# A Novel Carbon Fiber Composite Material for the Simulation of Damage Evolution in Thick Aquifers
## 4 Simulation Test of Damage and Fracture in Extremely Thick Sandstone Aquifers
### Damage Evolution Process of a Very Thick Sandstone Aquifer  
To understand the deformation and damage evolution process of the thick sandstone aquifer, the displacement and damage coefficient (\(D\)) in the central region of the thick sandstone aquifer were selected for analysis. This measurement point also represents the location of the maximum displacement along the measurement line in the thick sandstone aquifer. Figure 13 presents the variation curves of displacement and damage coefficient \(D\) of the thick sandstone aquifer during the workface advancement process.  
Figure 13a illustrates the response of Line 3, located at the bottom of the very thick sandstone aquifer. As mining progresses to 85 m, vertical displacement is first observed at the central measurement point, indicating early deformation. The damage variable \(D\) increases rapidly, ultimately reaching 0.69, while the damage increment \(\Delta D\) exhibits a concentrated distribution in the middle and lower regions. This reflects sustained crack development and significant damage accumulation, although complete structural failure  
Figure 12: Damage distribution of the very thick aquifer.  
has not occurred. This confirms that the basal interface is the most vulnerable zone during the early mining stages.  
Figure 13: Damage accumulation characteristics of very thick sandstone aquifers during coal mining; (**a**) Line 3; (**b**) Line 4; (**c**) Line 5.  
Figure 13b illustrates the response of Line 4, located in the middle section of the very thick sandstone aquifer. When the mining depth reached 95 m, deformation began at the central measurement point, and the \(RCR\) gradually increased. The damage variable \(D\) subsequently rose more rapidly, reaching a final value of 0.34. The damage increment \(\Delta D\) was concentrated primarily in the later stages of mining, indicating delayed fracture development compared with the bottom layer. Overall, the middle section experienced moderate damage evolution, maintaining better structural integrity than the lower zone.  
Figure 13c illustrates the response of Line 5, located at the top of the very thick sandstone aquifer. When mining reached a depth of 110 m, minor deformation occurred in the overburden, with the \(RCR\) slowly increasing. The damage variable \(D\) reached a final value of 0.12, indicating a relatively slow damage evolution at the top compared with the bottom and middle sections. The damage increment \(\Delta D\) exhibited small fluctuations, suggesting a slower and more moderate fracture development at the top, with the structure remaining relatively stable during mining.  
In summary, the thick aquifer exhibits a bottom-to-top progressive damage and fracture pattern under mining disturbance. The basal interface experiences early-stage fracturing and sustained crack development, showing significant damage and fracture propagation. The middle layer enters an accelerated fracture phase in the later stages of mining, characterized by dense crack formation, while the top layer remains relatively intact, exhibiting a slower rate of damage evolution.