## 3 Brief Introduction to the Physical Test for Limestone Specimens
### Evolution Law of the Permeability Coefficient in the Whole Process of Model Stress-Strain
(**a**) Whole process curve of load step-stress-permeability coefficient. (**b**) Presupposed weak zone model failure process stress results (MPa). (**c**) Results of water pressure distribution during crack formation (**m**). Figure 8: Numerical results for Sample 3. (**a**) Whole process curve of load step-stress-permeability coefficient. (**b**) Stress results for the intact specimen during the failure process (MPa). (**c**) Results of water pressure distribution during crack formation (m). The four stages are shown in Figures 6a, 7a and 8a. With the increase in load step, the stress increased at the same time, a microfracture slowly initiated, and the permeability coefficient increased slowly from 0, which was the value at the first stage. As the load step continued to increase, the microfracture closed, and the step stage appeared, which was the second stage. After the step stage, the crack and permeability coefficient suddenly increased rapidly, which was the third stage. Then, due to the closed fracture permeability coefficient exhibiting a small decline, the fourth stage occurred. Figures 6a and 8a indicate that the specimen slipped along the prefabricated joint surface, so the stress decreased sharply. The permeability coefficient increased sharply because the crack had penetrated. Figure 7a shows that no obvious stress drop process occurred, so the development of cracks could not be judged by the stress-strain curve. Figures 6b, 7b and 8b fully display the crack formation process, allowing our timely  
Figure 9: Numerical load step-stress-acoustic emission curve. (**a**) Prefabricated weak surface model. (**b**) Prefabricated weak zone model. (**c**) Complete model. understanding of where the crack extended, whether it went through, and when it went through, which were difficult to determine in the experiment. In addition, the water pressure distribution results in the process of model failure are illustrated in different colors in Figures 6c, 7c and 8c to reflect the occurrence of seepage. Red represents high water pressure, and blue represents low water pressure, that is, water permeated from top to bottom and was sealed around. These figures fully show the process of water flowing from top (high water pressure) to bottom (low water pressure) along the crack. The head pressure at the crack was the largest, and a certain water pressure distribution formed near the crack area. The area that the seepage could reach was limited due to the low permeability of limestone itself, and almost no water pressure was observed outside the crack. The microfractures in Model 1 were concentrated near the prefabricated joints, so the width of the crack was small, and the water penetration rate was low. Loading to Step 122-1 (load step 122, step 1; when element failure occurred in each loading step, step-in-step cycle calculation was applied, but when no failure occurred in this step, the next loading step was initiated) could not fully penetrate the entire model (Figure 6c). Given that the microfractures in Model 2 (Figure 5b) were mainly distributed in the weak region, only a few microfractures occurred in the other regions, resulting in a wide seepage channel in the weak region and a high penetration rate. By Step 65-3 (Figure 7c), the water flow penetrated into almost the entire model height. The seepage velocity of the intact rock block model (Figure 5c) was relatively low because the crack formation time was long, and the microfracture had the characteristics of dispersion and disorder and was randomly distributed in the whole model. Therefore, loading at Step 18-1 (Figure 8c) did not penetrate into half of the height of the model. Figures 6a, 7a and 8a show that the seepage velocity of Model 2 was the largest, followed by those of Models 1 and 3 (the smallest). The reason was that the joints and weak zones of Model 1 and 2 connected the high water head area at the top, and water could easily pass through this area. The weak zone of Model 2 was wider than that of Model 1, and the permeability was stronger.