## 5 Results and Discussion
### Evolution Characteristics of Seepage Field  
As elucidated earlier, water accumulation in the goat significantly influences the mechanical damage of coal bodies. Specifically, water infiltration deteriorates the mechanical properties of waterlogged coal bodies through physical and chemical processes. Additionally, water permeates into the interior of the coal body, further affecting its deformation and failure processes through a coupled hydro-mechanical mechanism. Therefore, the distribution characteristics of the seepage field is crucial to the instability and failure of coal bodies.  
Under the condition where the water pressure in the goat is set at 0.26 MPa, the evolution characteristics of the seepage field in the coal body are illustrated in Figure 18.  
The figure reveals that the seepage field originates from the goat side and gradually extends towards the goat-side roadway. A comprehensive analysis in conjunction with Figure 18 indicates a clear correspondence between the expansion of the seepage field and the development of the plastic zone.  
Under the combined influence of water accumulation in the goat and excavation disturbance in the roadway, plastic zones form on both sides of the coal body. Compared to the elastic zone, the permeability of the coal body in the plastic zone significantly increases, causing water from the goat to more easily infiltrate the disturbed plastic zone adjacent to the goat, thereby exacerbating the damage and deterioration of the coal body in that area. This process further expands the plastic zone, reducing its support capacity, while the water accumulation in the goat continues to spread with the expansion of the plastic zone.  
The water-immersed softening flow-solid coupling simulation method employed in this study provides a more rational simulation of the gradual instability and failure behavior of coal bodies during the excavation process in the presence of water accumulation.  
As excavation progresses, the distribution range of the seepage field gradually expands. It is observed that when the width is smaller, the expansion rate of the seepage field is faster. When the width is 6 m, the seepage field has already expanded to the surface of the goat-side roadway, indicating that water from the goat has permeated through the coal pillar, resulting in complete mechanical weakening of the coal body. If the water accumulation in the goat is not drained before excavation, there is a risk of sudden water  
Figure 18: Evolution characteristics of seepage field.  
inrush during excavation. When the coal pillar width is 9 m, the plastic zones are not penetrated, and there is a complete elastic zone of 2 m in the middle. The seepage field only extends 5 m into the coal pillar, indicating that the mechanical properties of the coal body have not been completely weakened and still possess strong bearing capacity. However, when the width exceeds 9 m, the seepage field only extends less than 5 m into the coal pillar, with a slight decrease trend. This indicates that with the increase of width, the complete elastic zone in the middle also expands, and the coal pillar's resistance to water infiltration enhances accordingly.  
Through comparative analysis of deviatoric stress, dissipated energy density, plastic zone, and seepage field distribution, it is ultimately determined that a coal pillar width of 9 m is most suitable.