A transparent soil modeling approach for investigating the performance of geosynthetic-encased granular columns in soft soils

Laboratory-scale physical modeling has established itself as an effective technique for investigating mechanisms underlying geotechnical problems. Recently, the use of transparent materials, combined with advances in digital imaging technologies, has emerged as an innovative and non-invasive approach for analyzing soil behavior. Transparent soils are biphasic systems composed of translucent solid particles and saturating fluids with closely matched refractive indices, enabling clear observation of internal processes. Digital Image Correlation (DIC) has proven particularly effective when integrated with transparent soil models. As a non-contact optical method, DIC minimizes interference and measurement inaccuracies associated with traditional instrumentation, allowing more reliable interpretation of displacement and strain fields. Recent studies have increasingly focused on transparent soils for modeling geosynthetic-reinforced soil structures. In this work, the behavior of embankments constructed over soft soils improved with geosynthetic-encased granular columns (GECs) was investigated using stratified transparent soil models. The study evaluated the mechanisms of load transfer from the embankment to the columns, as well as resulting deformations and differential settlements. Transparent soil modeling allowed observation of key load transfer mechanisms, such as soil arching within the PTC, and enabled evaluation of the soil arching ratio. Overall, transparent soil modeling proved to be a suitable and effective technique for simulating this type of geotechnical system, providing valuable insights into its behavior.

1 Introduction

Laboratory-scale physical modeling is a well-established approach for investigating the mechanisms in geotechnical problems. By reproducing prototype conditions at reduced scales, such models enable rapid acquisition of data on soil response while allowing precise control over experimental parameters, advantages that are often unattainable in full-scale testing. Nevertheless, the insertion of internal sensors into physical models can disturb soil behavior and compromise measurement accuracy. To overcome this limitation, researchers have increasingly adopted transparent soils in combination with digital imaging techniques. This non-invasive methodology enables direct visualization of soil deformation and internal mechanisms without altering the material’s properties, offering a alternative for studying complex geotechnical problems (Iskander, 2010; Liu and Iskander, 2010; Iskander et al., 2015).

Transparent soils are biphasic materials consisting of translucent solids and pore fluids with matched refractive indices, which confer their transparent properties. Their mechanical behavior is comparable to that of natural soils, allowing their use as substitutes in the physical modeling of geotechnical processes. For the simulation of granular soils, fused quartz particles saturated with mineral oil mixtures have proven particularly effective. This material is produced by melting quartz crystals at approximately 2000 °C followed by cooling, and it is widely employed for industrial applications such as semiconductors, solar cells, telescope and microscope lenses, telecommunication equipment, and chemical glassware (Ezzein and Bathurst, 2011; Iskander et al., 2015). Ezzein and Bathurst (2011) characterized the shear strength of the synthetic transparent sand, confirming its close similarity to natural quartz sands.

In the case of cohesive soils, Laponite RD®, a synthetic magnesium–lithium phyllosilicate, has been identified as a suitable analogue material (Iskander et al., 2002; Liu et al., 2003; Wallace and Rutherford, 2015; Wallace et al., 2018; Ads et al., 2020b; Ads et al., 2020a; Kong et al., 2020; Ma et al., 2022). Structurally related to hectorite, this synthetic smectite clay forms a transparent suspension when hydrated and swollen in distilled water. Wallace and Rutherford (2015) performed vane shear, consolidation, and permeability tests on Laponite RD®, demonstrating that its macroscopic behavior closely resembles that of natural soft clays. Pierozan et al. (2022) examined its optical and physical properties, refining its application in geotechnical modeling.

When combined with transparent soil techniques, digital image correlation (DIC) has proven to be a powerful method for observing and quantifying soil deformations without the interference of external instruments. Its non-intrusive nature minimizes errors in displacement measurements (Lopes, 2019). DIC works by analyzing digital images to compute incremental displacement fields, tracking small regions across images with sub-pixel precision to produce high-resolution spatial maps of deformation. In the processing stage, images are represented as pixel matrices, where each pixel is located by planar coordinates, and the tracked regions correspond to selected subsets of this matrix (Take, 2015).

Physical modeling with transparent soils has been widely applied in recent geotechnical research. These models have also enabled the investigation of soil–geosynthetic interaction using alternative materials that simulate geosynthetic reinforcements, designed to reproduce tensile strength and stiffness equivalent to those of real geosynthetics when scaled to the model. De Guzman and Alfaro (2016), De Guzman and Alfaro (2018) examined the performance of embankments constructed over peat foundations reinforced with basal geotextiles, using a transparent soil composed of silica and a compatible saturating fluid. Ding et al. (2020) investigated soil arching in rigid pile foundations with fused quartz transparent soil, where settlement-induced arching was analyzed using a latex film as horizontal reinforcement above pile caps. Zhang et al. (2018) conducted a series of two-dimensional trapdoor tests with fused quartz transparent soil to analyze soil arching phenomena under cyclic footing loading, employing window screen material to simulate geogrid reinforcement at model scale. Zhou et al. (2019) analyzed bulging effects in encased granular columns using fused quartz transparent soil. Chen et al. (2019), Chen et al. (2021) conducted footing tests on both unreinforced and reinforced transparent soil models, employing geotextiles and biaxial polypropylene geogrids, with fused silica sand saturated in oil as the transparent medium. Gao et al. (2022a), Gao et al. (2022b) compared the bearing capacity and deformation characteristics of foundations reinforced with different geogrid types and reinforcement layers, and further examined reinforcement mechanisms and failure modes in models reinforced with polylactic acid biaxial geogrids. Souza et al. (2023) evaluated the load response of conventional and encased granular columns in soft clay, simulated with transparent soil prepared from Laponite RD®.

Recent studies have investigated the behavior of embankments constructed over soft soils. Santos (2023) applied both granular and cohesive transparent soils in a stratified physical model to investigate load transfer mechanisms in an embankment constructed over soft soil and reinforced with rigid inclusions, incorporating a load transfer platform (LTP) above the pile caps. Similarly, Vieira (2024) performed experiments on stratified models to evaluate the behavior of embankments supported by granular columns. Both studies examined the influence of the number of horizontal geosynthetic reinforcement layers within the LTP on the overall load transfer process.

Embankments constructed over granular columns have been the focus of extensive research in recent years (Almeida et al., 2013; Hosseinpour et al., 2014; Jamshidi et al., 2017; Moayed and Zade, 2017; Aghili et al., 2021; Mohamadi et al., 2023; Tizpa et al., 2023; Shafiee et al., 2024). More specifically, studies have shown that incorporating a granular layer at the base of the embankment, known as the load transfer platform (LTP), helps reduce stress concentration near the column heads, increases the bearing capacity of the soft soil, and minimizes differential settlements (Shahu et al., 2000; Deb, 2007). Furthermore, the inclusion of geosynthetic reinforcement within the granular platform has been demonstrated to significantly enhance the overall load-bearing performance of such systems (Abdullah and Edil, 2007; Deb et al., 2007; Deb et al., 2008; Deb et al., 2011).

Despite these advances, the mechanisms of load transfer in embankments constructed over soft soils and supported by granular columns remain not fully understood—particularly when geosynthetics are incorporated, either as horizontal layers within the LTP or as encasement around the granular columns. Currently, design methodologies for embankments over granular columns are largely based on approaches originally developed for piled embankments (BS 8006-1, 2010; EBGEO, 2011; FHWA, 2017). However, this analogy is not entirely appropriate, as the substantial difference in stiffness between granular columns and concrete piles leads to distinct load transfer mechanisms in each system. Therefore, experimental approaches are essential to investigate the complex interactions among soil, granular columns, and geosynthetic reinforcements. Transparent soil techniques offer a promising means to tackle this challenge, as they enable the concurrent simulation of soft soils and granular materials while allowing detailed visualization and quantification of deformation and load transfer processes.

To address this gap, the present study adopts a transparent soil modeling approach to evaluate the behavior of embankments constructed over soft soils supported by geosynthetic-encased granular columns GECs with a load transfer platform (LTP) reinforced with geosynthetics. The soft soil was simulated using Laponite RD® transparent clay, and fused quartz transparent sand was employed to represent the granular platform. Model behavior was evaluated using DIC technique with the Ncorr software.

2 Materials and methods

2.1 Transparent soils

The simulation of the soft foundation soil and the granular material of the LTP was carried out with transparent soils. The soft soil was reproduced using the synthetic clay Laponite RD®, whereas the LTP was represented by synthetic fused quartz sand. The transparent synthetic clay was obtained through the hydration of Laponite RD® combined with sodium pyrophosphate decahydrate (SPD). The inclusion of SPD reduced the viscosity of the mixture without compromising its strength, facilitating the release of air bubbles during preparation and improving the transparency of the final material. Transparent clay was prepared by first blending distilled water with PSD in a mechanical mixer. Laponite RD® was then gradually and uniformly incorporated, and mixing was continued to ensure homogeneity. The suspension was subsequently left to consolidate for 10 days, enabling it to develop the strength parameters required for application in the physical model. The materials used in the preparation of the transparent clay, the mixing process, and the final transparency of the material are shown in Figure 1.

Figure 1

Figure 1. (a) Laponite RD, (b) sodium pyrophosphate decahydrate, (c) transparent clay preparation, (d) transparent synthetic clay after 10 days of consolidation.

The transparent artificial sand employed in the load transfer platform consisted of coarse quartz particles saturated with an oil mixture. The fused quartz, obtained from waste generated during quartz glass manufacturing, was ground and sieved to achieve the particle size distribution required for the model. To ensure transparency, the granular material was saturated with a liquid whose refractive index closely matched that of fused quartz (n = 1.4585). For this purpose, a 1:1 mass mixture of liquid petrolatum (n = 1.4400) and turpentine oil (n = 1.4779) was used. Figure 2 illustrates the transparency level obtained with the mixture.

Figure 2

Figure 2. Transparency level of fused quartz: (a) Dry, (b) Partially saturated, (c) Saturated.

Fine-grained fused quartz was used as the filling material for the granular columns to maintain the transparency required for transparent soil techniques. The geosynthetic functions of column encasement and horizontal reinforcement in the LTP were reproduced at model scale using alternative materials. Column encasement was provided by a nonwoven fabric replicating a nonwoven geotextile (Figure 3a), while horizontal reinforcement was simulated with a commercial mesh with 2 mm (CMD) × 5 mm (MD) openings (Figure 3b).

Figure 3

Figure 3. Alternative materials used to simulate geosynthetics: (a) nonwoven fabric, (b) commercial mesh.

The synthetic clay was characterized through unconsolidated-undrained (UU) triaxial tests, in accordance with ASTM (2015a). The tests indicated that the transparent clay behaves as a soft soil, exhibiting an undrained shear strength of 2.21 kPa for the formulation containing 7% Laponite RD® and 0.075% SPD. Characterization of the transparent synthetic sand and the column fill material included grain size distribution (ASTM, 2009), maximum void ratio (ASTM, 2016), minimum void ratio (ASTM, 2015b) e and triaxial compression tests (ASTM, 2020). The key properties of these materials are summarized in Table 1. The mechanical behavior of the nonwoven fabric and commercial mesh was evaluated using wide-width tensile tests (ASTM, 2024). The results are summarized in Table 2 and the stress–strain curves and the Mohr envelope for both transparent soils are shown in Figure 4.

Table 1

Table 1. Summary of the properties of fused quartz.

Table 2

Table 2. Summary of wide-width tensile tests.

Figure 4

Figure 4. (a) Stress–strain curve of synthetic clay, (b) Stress–strain curve of synthetic sand, (c) Mohr–Coulomb envelope of synthetic clay, (d) Mohr–Coulomb envelope of synthetic sand.

2.2 Model assembly and test setup

The prototype configuration was defined according to EBGEO (2011). Variable properties and dimensions were selected within the recommended ranges, with typical design values used for embankment materials. For physical modeling, only the upper portion of the columns near the LTP was simulated, as the primary mechanisms of interest occur in this region (Figure 5a). A 1g model at a 1:20 scale (laboratory:prototype) was used (Figure 5b). The model was built in an acrylic box measuring 400 mm × 200 mm × 400 mm (length × width × height). The scaling factors used in the model were applied based on the scaling laws presented by Viswanadham and König (2004) and Garnier et al. (2007).

Figure 5

Figure 5. (a) Prototype embankment layout; (b) Physical model layout.

The physical model incorporated granular columns with a diameter of 4 cm and a length of 21 cm, which correspond to 80 cm diameter and 4.2 m length at prototype scale. Each column was encased in a tubular form with a 2 cm lateral overlap and the bottom sealed with adhesive. During model assembly, a 6 cm layer of D28 foam was placed at the base of the acrylic box. As noted by Santos et al. (2025), the transparent synthetic clay exhibits limited consolidation within the available testing timeframe. The foam layer was therefore introduced to allow the displacement of the soft soil under load. Circular openings, 4 cm in diameter, were cut into the foam to allow the granular columns to rest directly on the bottom of the acrylic box, providing rigid support (Figure 6a). After placing the foam and laying the acrylic box horizontally, the transparent clay layer was filled to half its height to allow particle deposition for the creation of a random particle plane, which was later used for DIC analysis. Once the first half of the synthetic clay layer had consolidated, the remaining layer was added, and the box was returned to an upright position. Following consolidation of the full clay layer, the granular columns were installed into the model (Figure 6b). A pressure cell was positioned on the top of the second column (from left to right) to measure the stress transferred to the column. A layer of mesh was placed horizontally above the columns to serve as horizontal reinforcement. The granular layer was then added in 1 cm-thick increments to facilitate the creation of the vertical particle plane (Figure 6c). A 100 mm-thick polystyrene layer was placed over the LTP to replicate the flexible behavior of the embankment body. The final configuration of the physical model is shown in Figure 6d.

Figure 6

Figure 6. Assembly of the physical model: (a) foam placement, (b) addition of soft soil and installation of granular columns, (c) LTP installation, (d) final configuration.

The model was subjected to a compression test at 0.5 mm/min, with loads applied via a rigid plate and measured using a load cell. Images were captured every three seconds using a Canon T6i camera (24.2 MP). Displacements and strains in the load transfer platform and underlying soft soil were analyzed using Ncorr software, which applies digital image correlation (DIC) to generate displacement and strain fields. The software, developed in MATLAB with C++/MEX algorithms, allows observation of soil arching and stress transfer. The test setup is shown in Figure 7.

Figure 7

Figure 7. Test setup configuration.

3 Results and discussion

3.1 Displacement analysis

The model response was evaluated using digital image analysis with Ncorr software, which provides displacement fields as well as strain fields. Analyses were performed for applied stress levels of 5, 15, 25, and 35 kPa. To minimize boundary effects, the region of interest was confined to the area between the centers of the first and last columns, thereby excluding the model edges. The vertical displacement field obtained from the analysis is shown in Figure 8. The displacement maps are displayed with a color scale, where blue represents areas of minimum vertical displacement and red corresponds to areas of maximum vertical displacement within the model.

Figure 8

Figure 8. Vertical displacement: (a) 5 kPa, (b) 15 kPa, (c) 25 kPa, (d) 35 kPa.

In addition, settlements were measured at the top of the two central columns and at the surface of the soft soil layer between the columns to evaluate differential settlements between GECs and the soft soil within the model throughout the stress application. The results are shown in Figure 9a, and the corresponding monitoring point locations are presented in Figure 9b.

Figure 9

Figure 9. Vertical settlements: (a) Measured vertical settlements under different applied stress levels; (b) Measurement point locations.

The vertical displacement distribution revealed that maximum settlements occurred in the central portion of the model. This response is consistent with beam-like behavior of the load transfer platform (LTP), attributable to the high stiffness of the geosynthetic reinforcement at its base, which results in the stiffening of the platform. As a consequence, the LTP underwent flexural deformation, concentrating settlements on the two central columns. The underlying soft soil layer exhibited a comparable response, with the largest vertical displacements also located at the center of the model.

In the initial stages of loading, until 15 kPa, the largest displacements occurred in the soft soil due to its lower stiffness relative to the GECs. The soft soil undergoes greater deformation under the same applied stress, resulting in pronounced differential settlements between the GECs and the surrounding soil. This differential settlement mobilizes the shear resistance of the granular LTP layer, inducing the arching effect, which progressively transfers a larger portion of the applied load to the columns while reducing stress in the soft soil. Consequently, settlements measured at the columns increase after the 15 kPa loading stage and exceed those of the soft soil at the 35 kPa load. These column settlements are necessary to mobilize their ultimate bearing capacity, as the GECs function as semi-rigid supports.

3.2 Strain analysis

The behavior of the model can be better understood by examining the vertical strain field, shown in Figure 10. In this representation, compressive regions are indicated in blue, while tensile zones are shown in red. Areas depicted in green correspond to regions where the strain is zero.

Figure 10

Figure 10. Vertical strain: (a) 5 kPa, (b) 15 kPa, (c) 25 kPa, (d) 35 kPa.

The deformation fields within the physical model were analyzed to understand the stress distribution response under increasing load. Figure 10a, corresponding to an applied stress of 5 kPa, exhibits minimal vertical deformation across the model, with compression concentrated only in the central region above the two middle columns. This response is consistent with the beam-type behavior of the granular LTP layer previously described. When the applied stress reaches 15 kPa (Figure 10b), the compressive zone extends across all columns, resulting in a more uniform deformation pattern within the model and still showing no evidence of differential settlements. At 25 kPa (Figure 10c), the onset of an arching configuration becomes evident, linking the column heads and inducing compression in the upper portion of the soft soil layer, thus marking the initiation of the soil arching mechanism. At the final loading stage of 35 kPa, the arching effect is fully developed, accompanied by a pronounced compression zone in the surrounding soil near the column heads (Figure 10d). These results confirm both the mobilization of the granular layer through arching and the vertical compression of the GECs, mechanisms that are fundamental to the redistribution of stress and the mobilization of the columns bearing capacity.

The soil arching effect can also be verified through the analysis of shear strain. The shear strain field of the model during the loading stages is shown in Figure 11. The analysis was limited to LTP, as this is the region where the mechanisms occur. In this representation, blue and red zones denote equal magnitudes of shear strain in opposite directions, whereas green zones represent areas of zero shear strain.

Figure 11

Figure 11. Shear strain: (a) 5 kPa, (b) 15 kPa, (c) 25 kPa, (d) 35 kPa.

In the literature, the arching phenomenon is identified by the formation of shear bands that originate at the edges of rigid supports and incline toward the active mass (Zhang et al., 2018; Khatami et al., 2021; Zhao et al., 2021; Santos, 2023). In the case of granular columns, however, these planes are not clearly defined and instead form broader zones of concentrated deformation (Vieira, 2024). In the present study, such deformation zones are inclined outward from the column heads and extend through the entire thickness of the LTP layer. These shear zones begin to develop under an applied stress of 25 kPa, in agreement with the results previously discussed, and become clearly visible at the 35 kPa loading stage.

3.3 Load transfer

The applied stress corresponding to the imposed displacement during the test was monitored using a load cell and compared with the vertical stress transferred to one of the columns, as measured by a pressure cell installed at the column head (Figure 12). The loading stages were numbered from 1 to 7, with Stage 1 corresponding to 5 kPa and subsequent stages defined in increments of 5 kPa, up to Stage 7 at 35 kPa.

Figure 12

Figure 12. Total applied stress versus stress transferred to the column.

In the initial stages, the stress transferred to the column was equivalent to the total applied stress, indicating a uniform distribution within the model. From approximately 15 kPa (stage 3) onward, this distribution began to change, with a greater portion of the applied stress being transferred to the column. This behavior reflects a reduction in stress within the soft soil and marks the onset of soil arching. As loading progressed, the stress carried by the column continued to increase. At 25 kPa (stage 5), however, the rate of stress transfer to the column began to decrease, likely due to the initiation of vertical displacement of the column. Between 30 kPa and 35 kPa (stages 6 and 7), when soil arching was already well developed and the column’s load-bearing capacity was enhanced by its vertical displacement, the growth rates of both the applied stress and the stress transferred to the column became nearly identical.

Based on stress measurements, the load distribution between the columns and the soft soil was quantified, allowing the calculation of the soil arching ratio (SAR), as shown on Figure 13. This parameter, defined as the ratio of the vertical load carried by the columns to the total load applied to the model, provides a quantitative assessment of the load-sharing mechanism and the efficiency of stress redistribution within the reinforced foundation system.

Figure 13

Figure 13. Load distribution and soil arching ratio.

When evaluated in terms of load, the soft soil carried a greater portion of the applied stress than the column, reflecting its larger cross-sectional area relative to the GECs. The maximum soil arching ratio occurred at the first loading stage (5 kPa). As loading progressed, the SAR gradually decreased, likely due to compression of the GEC, which reduced its relative contribution to stress transfer. Between Stages 6 and 7 (30–35 kPa), the SAR stabilized and remained nearly constant, indicating that soil arching was fully developed and the column had reached its full load-bearing capacity. This behavior highlights the transition from initial uniform stress distribution to a system governed by soil arching.

4 Main conclusions

A stratified transparent soil system was developed to investigate the behavior of embankments supported by geosynthetic-encased granular columns over soft soils. This approach enabled both direct visualization of the physical model and accurate measurement of deformations, strain fields, and load transfer mechanisms. The transparent medium allowed real-time observation of internal displacements and strain development. When combined with digital image correlation (DIC) techniques, this methodology provided a comprehensive assessment of soil response in small-scale physical models without the need for embedded sensors that could alter the natural behavior of the material. This non-intrusive approach made it possible to analyze deformation patterns, load transfer processes, and soil–structure interactions that are difficult to capture using conventional measurement methods. Based on the experimental results, the following key conclusions can be drawn:

• The use of a geogrid with high stiffness at the base of the load transfer platform (LTP) results in the stiffening of the layer, reflecting a beam-like response

• Differential settlements increased with applied stress, as the soft soil deformed more than the columns at low stress levels, while at higher loads, soil arching progressively transferred the load to the columns, resulting in greater column settlements.

• Soil arching is activated with increasing load, initiating at 25 kPa and fully developing at 35 kPa, as indicated by compression zones in the soft soil and concentrated shear zones in the LTP, which demonstrate mobilization of the granular layer and stress redistribution toward the columns.

• Shear strain zones confirm arching behavior, with broad, inclined deformation extending outward from the column heads through the LTP thickness rather than forming distinct shear planes, becoming clearly evident at higher stress levels (≥25 kPa) and reflecting the characteristic behavior of granular columns under load.

• Analysis of load distribution indicates that the soil arching ratio decreases as the GECs compress and stabilizes at higher loads (30–35 kPa), reflecting the transition to a system dominated by soil arching and the columns achieving their full load-bearing capacity

Overall, integrating transparent soils with Digital Image Correlation enables detailed observation and measurement of critical load transfer mechanisms—including vertical compression of the columns, LTP flexural response, and soil arching—providing a comprehensive understanding of stress redistribution and column performance.

Data availability statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author contributions

AL: Data curation, Formal Analysis, Investigation, Methodology, Validation, Writing – original draft, Writing – review and editing. FP: Conceptualization, Data curation, Funding acquisition, Project administration, Validation, Writing – original draft, Writing – review and editing.

Funding

The authors declare that financial support was received for the research and/or publication of this article. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001.

Acknowledgements

The authors thank the Laboratory of Geotechnics and Geosynthetics and the Vitreous Materials Laboratory of the Federal University of São Carlos for their support. Special thanks are extended to Huesker – Brazil for their assistance during this research.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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The authors declare that no Generative AI was used in the creation of this manuscript.

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