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Review

Material Systems and Applicability Evaluation of Transparent Soil: Toward Transparent Model Testing in Geotechnical Engineering

by
Shifu Wang
1,2,
Changxing Zhang
1,2,*,
Biao Xia
3,
Meiqian Wang
1,2,
Zhiyi Tang
1,2 and
Wei Xu
1,2
1
Faculty of Civil Engineering and Mechanics, Kunming University of Science and Technology, Kunming 650500, China
2
Intelligent Infrastructure Operation and Maintenance Technology Innovation Team of Yunnan Provincial Department of Education, Kunming University of Science and Technology, Kunming 650500, China
3
School of Intelligent Manufacturing, Changde University, Changde 415000, China
*
Author to whom correspondence should be addressed.
Infrastructures 2026, 11(7), 212; https://doi.org/10.3390/infrastructures11070212 (registering DOI)
Submission received: 22 May 2026 / Revised: 14 June 2026 / Accepted: 22 June 2026 / Published: 24 June 2026
(This article belongs to the Special Issue Advanced Sensing Technologies and Smart Construction Materials for Structural Health Monitoring)
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Versions Notes

Abstract

Transparent soil technology provides a non-invasive experimental approach for visualizing internal processes in geotechnical infrastructure systems, where soil deformation, seepage, erosion, and failure evolution are often difficult to observe using conventional model tests. This review examines the material systems and applicability of transparent soil with emphasis on infrastructure-related applications, including foundation engineering, underground construction, seepage and grouting, internal erosion, slope failure, disaster mitigation, and thermal monitoring. The discussion focuses on transparent sand and transparent clay, comparing their engineering relevance, typical application scenarios, and main limitations rather than treating transparency as the sole criterion for material selection. Based on the reviewed studies, a four-dimensional applicability framework is proposed, consisting of mechanical similarity, optical measurability, system compatibility, and scenario matching. This framework is used to clarify how transparent soil can support mechanism interpretation, model calibration, and scheme comparison in infrastructure-related geotechnical experiments. The review indicates that transparent soil is particularly useful for revealing displacement fields, flow paths, localized deformation, and progressive failure processes in foundations, tunnels, slopes, and other geotechnical systems. However, direct extrapolation of model test results to engineering design parameters remains constrained by material equivalence, optical measurement conditions, model scale, and similarity calibration. Overall, the proposed framework and synthesis provide a systematic reference for transparent soil material selection, infrastructure-oriented scenario matching, and the assessment of applicability boundaries in transparent soil model tests.

1. Introduction

Geotechnical engineering problems in infrastructure systems commonly involve internal processes that are difficult to observe directly, strong material heterogeneity, and significant multi-field coupling. These problems are particularly relevant to foundations, tunnels, retaining structures, embankments, slopes, and underground utilities, where subsurface deformation, seepage migration, and localized failure may directly affect construction safety, serviceability, and long-term infrastructure performance. Recent studies on geotechnical materials have further shown that internal fracture, pore-structure evolution, and strength degradation can be strongly affected by loading conditions, defect geometry, and fluid–solid interactions [1,2]. The evolution of internal displacement, the migration of seepage paths, and the formation of local failure zones in soils have long been difficult to capture effectively. Conventional physical model tests are limited by their “black-box” nature and usually provide only external responses or data from a few discrete measurement points. Internal observation methods, such as X-ray imaging [3,4,5], computed tomography (CT) [6,7,8], and magnetic resonance imaging (MRI) [9], can be used for visualization studies. However, they are costly, operationally complex, and limited by specific application conditions. Conventional optical measurement methods are mainly applied to soil surfaces and cannot continuously characterize internal responses. Therefore, non-invasive acquisition of internal soil processes under controlled test conditions has remained a key issue in geotechnical experimental research. The development of transparent soil technology provides a new experimental approach to overcome the difficulty of observing internal processes in conventional model tests.
Transparent soil is not a natural transparent geomaterial. It is an artificial porous medium constructed through refractive-index matching. Its basic principle is to select transparent or semi-transparent solid particles and prepare a pore fluid with a similar refractive index. This reduces light scattering at the particle–pore fluid interface and gives the model soil optical visibility within a certain thickness. On this basis, image-based measurement techniques, such as laser slicing, digital image correlation (DIC) [10,11,12,13,14,15,16,17], particle image velocimetry (PIV) [18,19], and planar laser-induced fluorescence (PLIF) [20], can be used to obtain internal displacement fields, flow paths, and local deformation zones. With continuous development, transparent soil has evolved from early material exploration into an integrated experimental system involving material preparation, optical observation, image processing, and model testing. Its applications have also expanded from traditional problems, such as pile–soil interaction and underground engineering, to grouting diffusion, seepage erosion, slope hazards, thermal monitoring, and interdisciplinary fields related to environmental and biological processes [21,22,23,24,25]. Recent studies published in the past three years have further strengthened both material development and observation methods in transparent soil testing. For example, recent work has summarized visualization experiment technologies based on transparent geotechnical materials and their engineering applications [26]. Transparent soil has also been combined with PLIF and PIV to quantify internal pore structures, flow paths, and three-dimensional soil structures [27]. In addition, new transparent cemented soil materials have been developed and evaluated in terms of physicomechanical characteristics, mix proportions, and similarity requirements [28]. These advances indicate that transparent soil research is moving from qualitative visualization toward material-parameter control, multi-field imaging, and more quantitative scenario-oriented model testing. Many of these applications are closely associated with the construction, operation, maintenance, and risk mitigation of infrastructure systems, especially when internal soil responses control the performance of foundations, underground structures, slopes, and seepage-related protection works. Therefore, the core value of transparent soil technology does not lie in obtaining transparent specimens themselves. Rather, it lies in visualizing internal displacement, flow paths, and local failure processes under specific scales and observation conditions. This provides key evidence for mechanism analysis, model calibration, and scheme comparison.
In recent years, review studies on transparent soil have summarized this technology from the perspectives of materials, optics, and applications. However, most existing reviews focus on a single main topic, such as material and physicomechanical properties, optical measurement and image processing, or applications in specific engineering scenarios [29]. In contrast, a unified discussion of material selection, scenario matching, and the boundaries of engineering extrapolation remains insufficient, particularly for infrastructure-oriented geotechnical applications where model observations are expected to support mechanism interpretation, construction assessment, and risk analysis. Existing studies have shown that limitations in material equivalence, constraints of transparency on model scale, insufficient stability in speckle preparation and imaging, and weak standardization of testing systems can all affect the comparability of results among different studies and their applicability to engineering analogy [30,31,32]. Thus, the focus of transparent soil research has shifted from whether transparency can be achieved to whether the material is comparable, interpretable, and extrapolatable under specific working conditions.
Based on this background, this paper presents a systematic review of transparent soil material systems and their applicability evaluation in infrastructure-related geotechnical research. First, the main material routes and basic characteristics of transparent sand and transparent clay are summarized. Second, typical application scenarios are reviewed, including foundation engineering, underground engineering, seepage, grouting, internal erosion, slope stability, disaster prevention and mitigation, and thermal monitoring. On this basis, a four-dimensional analytical framework is proposed, consisting of mechanical similarity, optical measurability, system compatibility, and scenario matching. This framework is then used to discuss the applicability boundaries of different transparent soil materials under specific test scales and observation conditions. This paper aims to shift the focus from “whether the material is transparent” to “whether the material is applicable,” and to provide a systematic perspective for material comparison, infrastructure-oriented scenario selection, and applicability assessment of transparent soil model tests.

2. Material Systems and Basic Characteristics of Transparent Soil

Transparent soil is not a single material. It is an artificial porous medium composed of solid particles, pore fluid, and their refractive-index matching relationship. Different material routes vary in particle composition, structure-forming mechanisms, pore fluid types, mechanical responses, and optical performance. Their applicable target soils also differ. Existing transparent soil materials mainly include transparent sand and transparent clay. The former is mainly used to simulate the packing, friction, and dilatancy of granular media. The latter is more often used to simulate the compression, consolidation, and seepage characteristics of fine-grained soils. Some studies have also included transparent rock-like materials [33,34,35], transparent cemented soils, and soil–rock mixtures [36,37,38] in a broader category of transparent geomaterials. However, this review focuses on the applicability comparison of transparent soil materials. Therefore, the following discussion is mainly organized around two main systems: transparent sand and transparent clay.

2.1. Transparent Sand System

Transparent sand should meet two basic requirements. It should provide optical visibility and approximate the fundamental mechanical responses of sandy soils. Among existing materials, fused quartz sand is the most widely used and well-validated route. Other materials, such as silica gel particles, 3D-printed transparent particles, hydrogel beads, and aquarium beads, are mainly used for specific problems or visual demonstration. The differences among these materials are mainly reflected in particle strength, shape controllability, transparency stability, and equivalence to natural sand.
Fused quartz sand has few internal pores, high purity, good chemical stability, and a chemical composition close to that of natural siliceous sand [39]. Previous one-dimensional compression tests [40], direct shear tests [41], and triaxial tests [42] have shown that its strength, deformation, and dilatancy characteristics are close to those of natural sand. It is therefore one of the most well-validated transparent sand materials. However, fused quartz sand particles are usually angular. Under certain gradation and confining pressure conditions, they may show a relatively high friction angle or a tendency for local particle breakage. Therefore, its advantage lies mainly in approximating the key behaviors of natural sand, rather than fully replacing all parameters of natural sand.
Other transparent sand materials have more specific uses. Silica gel particles were commonly used in early studies. However, their application in equivalent modeling of general natural sand is limited by their porous structure, hygroscopicity, low particle strength, and tendency to break [43]. The main advantage of 3D-printed transparent particles is that their shape, size, and surface characteristics can be controlled. They are more suitable for studying particle-shape effects and constructing designed specimens [44]. Hydrogel beads and aquarium beads have high transparency and can be used to visually demonstrate particle movement and flow phenomena. However, their strength is low, which makes them unsuitable as equivalent materials for general sandy soils [45]. The applicability characteristics of different transparent sand materials are compared in Table 1.

2.2. Transparent Clay System

Compared with transparent sand, transparent clay has a more diverse material system. Its properties are more strongly affected by particle structure, pore fluid system, and sample preparation method. At present, transparent clay has not yet formed a single dominant route comparable to fused quartz sand. Materials such as amorphous silica, fumed silica, fused silica powder, Laponite RD (BYK-Chemie GmbH, Wesel, Germany), and Carbopol® Ultrez 10 (U10; Lubrizol Corporation, Wickliffe, OH, USA) have all been used. However, their target soils and applicability boundaries differ.
Among silica-based transparent clay materials, amorphous silica, fumed silica, and fused silica powder are representative routes. Amorphous silica has strong adsorption capacity, which helps remove residual gas between particles. Its strength, permeability, and consolidation characteristics are close to those of natural low-plasticity clay. It is therefore one of the well-validated classical materials in transparent clay research [50,51,52].
Fumed silica has finer particles, a larger specific surface area, higher transparency, and stronger compressibility. It is more suitable for modeling highly compressible clay, organic soil, and marine soft clay [53,54].
Fused silica powder has high purity and stable properties. It can be used for low-compressibility silty clay or general clay problems. However, its transparency is easily affected by particle morphology and light scattering. This limits its use in high-transparency or large-scale model tests [55].
In addition to silica-based materials, Laponite RD and U10 represent the development of water-based transparent clay materials. Laponite RD shows clear thixotropy and high compressibility. It can be used for high-water-content, low-strength soft clay or marine sediments. However, its low strength and long consolidation time limit its applicability [56]. U10 is easy to prepare and has high light transmittance. It can form a high-water-content system. However, its high void ratio and low strength limit its use as a substitute for general clay. It is more suitable for specific highly compressible fine-grained soil problems [57]. The applicability characteristics of different transparent clay materials are compared in Table 2.

3. Typical Application Scenarios of Transparent Soil Materials

The application of transparent soil technology has expanded from early pile–soil interaction and underground engineering model tests to seepage erosion, slope hazards, thermal monitoring, and interdisciplinary problems related to environmental and biological processes. Although these scenarios all rely on visualization of internal processes, their research focuses and applicability requirements differ. Foundation engineering focuses on deformation around piles and load transfer. Underground engineering focuses on excavation-induced disturbance, loosening zone development, and ground instability. Seepage, grouting, and internal erosion studies emphasize fluid migration, particle transport, and local erosion. Slope stability and disaster prevention studies focus on sliding failure and hazard evolution. Thermal and interdisciplinary applications further involve temperature, chemical, or biological processes.
Therefore, the selection of transparent soil materials should not be based only on transparency. It should also consider the target soil, observation object, test system, and engineering scenario. The following sections summarize the application characteristics and main applicability concerns of transparent soil materials in typical scenarios.

3.1. Foundation Engineering and Soil–Structure Interaction

Foundation engineering is one of the earliest and most mature application fields of transparent soil technology. Pile–soil interaction is the most representative topic. Existing studies mainly focus on single-pile bearing behavior, penetration disturbance, pull-out response, and local ground improvement. The purpose is not to reconstruct the complete prototype ground. Instead, it is to visualize the internal response of soil around piles, including displacement fields, compacted zones, shear zones, and load transfer paths.
Transparent soil visualization has been combined with mechanical loading, sensor measurements, and image-based displacement tracking to reveal pile–soil interaction mechanisms. For example, Chen et al. [61] used fused quartz sand, displacement meters, strain gauges, and a multi-laser and multi-camera PIV system to compare the lateral bearing behavior of different pile types, obtaining displacement fields around the pile, internal pile forces, and load–displacement responses. Ma et al. [62] designed a transparent soil visualization model test for inclined variable cross-section piles, in which a transparent model box, automatic loading system, laser source, CCD camera, and PIV-based image processing were integrated to capture soil displacement around piles. A representative configuration of such a transparent soil pile-testing system is shown in Figure 1. These studies indicate that transparent soil tests can be used to analyze the effects of pile geometry, pile stiffness, loading mode, and interface interaction on near-pile soil deformation and bearing behavior. Similar methods have also been applied to rigid drainage piles, geogrid-reinforced gravel piles, and post-grouted micropiles [63,64,65,66]. These studies extend the use of transparent soil in construction disturbance, local reinforcement, and bearing mechanism analysis.
From the perspective of applicability, foundation engineering is well matched with transparent soil technology. Deformation around piles is usually localized, and the observation target is clear. PIV/DIC can also be combined with displacement and strain measurements. In terms of materials, fused quartz transparent sand is suitable for sandy foundations and medium- to small-scale single-pile models. Transparent clay can be used for soft soil or layered ground, but its strength, disturbance sensitivity, and imaging stability should be carefully controlled. Current studies still mainly focus on single piles and regular loading conditions. Group piles, cyclic loading, composite foundations, and realistic construction processes remain insufficiently investigated. With increasing model scale, light transmission thickness, boundary effects, and particle size effects may limit result extrapolation. Therefore, transparent soil tests in foundation engineering are more suitable for explaining pile–soil interaction mechanisms and comparing different schemes. They should not be directly converted into engineering bearing capacity parameters.

3.2. Underground Engineering and Excavation Support

Unlike foundation engineering, which focuses on local load transfer, underground engineering is more concerned with the propagation of excavation-induced disturbance in the ground. Existing studies mainly focus on tunnel excavation, shield tunnel face stability, ground deformation, surface settlement, and local instability. The research focus is not the response of a single structure, but the visualization of internal displacement, loosening zone development, and failure mode evolution induced by excavation.
In recent years, transparent soil has been used to study tunnel face stability, excavation-induced disturbance, and progressive failure in shield tunnelling. For example, Zhang and Gu [67] designed a transparent soil model test for longitudinal-slope shield tunnelling using fused silica sand, a refractive-index-matched pore fluid, a model box, a shield model, a laser emitter, a CCD camera, and PIV-based image processing. Their study investigated excavation face instability under different longitudinal slope gradients and setback amounts, and obtained displacement nephograms of the soil in front of the excavation face, as shown in Figure 2. The results showed that the instability process developed from localized inward displacement near the excavation face to a vertically extending failure zone, and that an upward slope condition produced a wider and more developed destabilized region. These observations indicate that transparent soil can clearly visualize excavation disturbance zones, loosening zone development, and instability paths in shield tunnelling. Similar methods have also been applied to tunnel face retreat, large-deformation failure, excavation-induced ground deformation, and deformation in composite strata [68,69,70,71].
From the perspective of applicability, underground engineering is well matched with transparent soil technology, but it places higher demands on the model system and observation conditions. Tunnel excavation usually involves large-range continuous deformation. Light transmission thickness, model box size, laser sheet quality, and image stability can all affect the reliability of the results. Material selection should also correspond to the prototype ground. Transparent sand is commonly used for sandy or gravelly strata. Transparent clay can be used for soft clay or highly compressible strata. For surrounding rock, joints, or hard media, transparent rock-like materials should be considered. Therefore, there is no single optimal material for this scenario. Transparent soil is more suitable for process identification, mechanism analysis, and numerical model calibration in underground engineering. It should not be used directly to provide engineering design parameters.

3.3. Seepage, Grouting, and Internal Erosion

Unlike underground engineering, which focuses on excavation-induced disturbance, seepage, grouting, and internal erosion scenarios are more concerned with fluid migration and the internal responses induced by it. The key issues include the identification of flow paths, velocity distribution, grout diffusion, particle migration, and erosion initiation zones. Transparent soil can directly visualize internal processes in porous media. It therefore has clear advantages in this type of problem.
In recent years, transparent soil has been used to study grout diffusion and fluid migration. For example, Zhang et al. [72] used a transparent clay model test system combined with PIV to investigate grout diffusion and the displacement response of surrounding soil during the construction of grouted gravel piles. Guo and Zhao [73] designed a transparent soil grouting model for cover layers using fused quartz sand, refractive-index-matched pore fluid, stable slurry, a visual grouting device, and a DIC/PIV-based monitoring system. Their study visualized the progressive migration of the slurry front and analyzed the velocity field of transparent soil during grouting. A representative slurry-front diffusion process is shown in Figure 3. The observed process included vertical expansion, horizontal expansion, and slurry overflow, indicating that transparent soil can be used to identify the spatial evolution of grout diffusion and the interaction between slurry migration and soil deformation. In addition to grouting, transparent soil has also been applied to pore flow, seepage erosion, unsaturated seepage, and particle migration [74,75,76]. These studies show that it can be used to identify preferential flow paths, particle loss processes, local erosion zones, and grouting-induced deformation.
From the perspective of applicability, this scenario requires close matching among the material, pore fluid, and time scale. For sandy soils and general granular media, transparent sand remains the main material route. For weak fine-grained soils, highly compressible media, or special fluid migration problems, transparent clay or specially designed materials can be used. The pore fluid system directly affects seepage similarity. Oil-based systems usually provide good transparency, but they have high viscosity and are sensitive to temperature. Water-based systems are closer to real water-mediated behavior, but their stability and operating conditions remain limited. Therefore, transparent soil is more suitable for identifying fluid migration, grout diffusion, and internal erosion mechanisms in this scenario. If pore fluid viscosity, medium permeability, or time scale does not match the actual working conditions, the test results should not be directly extrapolated to engineering practice.

3.4. Slope Stability, Sliding Failure, and Disaster Prevention and Mitigation

Slope stability and disaster prevention scenarios partly overlap with seepage problems, but their research focuses are different. Seepage studies emphasize fluid migration and particle transport. In contrast, slope-related studies focus more on slope deformation and instability under water level changes, loading, or supporting structures. The key issues include identifying hazard initiation locations, slip surface propagation, local failure zones, and the interaction mechanisms between supporting structures and slopes. Transparent soil can visualize internal displacement and failure evolution in slopes. It is therefore suitable for identifying slope hazard processes.
In recent years, transparent soil has been used to study landslides under water level changes, anchored slopes, and anti-slide structures. For example, Zhou et al. [77] used transparent soil and PIV to study the deformation process of a landslide reinforced by umbrella-shaped anchors under rapid reservoir drawdown. Cong et al. [78] used fused quartz transparent sand, PIV, and 3D printing to conduct model tests on single-row and double-row circular anti-slide piles. Wang et al. [79] constructed a transparent soil slope model reinforced by micropile groups and used PIV to investigate the effects of pile arrangement and connecting beams on pile–soil interaction. Their study obtained soil displacement fields, shear strain fields, soil pressure responses, and three-dimensional displacement isosurfaces behind the piles. A representative displacement field of the slope soil reinforced by three-row micropile groups is shown in Figure 4. These studies indicate that transparent soil can be used to visualize slope deformation, soil arching, slip-related displacement, and the interaction between supporting structures and slope soil.
From the perspective of applicability, the main advantage of this scenario lies in the visualization of hazard evolution. Transparent soil can be used to observe the development of slopes from local deformation to global instability. It can also be used to compare responses under different slope angles, water level change rates, support forms, and structural arrangements [80,81,82]. In terms of materials, transparent sand is commonly used for granular slopes, anti-slide pile problems, and erosion-related cases. Transparent clay can be considered for high-water-content weak soils or fine-grained slopes, but related applicability studies remain limited. The main limitations of this scenario arise from large deformation, rapid instability, and complex boundary conditions. Imaging frequency, tracer method, model boundary, and material heterogeneity can all affect result accuracy. Therefore, transparent soil is more suitable for identifying hazard processes and analyzing support mechanisms in slope stability and disaster prevention studies. Without field monitoring or numerical validation, the results should not be directly extrapolated as slope stability parameters.

3.5. Thermal Monitoring and Interdisciplinary Applications

Beyond traditional scenarios such as foundation engineering, underground engineering, and seepage erosion, transparent soil technology has also been extended to thermal monitoring and interdisciplinary applications. These studies are no longer limited to displacement or flow fields. They further focus on internal responses such as temperature fields, thermo–hydro–mechanical coupling, contaminant transport, and biological processes. Their core value lies in using optical changes or visualization features of transparent soil to obtain internal field information that is difficult to capture continuously with conventional sensors.
Thermal monitoring is a representative application in this direction. For example, Y. Zhou et al. [83] used the temperature-dependent refractive index of pore fluid in transparent sand, which changes soil transparency with temperature. They established a relationship between pixel intensity and soil temperature and conducted two-dimensional model tests on energy piles to analyze the thermal interference process between two piles. Li et al. [84] further developed a non-contact visualization method for soil temperature fields based on transparent soil and digital image processing. They calibrated the relationship between normalized pixel intensity and temperature, visualized the temperature distributions around single and double energy piles under different pile spacings, and introduced a thermal interference coefficient to quantify the interaction between adjacent piles, as shown in Figure 5. Their results showed that thermal interference decreased significantly with increasing pile spacing; when the spacing increased to 6D, the thermal interference coefficient decreased to approximately 2.5%. These studies show that transparent soil can visualize temperature fields through image–temperature calibration. In addition to thermal monitoring, transparent soil has also been used in plant root growth and other interdisciplinary visualization applications [85,86]. These studies indicate that its application boundary is expanding from traditional geomechanics to multi-field coupling and long-term evolution processes.
From the perspective of applicability, thermal monitoring and interdisciplinary applications are still at the stage of methodological exploration. This direction places greater emphasis on material functionalization and integrated adaptation of the observation system. Material selection is jointly affected by temperature range, chemical environment, fluid type, biocompatibility, and long-term stability. Changes in temperature or chemical conditions may alter refractive-index matching and affect transparency and imaging quality. Therefore, transparent soil is more suitable as a platform for mechanism exploration and method development in this scenario. Related results can be used for thermal field analysis and model calibration, but they should not be used alone to determine engineering design parameters.
Overall, the application of transparent soil materials has expanded from traditional geotechnical model tests to multi-field coupling and interdisciplinary problems. Foundation engineering and underground engineering are relatively mature core scenarios. They are mainly used to identify internal soil displacement, load transfer, and excavation-induced disturbance. Seepage, grouting, internal erosion, and slope disaster prevention place greater emphasis on fluid migration, particle transport, sliding failure, and hazard evolution. Thermal monitoring and environmental or biological applications further reflect the functional extension of transparent soil technology.
Different scenarios impose different requirements on materials and test systems. Foundation engineering places more emphasis on strength stability and local displacement field identification. Underground engineering is more strongly limited by light transmission thickness, model scale, and system integration. Seepage and internal erosion problems depend on pore fluid properties and seepage similarity. Slope hazard studies require reliable tracking of large deformation processes. Interdisciplinary applications also need to consider long-term stability under thermal, chemical, and biological conditions.
To clarify the scenario-specific requirements for transparent soil material selection and test design, the key observable processes, material parameters, and experimental controls in different application scenarios are summarized in Table 3. Since the target values of these parameters depend on the prototype soil, model scale, pore fluid system, and testing purpose, Table 3 does not assign fixed numerical ranges for each scenario. Instead, it identifies the key material parameters and experimental controls that should be calibrated or verified before applying transparent soil materials to a specific scenario.

4. Applicability Evaluation Framework for Transparent Soil Materials

4.1. Basic Logic of Applicability Evaluation

The applicability of transparent soil materials should not be understood simply as whether the material is transparent nor should it be determined by a single mechanical index. A more reasonable criterion is whether the material can reliably visualize the key processes of the target engineering problem under specific test scales and observation conditions. It should also provide reliable evidence for mechanism analysis, model calibration, or scheme comparison.
As shown by the material systems and application scenarios discussed above, the effectiveness of transparent soil testing is jointly controlled by material properties, optical conditions, measurement systems, and engineering problems. If the mechanical behavior of the material differs greatly from that of the target soil, effective modeling is difficult, even with high transparency. Conversely, if the material has mechanical properties close to those of natural soil but has insufficient light transmission thickness or unstable image quality, reliable internal information cannot be obtained. Therefore, applicability evaluation of transparent soil materials should shift from single material-property assessment to integrated assessment of material, observation, system, and scenario.
Based on this understanding, this paper summarizes the applicability of transparent soil materials into four dimensions: mechanical similarity, optical measurability, system compatibility, and scenario matching. These four dimensions address four key questions: whether the material behaves like the target soil, whether the internal process can be clearly observed, whether the process can be accurately measured, and whether the material can serve the target scenario. Together, they determine the applicability of transparent soil materials in specific engineering scenarios.

4.2. Evaluation Dimensions and Main Criteria

The four evaluation dimensions are not independent single indices. Instead, they jointly act on the complete chain of test results. Mechanical similarity determines whether the material can approximate the main response of the target soil. It is the basis of applicability evaluation. Optical measurability determines whether internal processes can be clearly and stably captured. It is the key feature that distinguishes transparent soil technology from conventional model tests. System compatibility concerns the coordination among the material, model box, light source, camera, speckle or tracer system, and image-processing algorithm. It directly affects the reliability of the measurement results. Scenario matching further defines the engineering significance of material selection and test conclusions.
The importance of each dimension varies among application scenarios. For example, foundation engineering places greater emphasis on mechanical similarity and identification of displacement fields around piles. Underground engineering is more strongly limited by light transmission thickness and model scale. Seepage and internal erosion problems depend more on pore fluid properties and seepage similarity. Slope hazard studies require reliable tracking of large deformation processes. Therefore, applicability evaluation should not rely on good performance in only one dimension. Instead, it should assess whether the four dimensions together can support the target problem. The main criteria of the four dimensions are listed in Table 4.

4.3. Evaluation Procedure and Applicability Classification

The applicability evaluation procedure for transparent soil materials is shown in Figure 6. This procedure is guided by the four-dimensional framework of mechanical similarity, optical measurability, system compatibility, and scenario matching. It can be summarized as two stages: problem definition and integrated calibration, followed by scenario-based validation and applicability classification. Parameter optimization and scheme iteration are also included in this procedure.
The first stage includes prototype problem definition, evaluation scope determination, and integrated calibration of key indicators. First, the target soil type, engineering scenario, dominant mechanism, and observation objective should be clarified. Examples include displacement fields around piles, tunnel face instability, grout diffusion, and slope sliding processes. The evaluation scope should then be defined. This includes target material properties, key processes, observation objects, and test purposes. Based on this, key indicators should be selected according to the four-dimensional framework. The dominant evaluation dimensions that require priority control should also be identified. For sand-related problems, particle characteristics, strength, and dilatancy should be calibrated. For clay-related problems, compressibility, consolidation behavior, and permeability should be emphasized. For seepage or thermal problems, pore fluid viscosity, temperature sensitivity, or time scale should also be calibrated. If the dominant criteria are not satisfied, the material proportion, refractive-index matching, sample preparation method, imaging conditions, or observation system should be adjusted. The procedure should then return to the calibration step for further optimization.
The second stage includes scenario-based validation, extrapolation boundary determination, and applicability classification. After preliminary calibration, the transparent soil material should be tested in representative model experiments. This step examines whether the target process can be captured stably. It also defines the applicable model scale, boundary conditions, and engineering extrapolation range of the test results. If the material and observation system can reliably identify the target process within the valid extrapolation boundary, applicability classification can be performed. If not, the procedure should return to parameter optimization and scheme iteration. Based on this process, the applicability of transparent soil materials can be divided into three categories. If the material is generally coordinated across the four dimensions and can stably reflect the target process, it can be classified as having high applicability. If the material is consistent with the target problem only in the main trend and still requires scale correction, system compensation, or boundary limitation, it can be classified as conditionally applicable. If the material performs well in one dimension but shows clear mismatch with the target soil, observation object, or engineering scenario, it should be classified as having limited applicability.

4.4. Evaluation of Representative Material–Scenario Combinations

Based on the material characteristics, typical application scenarios, and four-dimensional evaluation framework discussed above, several representative material–scenario combinations are selected for illustrative evaluation, as shown in Table 5. Table 5 is not intended to provide an absolute ranking of transparent soil materials. Instead, it illustrates how the four-dimensional framework can be applied. In actual tests, applicability levels may change with material proportion, pore fluid system, model scale, sample preparation method, imaging conditions, and observation target. Therefore, the evaluation results in Table 5 should be understood as qualitative judgments under typical conditions. They cannot replace material calibration and scenario-based validation in specific experiments.
In Table 5, “High” and “Moderate” indicate the relative matching degree between each evaluation dimension and the target scenario. “High” means that the overall match is good. “Moderate” means that the requirement is basically satisfied, but the result is still limited by material properties, model scale, pore fluid system, sample preparation method, or imaging conditions. The overall judgment is divided into high applicability, conditional applicability, and limited applicability according to the coordination of the four dimensions and the results of scenario-based validation. This table provides an illustrative evaluation. Specific applicability should still be further calibrated according to actual test conditions.

5. Existing Challenges and Future Directions

Transparent soil technology has shown strong capability for process visualization in foundation engineering, underground engineering, seepage erosion, and slope hazards. However, from the perspective of applicability, existing studies still face limitations in material equivalence, test standardization, scale extrapolation, and complex scenario simulation. These issues indicate that transparent soil tests are currently more suitable for providing mechanistic evidence and model calibration support, rather than directly replacing prototype engineering parameters.

5.1. Insufficient Material Equivalence and Scenario Adaptation

Differences still exist between transparent soil materials and natural soils. Transparent sand cannot fully reproduce the particle shape, gradation, dilatancy, and breakage characteristics of natural sand. Transparent clay is more strongly affected by the pore fluid system, sample preparation method, compressibility, and thixotropic behavior. Therefore, the fact that a transparent soil material can be prepared does not mean that it can replace natural soil. Its applicability should still be judged according to the target soil and the target process.
Future material development should shift from “general transparent materials” to “target-oriented material design.” For sand-related problems, particle morphology, strength level, and dilatancy should be made more controllable. For clay-related problems, the matching of compressibility, consolidation behavior, and permeability should be improved. For special soils, layered soils, rock-like materials, or soil–rock mixtures, more targeted transparent geomaterial systems are needed. Material optimization should not focus only on transparency. It should serve the identification of key mechanisms in specific engineering scenarios.

5.2. Limited Test Standardization and Scale Extrapolation

Transparent soil tests still lack unified standards for material preparation, transparency evaluation, optical measurement, and image processing. Different studies vary in solid particles, pore fluid proportions, refractive-index matching, speckle preparation, model box size, and image algorithms. These differences make direct comparison among results difficult. Even when similar materials are used, differences in light sources, cameras, laser sheets, and image-processing procedures may lead to clear differences in the obtained displacement or flow fields.
Model scale is another important factor limiting engineering extrapolation. As the model size increases, light transmission thickness, boundary effects, particle size effects, and image attenuation become more prominent. These limitations are especially important in tunnels, pile groups, composite strata, and large-scale construction disturbance scenarios. Therefore, transparent soil tests are more suitable for medium- and small-scale process identification and model calibration. To improve engineering extrapolation, future studies should strengthen transparency evaluation standards, similarity calibration procedures, three-dimensional visualization methods, and joint validation with field monitoring and numerical simulation.

5.3. Need for Improved Multi-Field Coupling and Data Integration

Existing transparent soil studies have been widely applied to single mechanical or seepage problems. However, applications involving thermo–hydro–mechanical coupling, multiphase migration, contaminant transport, root growth, and extreme environments are still at an exploratory stage. These complex scenarios impose higher requirements on materials. Temperature changes, chemical environments, long-term immersion, biological effects, and multiphase flow may all affect refractive-index matching, material stability, and image recognition quality. Therefore, transparent soil tests in complex scenarios cannot simply use conventional materials and observation methods. Material functionalization, observation system adaptation, and multi-field parameter calibration should be considered together.
In addition, transparent soil tests can generate large amounts of internal displacement, flow, and temperature field data. However, these data are usually scattered among individual tests and lack unified formats or sharing platforms. Future studies should improve the standardized recording of raw images, processing algorithms, material parameters, and model conditions. Transparent soil testing should also be integrated with numerical simulation, parameter inversion, and intelligent image analysis. This will help transparent soil develop from a single visualization test into a cumulative, comparable, and verifiable research platform.
Table 6 summarizes the main existing challenges and corresponding future directions.

6. Conclusions

(1) Transparent soil materials have developed from an early exploratory stage focused mainly on achieving transparency into a multi-route material system that considers optical properties, physicomechanical responses, and compatibility with testing systems. For transparent sand, fused quartz sand remains the most mature and well-validated mainstream material. In contrast, transparent clay still involves multiple material routes, and no unified material system has been established to cover different target soils and working conditions.
(2) The main value of transparent soil technology does not lie in obtaining transparent specimens themselves. Rather, it lies in visualizing internal soil displacement, flow paths, local deformation, and failure evolution. Existing studies show that transparent soil has a solid application basis in foundation engineering and underground engineering. It also shows potential in seepage erosion, slope hazards, and thermal monitoring. However, different scenarios involve different dominant processes. A material that is “usable” is not necessarily “applicable” to all problems.
(3) The applicability of transparent soil materials should be evaluated based on the integrated matching among material, observation, system, and scenario. This paper proposes a four-dimensional evaluation framework consisting of mechanical similarity, optical measurability, system compatibility, and scenario matching. It also develops an evaluation procedure involving prototype problem definition, key indicator calibration, scenario-based validation, and extrapolation boundary determination. This framework helps clarify the applicable conditions and extrapolation boundaries of different materials in specific scenarios.
(4) Current transparent soil research still faces several challenges, including insufficient material equivalence and scenario adaptation, limited test standardization and scale extrapolation, and inadequate multi-field coupling and data integration. Future studies should strengthen target-oriented material optimization, establish unified standards for sample preparation, transparency evaluation, and image processing, and integrate transparent soil testing with field monitoring, numerical simulation, and intelligent image analysis. These efforts will improve the comparability, interpretability, and engineering reference value of transparent soil test results.

Author Contributions

Conceptualization, C.Z. and S.W.; methodology, C.Z. and S.W.; software, S.W.; formal analysis, B.X.; investigation, B.X.; resources, M.W.; data curation, S.W.; writing—original draft preparation, S.W.; writing—review and editing, Z.T.; super vision, W.X.; project administration, M.W.; funding acquisition, C.Z. and Z.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Grant No. 12162017, 52508348), Yunnan Fundamental Research Projects (Grant Nos. 202601AT070012, 202301AT070394, 202301BE070001-037).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Visualization model test system of pile driving: (a) model test device, (b) model test system diagram, (c) laser, and (d) CCD camera. The Chinese text visible on the blue device in panel (a) identifies the fully automatic loading system [62].
Figure 1. Visualization model test system of pile driving: (a) model test device, (b) model test system diagram, (c) laser, and (d) CCD camera. The Chinese text visible on the blue device in panel (a) identifies the fully automatic loading system [62].
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Figure 2. Displacement nephograms of the tunnel-face instability process in the longitudinal section under different setback and slope conditions: (a) S = 3 mm, i = −15%; (b) S = 3 mm, i = 0%; (c) S = 3 mm, i = 15%; (d) S = 6 mm, i = −15%; (e) S = 6 mm, i = 0%; (f) S = 6 mm, i = 15%; (g) S = 9 mm, i = −15%; (h) S = 9 mm, i = 0%; and (i) S = 9 mm, i = 15%. Here, S denotes the setback amount, and i denotes the longitudinal slope gradient [67].
Figure 2. Displacement nephograms of the tunnel-face instability process in the longitudinal section under different setback and slope conditions: (a) S = 3 mm, i = −15%; (b) S = 3 mm, i = 0%; (c) S = 3 mm, i = 15%; (d) S = 6 mm, i = −15%; (e) S = 6 mm, i = 0%; (f) S = 6 mm, i = 15%; (g) S = 9 mm, i = −15%; (h) S = 9 mm, i = 0%; and (i) S = 9 mm, i = 15%. Here, S denotes the setback amount, and i denotes the longitudinal slope gradient [67].
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Figure 3. Diffusion transport process of the slurry front in transparent soil grouting: vertical expansion stage, horizontal expansion phase, and slurry overflow stage [73].
Figure 3. Diffusion transport process of the slurry front in transparent soil grouting: vertical expansion stage, horizontal expansion phase, and slurry overflow stage [73].
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Figure 4. Displacement cloud images of slope soil reinforced by three-row micropile groups [79].
Figure 4. Displacement cloud images of slope soil reinforced by three-row micropile groups [79].
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Figure 5. Temperature distribution and thermal interference around energy piles in transparent soil under different pile spacings: (a) temperature distribution and interaction around the energy piles in transparent soil; (b) horizontal temperature distribution around the energy piles; and (c) variation in thermal interference effects at different pile spacings [84].
Figure 5. Temperature distribution and thermal interference around energy piles in transparent soil under different pile spacings: (a) temperature distribution and interaction around the energy piles in transparent soil; (b) horizontal temperature distribution around the energy piles; and (c) variation in thermal interference effects at different pile spacings [84].
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Figure 6. Applicability evaluation procedure for transparent soil materials guided by the four-dimensional framework.
Figure 6. Applicability evaluation procedure for transparent soil materials guided by the four-dimensional framework.
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Table 1. Representative physical properties and applicability of natural sand and typical transparent sand materials.
Table 1. Representative physical properties and applicability of natural sand and typical transparent sand materials.
Material TypeGrain Shape/StructureVoid Ratio or PackingStrength/CompressibilityPermeability/Pore Fluid IssueOptical and System CompatibilityApplicability Judgment
Natural sandRounded–angular mineral particles; morphology depends on source, transport history, and weathering e ≈ 0.43–0.85 for typical dense/loose and uniform/well-graded sands [46]Fujian standard sand: φp ≈ 30–37° at Dr = 30–70% in comparison tests with transparent sand [47]k ≈ 10−6–10−2 m/s, depending on gradation, fines content, and density [46]Opaque; internal fields cannot be directly observed opticallyReference material for evaluating mechanical similarity
Fused quartz sandAngular–subangular; high purity; composition close to siliceous sandTypical particle size: 0.5–1.0 mm; Dr = 30–70% in representative tests [40,42]φ ≈ 38.8°; φp ≈ 40.9–47.3°; α1–2 ≈ 0.192 MPa−1 [41]Intrinsic permeability ≈ 18.7–31.5 D; RI-matched pore fluid required [48]High transparency; compatible with laser slicing, PIV, and DICBest match for sandy soil model tests; suitable for pile, tunnel, deformation, and seepage visualization
Silica gel particlesPorous silica-based particles; hygroscopic; lower particle strength than mineral sandTypical particle size: approximately 0.5–5.0 mm [43]φ ≈ 29–42°; c ≤ 1 kPa; compressibility higher than fused quartz and many natural sands [43]No universal hydraulic conductivity; seepage response affected by interparticle voids, internal porosity, and pore fluid absorption [43]Acceptable transparency, but optical stability may be affected by hygroscopicity and fluid interactionConditionally applicable; requires calibration before replacing natural sand
Hydrogel beads/aquarium beadsSmooth, regular, and highly deformable particlesPorosity around 0.45 in representative hydrogel/aqua-bead systems [29]Very low strength: c or compressive strength ≈ 0.005–0.012 kPa; Cc ≈ 0.10–0.15 [29]Hydraulic conductivity k ≈ 6 × 10−10–7 × 10−4 m/s; strongly affected by swelling and fluid interaction [29]High transparency; suitable for flow, migration, and diffusion visualization, including reagent migration tests [49]Poor mechanical similarity; mainly suitable for qualitative visualization and flow/migration observation
3D-printed transparent resin particlesDesigned particle shape, size, and surface morphology [44]emin ≈ 0.598; emax ≈ 0.721–0.735 for selected printed particles [44]φp ≈ 34.03–43.14° in direct shear tests; φp ≈ 35.76–44.20° in CD triaxial tests [44]No universal k; depends on particle design, packing density, and pore fluid compatibilityGood designability; compatible with controlled particle-shape and image-based studiesSuitable for mechanism-oriented particle-shape studies; not a direct substitute without calibration
Note: The listed values are representative values or ranges reported in selected studies rather than universal constants. They may vary with particle size distribution, relative density, pore fluid type, refractive-index matching, sample preparation method, and testing conditions. For parameters without generally accepted numerical ranges, the table indicates the controlling factors and the need for material-specific calibration rather than assigning uncertain values.
Table 2. Representative physical properties and applicability of natural clay and typical transparent clay materials.
Table 2. Representative physical properties and applicability of natural clay and typical transparent clay materials.
Material TypeParticle/Structure FeatureWater Content or Void RatioStrength/CompressibilityPermeability/Consolidation BehaviorOptical and System CompatibilityApplicability Judgment
Natural clayPlate-like clay minerals; fabric affected by mineralogy, deposition, stress history, and organic contente ≈ 0.6 for stiff glacial clay; e ≈ 1.2 for soft glacial clay; e ≈ 1.9–5.2 for organic or montmorillonitic soft clay [46]Strength and compressibility vary strongly with water content, void ratio, plasticity, and stress historyRepresentative fine-grained/plastic soils: saturated hydraulic conductivity k ≈ 1.2 × 10−11–3.9 × 10−6 m/s [58]Opaque; internal deformation and seepage cannot be directly observed opticallyReference material for evaluating transparent clay similarity
Amorphous silicaSilica-based fine particles; median aggregate size ≈ 1.4–175 μm; specific gravity ≈ 2.0–2.1 [59]Mixture-dependent; high apparent void ratio due to internal porosity of silica aggregatesClay-like strength and compressibility under selected mixtures; Cα/Cc ≈ 0.03–0.05 in reported consolidation tests [51,52,59]Hydraulic conductivity k ≈ 2 × 10−8–1 × 10−5 m/s in flexible-wall tests; consolidation behavior similar to organic clay or peat [51,52,59]Good transparency under RI matching; compatible with DIC/PIV and laser slicingSuitable for low-plasticity clay, organic-clay-like, or conventional transparent clay model tests after calibration
Fumed silicaVery fine silica particles; large specific surface area; gel-like structureHigh pore fluid content; structure strongly affected by concentrationHigh compressibility; suitable for highly compressible clay or marine soft clay analogs [53,54]Consolidation and permeability are sensitive to mixture ratio and preparation method [53,54]High transparency, but specimen preparation and imaging stability need controlConditionally applicable for highly compressible fine-grained soils
Fused silica powderHigh-purity silica powder; particle morphology affects scatteringMixture-dependent; lower-compressibility clay-like specimens can be preparedMechanical response depends on particle size, pore fluid, and preparation method [55]Permeability and consolidation behavior require mixture-specific calibration [55]Transparency may decrease due to particle morphology and light scatteringSuitable for low-compressibility silty clay or general clay-like problems after calibration
Laponite RDSynthetic smectite-like nanoparticles; clear thixotropyHigh-water-content system; concentration and aging effects are significant [56]Low strength and high compressibility; thixotropic behavior is important [56]Long consolidation time; strength and permeability may evolve with aging [56]Good transparency, but time-dependent behavior affects repeatabilitySuitable for high-water-content soft clay or marine sediment analogs
U10Carbopol-based water-transparent clay; hydrogel-like structure0.75% U10 mixture has been used in representative transparent clay tests [60]Very low strength: Su,peak ≈ 0.26–0.30 kPa for 0.75% U10 mixture; sensitivity St ≈ 2.8–3.4 [60]Hydraulic conductivity k ≈ 5.3 × 10−9 m/s; deformation response requires calibration for each mixture [57]High light transmittance; suitable for water-based transparent clay systemsSuitable for highly compressible fine-grained soil problems; not a general clay substitute
Note: The listed values and descriptions are representative values or characteristics reported in selected studies rather than universal constants. Transparent clay properties may vary with solid concentration, pore fluid type, refractive-index matching, sample preparation method, aging time, consolidation state, and testing conditions. For parameters without generally accepted numerical ranges, the table indicates the controlling factors and the need for mixture-specific calibration rather than assigning uncertain values.
Table 3. Key parameters and experimental controls for transparent soil testing in typical application scenarios.
Table 3. Key parameters and experimental controls for transparent soil testing in typical application scenarios.
Application ScenarioDominant Process to Be CapturedKey Material Parameters to Focus onKey Experimental Controls
Foundation engineering and soil–structure interactionPile penetration, load transfer, local compaction, shear-zone developmentStrength, stiffness, density/void ratio, dilatancy, particle size, interface frictionDisplacement-field resolution, pile–soil interface condition, boundary effect, model scale
Underground engineering and excavation supportExcavation disturbance, face instability, loosening zone, ground deformationStrength, stiffness, density/void ratio, dilatancy or compressibility, target-stratum similarityLight transmission thickness, model box size, laser-sheet stability, two- or three-dimensional deformation tracking
Seepage, grouting, and internal erosionFlow path, grout diffusion, particle migration, erosion initiationPermeability, pore fluid viscosity, void ratio, particle size distribution, fines content, erodibilityFlow-rate control, refractive-index matching, tracer reliability, flow-field visualization method
Slope stability, sliding failure, and disaster preventionSlope deformation, slip-surface evolution, local collapse, support interactionStrength, stiffness/compressibility, density/void ratio, permeability under water level change, deformation capacityLarge-deformation tracking, imaging frequency, boundary condition, speckle or tracer stability
Thermal monitoring and interdisciplinary applicationsTemperature field, thermal interference, contaminant migration, root growth, long-term responseRefractive-index temperature sensitivity, thermal/chemical stability, pore fluid stability, biocompatibility, long-term stabilityImage–temperature calibration, illumination stability, environmental control, long-term imaging repeatability
Table 4. Four-dimensional evaluation framework for the applicability of transparent soil materials.
Table 4. Four-dimensional evaluation framework for the applicability of transparent soil materials.
Evaluation DimensionCore QuestionMain CriteriaApplicability Implication
Mechanical similarityIs it close to the target soil?Compressibility, shear strength, consolidation behavior, permeability, and dominant deformation modeDetermines whether the material can support mechanism analysis of the target problem
Optical measurabilityCan it be observed stably?Transparency, effective light transmission thickness, refractive-index stability, and image recognition qualityDetermines whether internal displacement, flow, or failure processes can be effectively captured
System compatibilityCan it be measured accurately?Model box size, light source arrangement, camera resolution, speckle/tracer method, and image-processing algorithmDetermines the accuracy, repeatability, and reliability of test results
Scenario matchingDoes it serve the target problem?Consistency among the material system, observation object, and dominant engineering mechanismDetermines whether material selection and test conclusions have engineering significance
Table 5. Applicability evaluation examples of representative material–scenario combinations.
Table 5. Applicability evaluation examples of representative material–scenario combinations.
Material–Scenario CombinationMechanical SimilarityOptical MeasurabilitySystem CompatibilityScenario MatchingOverall JudgmentMain Basis
Fused quartz sand–foundation engineeringHighHighHighHighHigh applicabilityThe material properties are relatively stable. It is suitable for single-pile and medium- to small-scale models in sandy foundations, and can effectively identify pile-side deformation and load transfer processes.
Fused quartz sand–underground engineeringModerateModerateModerateModerateConditionally applicableIt can be used to identify excavation-induced disturbance and instability processes. However, large-scale tunnel models are limited by light transmission thickness, model box size, and continuous displacement field recognition.
Transparent sand–seepage and internal erosionModerateModerateModerateHighConditionally applicableIt can visualize flow paths and particle migration. However, pore fluid viscosity, medium permeability, and time scale require careful calibration.
Amorphous silica–low-plasticity clay problemsHighModerateModerateHighConditionally applicableIts strength, permeability, and consolidation characteristics are close to those of low-plasticity clay. However, plasticity and thixotropic behavior are still affected by the pore fluid system and sample preparation conditions.
Fumed silica–highly compressible clay/marine soft clayHighModerateModerateHighConditionally applicableIt is suitable for modeling highly compressible fine-grained soils. However, its strong compressibility means that sample preparation and imaging conditions strongly affect the results.
Laponite RD/U10–high-water-content weak fine-grained soilModerateHighModerateModerateConditionally applicableIt has good transparency and is suitable for low-strength and high-water-content problems. However, its low strength and long consolidation time make it unsuitable as a general substitute for clay.
Functionalized transparent soil–thermal and interdisciplinary applicationsModerateModerateModerateModerateLimited applicabilityIt can be used for thermal field or multi-field process visualization. However, the image–temperature relationship must be strictly calibrated, and the effects of temperature, chemical environment, and long-term stability must be controlled.
Table 6. Main challenges and future directions in transparent soil research.
Table 6. Main challenges and future directions in transparent soil research.
Main ChallengeSpecific ManifestationFuture Direction
Insufficient material equivalenceParticle morphology, compressibility, permeability, and soil structure still differ from those of natural soilsOptimize materials for target soils and engineering scenarios
Insufficient scenario adaptationA single material is difficult to apply across foundation engineering, underground engineering, seepage, slope stability, and multi-field coupling problemsEstablish material–scenario relationships and clarify applicability boundaries
Lack of standardizationMaterial preparation, transparency evaluation, optical measurement, and image-processing procedures are not unifiedEstablish repeatable and comparable test standards
Limited scale extrapolationLight transmission thickness, boundary effects, and particle size effects limit large-scale model applicationsDevelop three-dimensional visualization techniques and validate test results using field monitoring and numerical simulation
Insufficient multi-field coupling capabilityMaterial stability and observation accuracy are limited under thermal, chemical, biological, and multiphase flow conditionsDevelop functionalized transparent soils and integrated multi-field testing methods
Insufficient data accumulationRaw images, displacement fields, and material parameters lack sharing and reuse mechanismsEstablish transparent soil test databases and intelligent analysis platforms
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Wang, S.; Zhang, C.; Xia, B.; Wang, M.; Tang, Z.; Xu, W. Material Systems and Applicability Evaluation of Transparent Soil: Toward Transparent Model Testing in Geotechnical Engineering. Infrastructures 2026, 11, 212. https://doi.org/10.3390/infrastructures11070212

AMA Style

Wang S, Zhang C, Xia B, Wang M, Tang Z, Xu W. Material Systems and Applicability Evaluation of Transparent Soil: Toward Transparent Model Testing in Geotechnical Engineering. Infrastructures. 2026; 11(7):212. https://doi.org/10.3390/infrastructures11070212

Chicago/Turabian Style

Wang, Shifu, Changxing Zhang, Biao Xia, Meiqian Wang, Zhiyi Tang, and Wei Xu. 2026. "Material Systems and Applicability Evaluation of Transparent Soil: Toward Transparent Model Testing in Geotechnical Engineering" Infrastructures 11, no. 7: 212. https://doi.org/10.3390/infrastructures11070212

APA Style

Wang, S., Zhang, C., Xia, B., Wang, M., Tang, Z., & Xu, W. (2026). Material Systems and Applicability Evaluation of Transparent Soil: Toward Transparent Model Testing in Geotechnical Engineering. Infrastructures, 11(7), 212. https://doi.org/10.3390/infrastructures11070212

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