# Experimental Study on Grouting Visualization of Cover Layer Based on Transparent Soil
## 2 Visualization Grouting Test System
### Transparent Test Chamber  
Considering the boundary effects in the model test, a preliminary grouting test was conducted to determine the slurry diffusion distance, \(d\), as 130 mm, aiming to minimize the boundary-induced interference in the diffusion process. Accordingly, the length and width of the grouting model box were set to 700 mm (>5 \(d\)), and the thickness to 80 mm, considering the space requirements for laser irradiation and transparent soil filling. The model box was constructed using acrylic glass. To enhance the overall strength of the  
Figure 1: Grading characteristics of transparent soil particles: (**a**) Basic particle size of fused quartz sand. (**b**) Schematic diagram of particle skeleton distribution.  
Figure 2: Transparent soil material configuration process.  
model box and ensure its stability and sealing under varying grouting pressures, a clamp frame was installed around the exterior. The frame consisted of four \(\angle\)30 \(\times\) 4 mm angle steel columns, welded perforated plates, and was clamped together using 12 long bolts. A 3 mm thick rubber pad was placed between the frame and the model box to provide cushioning. The structure of the model box is illustrated in Figure 3.  
Due to the wide particle size distribution of the cover layer, with maximum particle sizes exceeding 1 m, conducting indoor grouting tests using prototype materials would present insurmountable challenges in model box construction, sample preparation, test control, and result accuracy, while also significantly increasing costs. In this study, a geometric scaling method was adopted for test design, with a similarity ratio \(S_{\text{L}}\) = 10 (i.e., a model size of 0.7 m corresponding to a prototype of 7 m), to simulate the slurry diffusion characteristics in real-world cover layers. The current scale model primarily satisfies geometric similarity and partial physical similarity. However, due to the small size of the model, slurry diffusion may be more significantly influenced by boundary effects and wall friction--especially near the boundaries, where atypical diffusion patterns are more likely to form. In addition, the model does not fulfill mechanical similarity requirements (such as stress fields and fluid inertia effects).  
Although the geometric scale method can bring convenience to the research, it is undeniable that different physical quantities cannot meet the similarity criteria at the same time (such as the gravity, slurry viscosity, and geometric scale ratio are different in this test). In addition, in the grouting test, minor factors such as the surface tension and particle roughness of the slurry under small size may dominate the slurry diffusion results. On the other hand, if the small error of the model is scaled up, it may lead to the deviation of the prototype prediction. Therefore, the experimental study in this paper is limited to the analysis of the diffusion pattern of slurry and the rule of visual flow in the overburden layer, and does not involve the influence of the size scale effect of the grouting model. The influence of the model scale effect on slurry diffusion will be the focus of the follow-up research.