## 4 Acoustic Emissions (AEs) of Fissured Sandstone during the Compression Process  
### AE Events  
Acoustic emissions (AEs) are utilized for the observation of the acoustic characteristics exhibited by fissured sandstone during the process of failure under uniaxial compression. These acoustic signals effectively capture both micro and macro cracking behavior, which correspond to the deformation experienced by the specimens during uniaxial compression. The analysis focuses on studying the failure process of fissured sandstone through the examination of acoustic emission events in conjunction with axial strain measurements. Figure 7 illustrates the acoustic emission (AE) events during the failure process under conditions of saturated, dried, and frozen-saturated states. The AE event data were pre-processed because of the non-uniform faces of the specimens and adjustments were made to the experimental equipment utilized for generating the AE signal. The figure clearly illustrates that the AE event corresponds to axial strain during the compression process of fissured sandstone, indicating that the peak value of the AE event reflects sudden changes in axial strain. This effectively captures information regarding the initial crack tip, crack propagation, and failure process during the compression process. All of the time-strain curves exhibit similar trends: (1) strain increases over time but at a gradually decreasing rate, and (2) even when reaching the final peak of AE event, some residual strain remains until brittle fracture of the specimen occurs.  
Under preliminary compression, the absence of an AE event count indicates the non-propagation of microcracks at this stage. The time of an AE event occurrence increases with the inclination angle in all three states, implying that higher inclination angles result in delayed microcrack appearance. In the dried and frozen-saturated states, there are three peaks of AE events during the compression process: The first peak corresponds to the initial crack formation and propagation between the three fissures near the inner tip; the second peak corresponds to the initial crack formation and propagation in fissures No. 1 and 3 near the outer tip; and the third peak represents specimen failure. However, in the saturated state, there are only two peaks of AE events during the compression process: one corresponding to initial crack formation and propagation between the three fissures near the inner tip, and another corresponding to specimen failure. The magnitude of the AE event peak is lower in the saturated state compared to in dried and frozen-saturated states due to deformation occurring during the compression process; pore water dissolves some mineral constituents, leading to the debonding of mineral grains, resulting in increased porosity. Consequently, continuous energy release occurs as a consequence of rock deformation throughout this process, causing a less significant increase in accumulated energy generating AE events.  
The data presented in Figure 7 also demonstrate a clear trend wherein the occurrence time of AE events increases with the inclination angles across all three states. However, it is observed that the appearance time of AE events follows an increasing order for the saturated state, dry state, and freezing-saturated state, corresponding to the initial crack time, which will be further discussed in Section 4.2.