative AE energy curve continues to increase. When the stress reaches its peak, both the AE energy counts and the slope of the cumulative AE energy curve reach their maximum values at this stage.