# A Comprehensive Review of Open Caisson Modeling Technology: Current Practices and Future Prospects
## 5 Challenges and Future Directions
### Future Directions
#### 5.2.4 Transparent Soil Mass Modeling  
Geotechnical model testing primarily monitors the internal deformation of the model using pre-embedded sensors that provide point-specific data. However, this method has limitations in capturing continuous stress and strain distributions over lines or surfaces. As a result, there is an urgent need for the development of non-intrusive geotechnical models [189] that may observe the continuous deformation of rock and soil without the requirement for sensor implantation, which is intrinsically limited in non-transparent materials. While it is possible to monitor deformation at the interface between the soil and the model box by replacing the box sidewalls with transparent glass, this method does not provide information about the soil's interior deformation. The idea of transparent soil had emerged as a result of this limitation. Transparent soil not only ensures the geotechnical attributes of soil, but also allows for the visualization of the internal deformation. A newly developed sort of material for model research is transparent soils [190]. Finding an analogous material for transparent soil has emerged as an innovative field of study for natural soil simulation. About 40 years ago, the study of transparent soil was initiated. Initially, [PERSON] [191] started with broken glass pieces as a transparent material to investigate the stress and strain distribution of materials under shear conditions. Subsequently, similar materials [192] were employed to analyze the stress variations in saturated dykes subjected to seismic loadings. However, due to the lack of absorption capacity in broken glass, it was impossible to remove the entrapped air from the glass voids. This limitation caused poor transparency and an inability to accurately replicate the strength and deformation characteristics of natural soils. To address these challenges, [PERSON] et al. [193] proposed the use of consolidating suspensions of amorphous silica as transparent soils which were synthesized by a pore fluid with a matched refractive index. Later on, [PERSON] et al. [194] investigated that amorphous silica and transparent silica gels have macroscopic geotechnical properties comparable to natural sands and clays, making them suitable for simulating soil behavior in model studies. Transparent soil is a two-phase medium composed of synthesized aggregate and fluid parts with matched refractive indices (RIs), intended to create optical transparency in the concluding soil [168]. For the successful manufacturing of a transparent soil that allows the entire penetration of visible light, it is essential that the refractive indices (RIs) of aggregate materials and pore fluids are similar [195; 193]. This transparency enables the direct visualization of soil behavior and continuum deformation in experimental models. When integrated with innovative photogrammetry and image processing techniques, transparent soils facilitate the accurate measurement of the spatial deformation, displacement and multi-phase flow in experimental model tests. These manufactured soils have been effectively utilized in geotechnical model tests, as their measured geotechnical properties replicate those of natural soils, making them a viable solution for simulating prototype behavior. The most commonly used aggregate materials for transparent synthetic soils include silica gel, amorphous silica powder, fused silica, fused quartz, aqua beads and gel beads. These constituents are matched with various fluids or fluid blends including white oil, mineral oils, parafhinic oil, brine mixture, sucrose solution and water. Previous studies have extensively explored the geotechnical properties of these transparent soils and verified that their properties align closely with those of natural soils [195; 196; 197; 198; 199]. Nowadays, transparent soils are widely utilized in laboratory settings to simulate soil-structure interaction problems, [200; 201; 202; 203] including soil deformation measurements [204; 205; 206], and tunnelling-induced movements [207; 208].