## 2 Transparent Geotechnical Test
### Test Materials  
Currently, the primary aggregates for transparent geotechnical similar materials include fused silica sand, amorphous silica powder, amorphous silica gel, and epoxy resin. Fused silica sand, in particular, is produced by melting natural high-purity silica in an electric furnace at temperatures exceeding 1760 \({}^{\circ}\)C, followed by rapid cooling. Its refractive index is essentially linearly related to its density; as the density of the quartz sand increases, its refractive index gradually rises.  
The product obtained after crushing or the further ultrafine grinding of fused silica is known as fused silica sand or fused silica powder. Silica gel powder is characterized by its high purity, high transparency, low coefficient of thermal expansion, strong chemical resistance, stable physical and chemical properties, and strong adsorption capacity. Its specific gravity is generally around 2.2 g/cm\({}^{3}\), and its refractive index ranges between 1.41  
Figure 5: Test setup and model details. (**a**) Schematic diagram of the transparent rock model test system; (**b**) picture of the test setup.  
Figure 6: Diagrams of artificial spot surface.  
and 1.46. The configuration of the pore liquid often employs the \"solution 1 + solution 2\" method, which involves combining two solutions with different refractive indices. This approach mixes the solutions at a specific ratio to achieve a refractive index similar to that of the aggregate. For example, the refractive index of the pore liquid and amorphous silica powder can be matched by mixing n-dodecane and liquid paraffin in a volume ratio of 2.5:1.  
In this test, it is assumed that the tunnel is located in a stratum composed solely of siltstone. Building upon previous research findings in transparent similar materials, transparent rock configured with epoxy resin (as the aggregate), a curing agent, and an accelerator exhibits higher transparency. However, it also shows poor anisotropy, higher plasticity, and low brittleness. Transparent soil configured with amorphous silica powder (as the aggregate), n-dodecane, and liquid paraffin possesses properties that are primarily similar to those of clay. On the other hand, transparent clays made from fused silica sand (as the aggregate) combined with n-dodecane and liquid paraffin, or fused silica sand with a calcium bromide solution, exhibit properties mainly akin to sandy clays. In this study, transparent clays were selected and configured from amorphous silica powder (as the aggregate), n-tridecane, and liquid paraffin. These transparent clays exhibit lower strength and resemble soft rock after consolidation. The basic physical and mechanical parameters of siltstone used in the test are presented in Table 1. The material used in the test and the material intended to be simulated in reality have the basic same physical and mechanical parameters. However, the transparency of the rock formulated with this material is limited. Based on the previous research results, this experiment involved using amorphous silica powder as the aggregate and a mixture of n-tridecane and liquid paraffin as the cementing material. Then, transparent materials were synthesized in a certain proportion to simulate natural soft rock. The effects of different refractive indices are shown in Figure 7. As can be seen from the figure, the number in the picture represents the refractive index; when the refractive index of the transparent similar material is 1.446, the transparency effect is the best.  
Figure 7: Transparent similar materials with varying refractive indices.