# 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  
### Experimental Background and Design  
The experimental mine is located in the Ordos mining area of China. The overlying strata of this mine widely contain a Cretaceous sandstone aquifer with a thickness ranging from approximately 112 to 262 m, characterized by its considerable thickness, widespread distribution, and high water content. The initial coal seam to be mined is the 2-2 seam, with a thickness of about 4.5 to 9 m, and it is situated at a distance of approximately 138 to 398 m from the aquifer. Mining this coal seam may damage or fracture the overlying thick aquifer, resulting in water resource loss and interference with underground safety production. To investigate the damage to the thick aquifer during coal seam mining, a similar physical simulation study was conducted. Considering the overburden conditions of multiple borehole mining sites and the thickness of the initial coal seam, the design of rock layer arrangement and material ratios was developed, as shown in Table 1. The geometric similarity ratio of the experiment is 1:200, the density similarity ratio is 1:1.60, the stress similarity ratio is 1:560, and the Poisson's ratio similarity is 1:1. The total model laying thickness is 170 cm, which corresponds to an actual thickness of 340 m according to the geometric similarity ratio. However, the actual burial depth of the initial coal seam is  
Figure 8: \(RCR\)–Strain Relationship of 2% CFSSM. approximately 800 m, necessitating the application of an additional load equivalent to 460 m (11.5 MPa) above the model to simulate the real overburden conditions. The experiment uses a steel-frame vertical model stand as the supporting platform. Various rock layer materials are prepared by mixing river sand, calcium carbonate (CaCO\({}_{3}\)), and gypsum powder (CaSO\({}_{4}\cdot\)1/2H\({}_{2}\)O) with water. River sand forms the primary structural framework, while calcium carbonate and gypsum powder play key roles in the consolidation and shaping of the material. By adjusting the proportions of these three components and the amount of water added, a simulated material is created that closely matches the mechanical properties of actual rock layers, providing a reliable material foundation for the experimental model. To reduce experimental errors, the river sand is pre-treated by sieving, using screens with different mesh sizes to control its particle size and ensure it meets the required standards. Next, river sand, calcium carbonate, and gypsum powder are accurately weighed according to the pre-designed proportions. After thorough mixing in a mixing tank, the materials are added layer by layer to the model stand and compacted using a compaction hammer. Due to the limited height of the model stand, which cannot fully simulate the entire thickness of the geological layers, a pressure pump is installed at the top of the model. By adjusting the applied pressure, the missing overburden weight is simulated, compensating for the lack of overlying stress in the model and ensuring the accuracy and realism of the experimental environment. During the model setup, CFSSM measurement lines were laid within the rock layers No. 4, No. 6, and No. 8. Specifically, one line was placed in each layer No. 4 and No. 6, while three lines were positioned at the top and bottom of the layer numbered No. 8. Copper rods were installed at 10 cm intervals within each monitoring layer to serve as electrodes. By observing changes in resistance between these electrodes, microfracture monitoring in localized areas can be effectively achieved. During the mining process in the model, a photogrammetry system was used to record the movement of the overburden above the working face, while the resistance of the CFSSM was monitored using a digital LCR bridge. The detailed arrangement plan is illustrated in Figure 9.