# Study on Failure Mechanism of Mudstone Based on Digital Core and Digital Volume Correlation Method
## 4 Evolution Characteristics of Digital Core Fracture Model
### Distribution and Volume Change of Pores and Fractures  
The change of internal pores in the process of rock loading can clearly reflect the time and space variation characteristics of rock instability and failure. Pore throat is a narrow channel in which pores are connected with each other in rock mass or soil, and pores are the space between solid mineral particles in rock and soil. In this paper, the porosity of CT data in the process of real-time in situ loading was analyzed, the pore connectivity in  
Figure 4: Microcracks and pores near mineral particles. (**a**) Mineral accumulation zone. (**b**) Quartz particles. (**c**) Microcracks near mineral particles.  
Figure 5: Gray image segmentation process. (**a**) Primitive mudstone. (**b**) Sub-block statistical feature. (**c**) Calculation of connectivity of mineral particles. (**d**) Extraction of aggregate mineral zone.  
the process of rock failure was located and quantitatively counted, the pore evolution data obtained from experiments were analyzed, and the crack propagation trend and change characteristics were studied.  
Figure 6 is a 3D distribution figure of pores and pore throats in mudstone at different loading stages. Through statistical analysis, we found 805 pores and 2 pore throats in mudstone in the initial stage, and the coordination number of pores was basically zero. No matter the spatial distribution and connectivity or the results of numerical statistics, the pore connectivity in the initial stage was poor. The initial pore volume was 0.804 mm\({}^{3}\) (the size of the pore ball is plotted according to the proportion of the equivalent diameter of the pore).  
With the progress of loading, the number of pore throats in the 3D pore network model increased, and the number of pores in the first loading stage (18.5 MPa) decreased. At this stage, the mudstone was in the compaction stage; a small number of pores were closed and compacted under the action of external load; and a small number of pores were connected, which was characterized by the enhancement of pore connectivity, and the fracture volume was 1.035 mm\({}^{3}\).  
When the loading continued, the pore connectivity in the second loading stage (37.5 MPa) was further enhanced, indicating that new pores were generated and the old pores were further developed and connected, and a macroscopic crack appeared on the surface of the sample. In the pore network model inside the sample, it can be seen that the pore network model was mainly developed into two clusters, and the positions of the two pore connecting clusters were very close to the distribution of the mineral zone. At this stage, the rock damage increased gradually, the number of pore development and penetration was also much higher than that in the previous loading stage, and the fracture volume also increased to 2.32 mm\({}^{3}\).  
With the continuous development of pores and the accumulation of penetration, the local microcracks gradually expanded into macroscopic cracks and further penetrated, reaching the maximum load limit of mudstone, and the damage of rock reached the peak. In the third loading stage, the fracture volume reached 180.75 mm\({}^{3}\), and there was an obvious stress drop, indicating that in the previous stage, the pores in the mudstone had developed to a critical state of failure, which was extremely prone to instability failure, and the adjacent pores penetrated into cracks. The mudstone was completely destroyed, as shown in Figure 7.  
Figure 6: Pores and pore throats distribution of mudstone in the third loading stage. (**a**) First loading stage. (**b**) Second loading stage. (**c**) Third loading stage.  
Table 2 shows the parameters of fracture development in different loading stages of mudstone.