## 3 Brief Introduction to the Physical Test for Limestone Specimens
### Prediction of Rock Permeability during Crack Propagation  
The analysis of the stress-strain-permeability curve revealed that the failure and seepage of the model could be judged by the stress drop. However, with hysteresis, passivity, and failure, predicting the catastrophic failure of the model was impossible. In fact, if the microfracture of the model is located and monitored, and crack coalescence is identified before microfracture coalescence, then crack coalescence and catastrophic seepage can be prevented effectively. When rock micro-fracture occurs under load, some strain energy is released in the form of elastic waves. These elastic waves can be monitored by the acoustic emission technology. Acoustic emission positioning technology is used to determine the location, time of occurrence, and strength of microfractures. In this way, we can analyze the mechanical behavior of the rock fracture process in terms of time, space, and strength and understand the defects of the current medium, both of which are crucial to obtain information about the precursors of the main fracture. Collection and analysis of the data obtained through the numerical model of acoustic emission (AE) monitoring showed that AE can effectively reflect a microfracture and that dense limestone materials mainly show permeability in the cracks. Therefore, if the development of cracks can be effectively predicted, then the occurrence of seepage can be effectively controlled. The underlying specific principle has been explained in [25]. Figure 9 shows the load step-stress-acoustic emission curve of the three models. With loading, the AE number increased slowly. Before the seepage velocity increased, AE suddenly occurred, and the permeability coefficient increased significantly. The relationship curve in Figure 10 indicates that AE showed certain advantages when judging the seepage characteristics. The AE location information in Figure 10 can well reflect the occurrence and penetration of microfractures and predict the permeability characteristics. In Figure 10, a circle represents an AE event. The size of the circle refers to the relative energy of AE. The white circle represents compressive failure, the red one represents shear failure, and the black one represents a historical AE event. Under axial load, oblique cracks were produced along the direction of the prefabricated weak plane in the prefabricated weak plane model, which was predictable. This phenomenon can also be seen clearly from the AE location information in Figure 10a. At Step 2-4, the model mainly exhibited minimal pressure damage, randomly distributed throughout the model. By Step 3-5, the compressive failure was still randomly distributed in the whole model, but the red tensile failure had begun to concentrate along a certain line of the model (the direction of the prefabricated weak surface). By Step 5-3, the small tensile failures formed a shear failure zone. Nevertheless, the failure point did not penetrate the upper and lower parts of the specimen, so the water at the upper end could not penetrate into the specimen in a wide range. Seepage along the crack occurred only when the crack passed through the whole sample at Step 8-25. Model 2 was mainly destroyed along the prefabricated weak zone. Figure 10b shows the AE location information of the prefabricated weak zone model. The AE distribution of the prefabricated weak zone model was basically consistent with that of the prefabricated weak surface model. At Step 5-37, the crack penetrated, and the microfractures were mainly distributed along the prefabricated weak zone. Comparison of Figure 10a,b showed that in the early stage of loading (Figure 10c), the microfractures of the model were randomly distributed throughout the model, and no obvious shear microfracture occurred until Step 12-30, during which some concentrated red damage points emerged in the middle.