## 3 Development of Fluid-Solid Coupling Similar Materials  
### Selection of the Similar Materials  
Fluid-solid coupling similar materials generally use organic coagulation materials as a binder to ensure the stability of its physical and mechanical strength and to ensure that its percolation water environment does not collapse [15; 16; 17; 18; 19; 20]. The geometric similarity ratio of the similarity model is 1:100, and the physics of the similar materials of the aquifuge layers are determined according to the fluid-solid coupling similarity theory of the homogeneous continuum. The requirements of the strength and hydrologic parameters are: the compressive strength range is 0.08-0.5 MPa, the permeability coefficient is \(10^{-7}\)-\(10^{-6}\) cm/s, and the water resistance is high. On the basis of previous studies of the fluid-solid coupling of similar materials, in this paper, paraffin wax and petroleum jelly were chosen as the waterproof cementing agents, river sand as the basic aggregate, and calcium carbonate powder and gypsum were added as auxiliary cementing agents to meet the requirements of the simulation of the seepage of underground water during the mining process. The components of the fluid-solid coupling of similar materials are shown in Figure 1.  
1. Aggregate: It directly affects certain parameters, such as the bulk density and physical strength of similar materials, and the property stability of the aggregate has a considerable impact on the properties of the similar materials. Therefore, this experiment selected river sand, with relatively stable physical and chemical properties, as the basic aggregate, having a particle size range of 0-2 mm and uniform grading.
2. Cementing agent: Its main function is to bond aggregates and to create impermeability. The mechanical strength and hydration strength of the auxiliary cementing agent largely determine the mechanical strength and plastic deformation of the similar materials. The hydrophobicity and flow ability of the waterproof cementing agent comprehensively affect the hydraulic parameters and plastic deformation of the similar materials. Based on the investigation of the properties of the main impermeable and waterproof cementing agents (such as paraffin wax, petroleum jelly, mineral oil, and silicone oil) and auxiliary cementing agents (such as gypsum, calcium carbonate, cement, and graphite), it was found that gypsum is a hygroscopic expansion regulator, calcium carbonate is a good cementing brittleness agent, and petroleum jelly is a chemically inert and hydrophilic waterproof agent, with the characteristics of not easily mixing with water. Solid paraffin wax at a regular temperature is also chemically inert and has excellent non-hydrophilicity and adhesiveness. When it is combined with petroleum jelly to bind river sand aggregate particles, it can disperse water better, and it can easily form a crack joint surface under plastic deformation. The water resistance of similar materials is improved by adding waterproof materials, which can change the crystalline porosity and solubility of the materials and prevent the invasion of water molecules.  
In order to meet the performance requirements of similar materials in this simulation experiment, calcium carbonate powder and gypsum were selected as the auxiliary cementing agents, and paraffin wax and petroleum jelly were selected as the main waterproof cementing agents. The characteristics of the selected cementing materials were as follows: No. 58 full-refined paraffin wax; medical-grade white petroleum jelly (Vasculine); calcium carbonate powder with heavy calcium carbonate and a fineness higher than 1250 mesh; and super-high-quality gypsum powder, with a fineness higher than 350 mesh.