## 2 Applications of RIM
### Geotechnical Engineering: Solid Soil Particle-Groundwater Interaction  
In the field of civil engineering, the measurement of the displacement field of granular media enables the understanding and prediction of dynamic forms of soil movement, which in turn facilitates the informed decision-making of engineers regarding displacement trends. Conventional soil measurement techniques frequently necessitate the removal of undisturbed soil samples for analysis, yet the opacity of the soil remains a persistent challenge that cannot be readily resolved. Transparent soil has been a subject of considerable interest in geotechnical engineering due to its unique advantages in visualizing soil behavior during various engineering processes. The introduction of transparent geotechnical experiment technology not only enables the visualization of deformation and seepage processes within rocks and soil but also eliminates the need for direct contact between measuring equipment and soil [51; 52]. Transparent soil models that replace real soil can be viewed as the interaction and dynamic changes between solid soil particles and groundwater under different engineering situations. It serves as a surrogate for natural geotechnical materials and has been instrumental in advancing the understanding of complex interactions between soil and structures, as well as in the study of soil deformation and failure mechanisms. Consequently, this approach offers distinct advantages including minimal interference with the experimental process, cost-effectiveness, reliable repeatability, high precision, and straightforward device operation. Different kinds of geotechnical transparent soils have been prepared in the published literatures [2; 31; 53; 54; 55; 56; 57; 58; 59; 60]. The pioneering work of [PERSON] and [PERSON] [2] laid the foundation for subsequent research in the field of transparent porous media with permeabilities and porosities similar to those of soils, aquifers, and petroleum reservoirs. The study by [PERSON] et al. [57], which focused on the preparation and geotechnical properties of transparent soil, is particularly noteworthy. They detailed the creation of a transparent soil with clay-like properties by mixing 6% tuned silica with a blend of 70% paraffin and 30% white spirit. This material, with its meticulously reported properties such as density, consolidation coefficient, and undrained shear strength, ensures reproducibility and reliability in experimental setups. Their work significantly contributes to the development of transparent soil models for geotechnical applications. Additionally, [PERSON] and [PERSON] [58] examined LAPONITE RD(r), a synthetic smectic clay, highlighting its unique properties as a transparent soil surrogate. This synthetic clay, when hydrated and consolidated, forms a transparent slurry that allows for non-intrusive observation of soil behavior within physical models. Their research provides valuable insights into the macroscopic geotechnical properties of synthetic clay, demonstrating its potential as a soft clay surrogate for geotechnical physical model testing, especially in offshore geotechnics. [PERSON] and [PERSON] [53] compared the boundary soil displacement fields under a model footing with those from a natural soil model to investigate the modeling capacity of transparent soil. A digital image correlation (DIC) technique was employed to quantify soil deformation, which revealed that transparent soil can be utilized to investigate natural soil, albeit with certain constraints. The study demonstrated the potential of transparent soil for advanced three-dimensional deformation measurements in soil-structure interaction problems. Similarly, [PERSON] et al. [54] conducted an experiment on the displacement field of layered soils surrounding laterally loaded piles using transparent soil. Two particle sizes (0.1-0.5 mm and 0.5-1 mm) of saturated transparent soil were achieved by mixing glass sand and transparent pore solution.