# Research on the Characteristics of Seepage Failure in the Surrounding Rock (Coal) of the Goafs
## 3 Results and Discussion  
### Test Results of Coal Samples  
The experimental investigation into the permeability of coal samples revealed a range from 0.09 \(\times\) 10\({}^{-3}\) to 0.24 \(\times\) 10\({}^{-3}\) um\({}^{2}\), highlighting considerable variability in permeability, particularly during the saturated seepage stage for certain samples (as detailed in Table 3). This range of permeability values is crucial as it reflects the inherent heterogeneity of the coal samples, which can be attributed to differences in their microstructure, pore distribution, and the presence of natural fractures [17; 18; 19]. By analyzing the permeability change curves over time, we can discern several key characteristics of the seepage process across different coal samples:  
1. Stable Stage and Mutation Stage:  
Most coal samples underwent progression from a saturated seepage stage, characterized by relatively consistent fluid flow, to a stable seepage stage, where permeability remained constant or varied minimally. This behavior suggests that the internal pore structure and fractures of the coal samples reached a state of equilibrium under the prevailing experimental conditions. This is also similar to the stress \"plateau period\" or primary pore and pore density stages mentioned by some scholars when conducting coal sample soaking experiments [19; 34], that is, the internal moisture of the coal sample has a certain buffering effect on the deformation of the coal sample, which will slow down the further expansion and damage of cracks or pores in a short period of time after the coal sample is saturated with water, which is what we call the stable stage. However, certain samples exhibited abrupt changes or fluctuations in permeability before stabilizing, indicating a mutation stage. This stage is particularly important as it signifies a transitional phase where the internal structure of the coal is adjusting to external pressures or fluid flow, leading to sudden changes in permeability. Mainly due to the swelling of the matrix in coal rock by water, the original structure becomes more compact. With the continuous increase in soaking time, the cement is gradually diluted and internal structures are mutually squeezed and destroyed, resulting in increased porosity and ultimately leading to changes in permeability [34]. For example, in samples M-6 and M-9, significant permeability mutations were observed in response to changes in confining pressure (illustrated in Figure 4a). These mutations are likely caused by the dynamic behavior of fractures within the coal, which can either open or close suddenly due to the applied pressure, drastically altering the permeability. 2. Effect of Confining Pressure:  
During the experiments, the fracture structures of several coal samples underwent significant changes, which notably impacted their permeability characteristics. These structural changes primarily manifested in two forms: (1) coal chips and blockages within the seepage channels, and (2) fissure expansion due to mechanical and hydraulic influences. One of the key phenomena observed was the presence of coal chips and the resulting blockages within the seepage channels of some coal samples. These blockages occurred as fragments of coal, dislodged during the experiments, entered the fluid pathways, obstructing the flow. Consequently, this led to noticeable fluctuations in permeability as the detached coal particles intermittently clogged the flow channels. In samples M-3 and M-5, for example, the permeability curves (Figure 4a) showed significant fluctuations during specific periods. These fluctuations were marked by sudden drops in permeability, followed by gradual recovery as the blockages cleared, either naturally or due to changes in pressure conditions, allowing the coal chips to move through the system. This dynamic interaction between the coal matrix and fluid flow indicates that the integrity of the seepage channels was periodically compromised by the presence of coal chips.