# Experimental Study on Grouting Visualization of Cover Layer Based on Transparent Soil
## 1 Introduction  
Grouting technology, a principal method for improving the mechanical behavior of loose rock and soil particles by injecting grout into their voids, is extensively employed in civil engineering, water conservancy, mining, transportation, and related fields to enhance stability, bearing capacity, and impermeability. Owing to its adaptability to complex geological conditions, flexible construction procedures, and precise controllability, grouting has become the core technique for seepage control and reinforcement of cover layers ([PERSON] et al., 2022) [1]. Investigating visualized slurry-diffusion methods within the cover layer not only deepens our understanding of the cover layer reinforcement mechanisms but also plays a vital role in optimizing grouting quality and performance. Grouting tests are an important method for studying grouting mechanisms. Currently, many researchers have conducted extensive experimental studies on the grouting process([PERSON] et al., 2019) [2], slurry material properties ([PERSON], 2005) [3], stratigraphic characteristics ([PERSON] et al., 2017) [4], grouting scheme design etc. ([PERSON] et al., 2014) [5], in order to analyze the hydraulic behavior of grouting in the cover layer. Due to the invisibility of natural rock and soil layers, it is difficult for the traditional grouting test to directly observe the diffusion process of slurry in soil. Even with numerous sensors, technical challenges remain, including limited spatial resolution, lack of temporal continuity, and difficulties in the coupling and decoupling of multiple fields ([PERSON] et al., 2022) [6]. With the rapid development of transparent soil testing technology, visualizing slurry diffusion in the cover layer has become achievable. Researchers have conducted extensive studies on the physical properties and geotechnical engineering applications of transparent soil materials. Through the triaxial consolidation undrained tests and direct shear tests on transparent glass sand and standard sand, it was found that under the same gradation, their shear strengths were similar, indicating that transparent glass sand can effectively simulate natural sand ([PERSON] et al., 2016) [7]. By setting visible particles in transparent soil slope, the instability process caused by internal deformation of transparent soil slope is successfully simulated ([PERSON] et al., 2019) [8]. In studying the seepage characteristics of transparent soil materials, the fluid-solid interaction process in debris flow was simulated by using transparent particles and corresponding mixed oil ([PERSON] et al., 2012) [9]. Based on the self-developed transparent soil permeameter, the piping mechanism of non-cohesive soil was studied by using glass sand, fluorescent agent, and mineral oil with different gradations ([PERSON] et al., 2017) [10]. Transparent sand was prepared with fused quartz particles and mixed mineral oil, and the feasibility of grouting diffusion visualization was verified by a simple grouting test ([PERSON] et al., 2024) [11]. The visualization of the chemical slurry infiltration grouting process in a transparent soil model was completed by using a laser and a CCD camera. The research showed that the chemical slurry was \"pear-shaped\" diffusion under the action of gravity; that is, the vertical diffusion distance was greater than the horizontal diffusion distance ([PERSON] and [PERSON], 2016) [12]. At present, transparent soil technology has been gradually applied to grouting test research. However, the existing research on grout in transparent soil mostly uses dye pigments as diffusion tracer materials, which makes it difficult to truly reflect the flow characteristics of Bingham fluid represented by cement grout in the overburden. In addition, due to the size of the model box, the slurry diffusion is easily affected by the boundary effect, which leads to the deviation between the research results and the actual situation, and most studies only qualitatively analyze the diffusion form of slurry in transparent soil.