# Remediation of soil subjected to suffusion using fine injection techniques combined experimental and numerical insights

Fan Chen, Abhijit Hegde, Antoine Wautier, Nadia Benahmed, Pierre Philippe, François Nicot

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Fan Chen, Abhijit Hegde, Antoine Wautier, Nadia Benahmed, Pierre Philippe, et al. Remediation of soil subjected to suffusion using fine injection techniques combined experimental and numerical insights. 12th International Conference on Scour and Erosion, Nov 2025, Chongqing, China. 2025. [hal-05469323](https://hal.inrae.fr/hal-05469323v1)

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# Remediation of soil subjected to suffusion using fine injection techniques - combined experimental and numerical insights

Fan Chen¹*, Abhijit Hegde¹, Antoine Wautier¹, Nadia Benahmed¹, Pierre Philippe¹
François Nicot²

¹NRAE, Aix-Marseille University, RECOVER, Aix-en-Provence, France.

²Savoie Mont-Blanc University, Laboratoire ISTerre - UMR CNRS 5275, Le Bourget du Lac, France.

E-mails: fan.chen@inrae.fr (Corresponding author; PhD Student).

abhijit.hegde@inrae.fr; antoine.wautier@inrae.fr; pierre.philippe@inrae.fr;

nadia.benahmed@inrae.fr; francois.nicot@univ-smb.fr.

## ABSTRACT

The phenomenon of suffusion, commonly observed in gap-graded soils, involves the detachment and transport of the finest grains under internal fluid flow through porous media. This can lead to significant loss of fines, potentially causing mechanical instabilities in the material and posing a risk of catastrophic failure in hydraulic structures. To mitigate this risk, a novel approach is investigated by introducing fines into loose skeleton composed of coarse sand grains. This study aims to: (1) examine the infiltration process of fines injection; (2) evaluate its effectiveness from point of view of the mechanical behavior of the suffused sample using both numerical and experimental methods. For the first objective, both experimental and numerical results show that the distribution of injected fine grains decay exponentially along the infiltration depth, allowing a straightforward characterization of the infiltration process via the mean infiltration distance $L_0$. A micro-mechanically based probabilistic model is put forward to interpret $L_0$ from the pore/constriction size distribution of the coarse sand column (matrix) across a board range of size ratios $R (D_{15}/d)$ between fine and coarse grains. Regarding the second objective, the mechanical properties of the soil before and after fines injection are analyzed using constant-volume triaxial tests. The role of a small amount of injected fines on macroscopic material properties, such as contractive behaviors and liquefaction resistance, is evaluated from both numerical and experimental aspects. The findings reveal the beneficial role of the fine grains inclusion in delaying/preventing the development of plastic strain in the coarse grain skeleton, thus enhancing the overall loose material stability.

## INTRODUCTION

Suffusion, a prevalent issue in geotechnical engineering, refers to the internal erosion process where finer soil particles are removed and washed away by seepage flow, resulting in the potential loss of the finer fraction of the soil (Wautier, et al, 2017, 2019a). Over time, soil suffusion can induce mechanical instabilities which, in turn, lead to serious and even catastrophic damages such as differential settlement, sinkholes, static liquefaction, and other forms of structural failure (Foster et al, 2000; Bonelli, S, 2013). To counteract suffusion, one strategy harnessing upstream hydraulic flow to reintroduce fine particles into the coarser soil matrix of a dam body (Chen et al, 2025) (see Figure 1). Effective remediation of eroded soil through fine particles migration/infiltration requires deep understanding of the influencing factors of this
behavior. However, the underlying mechanisms remain complex due to the interplay between the
multiple external factors such as flow conditions and internal features of the granular media,
particularly the geometry and connectivity of its pore space.

Research on grain infiltration has considered several variables such as the size ratio of coarse and fine particles R, the amount of injected fine particles (affecting collective arching), as well as particle interaction properties such as the coefficient of restitution and inter-particles friction coefficient. Traditionally, studies have focused on simplified scenarios using mono-dispersed, gravity-driven infiltration where the particle R is within a limited range, while overlooking the complexities of fluid-driven infiltration into poly-dispersed soils with varying packing density conditions. Thus, to address these gaps, this work attempts to study the dry and fluid-driven fine grain infiltration behaviors into poly-dispersed soil skeleton from numerical and experimental aspects. Suffusion permeameter device (SEPT) was modified and upgraded to allow for the injection of fluidized fine grains suspension into loose soil samples (Figure 2(a)). In parallel, a coupled Pore-scale Finite Volumes-Discrete Element Method (PFV-DEM) numerical model (Figure 2(b)) was constructed to simulate the fluid-driven infiltration process of fine particles into coarse-grain packing. The influence of size ratio R (D<sub>15</sub>/d) on the fine particles retention regimes was systematically studied through the exponential retention profiles and characteristic infiltration distances derived from both numerical and experimental works. A probabilistic model was developed to describe the infiltration distance of fines based on the constriction size distribution given by either the numerical model, or a modified Rayleigh distribution from particle size distribution (PSD) of the granular material. The model successfully reproduces the passing probabilities and infiltration distances of the fine grains from both numerical and experimental results.

<figure><img src="image_2.png" /><figcaption>Figure 1. Schematic of proposed remediation method for eroded dam through upstream fine sand injection.</figcaption></figure>

To assess the influence of a small amount of injected fine particles on the macroscopic
mechanical behavior of the materiel, constant-volume triaxial shear tests were performed both
experimentally and through Discrete Element Method (DEM) simulations. This loading path is
particularly effective to trigger collapses of force chains within the loose coarse-grain skeletons,
thus induces the static liquefaction (Wautier, et al, 2019b). Experimentally, two series of tests
were conducted on fine-coarse soil mixtures prepared with narrowly graded Hostun 1/2.5 coarse
sand and a small amount (5% by mass) of Fontainebleau fine sand. In the first series, the fine
sand was injected as a suspension into the coarse skeleton (injected samples), while in the
second, the fines were blende with the coarse sand using the moist tamping technique
(Benahmed et al., 2004) during the sample preparation stage (reconstituted samples). The
comparison reveals a remedial effect of the injected fines content (fc) in enhancing resistance to static liquefaction, while a contrasting result for the reconstituted samples. To better understand the micro-mechanism behind such remediation phenomenon of injected fines, DEM simulations were conducted at a representative elementary volume (REV) scale, where the fine sands are generated to form either a randomly-filled or clogged micro-configurations within the coarse grain skeleton. The numerical undrained triaxial tests successfully reproduced the observed improvement in liquefaction resistance. The combined numerical and experimental findings demonstrate how a low fines content can inhibit the plastic behavior during static liquefaction of initially-loose granular materials, by introducing them according to a specific micro-configurational pattern.

<figure><img src="image_3.png" /><figcaption>Figure 2. Experimental (a) and simulation (b) setup for fine-suspension infiltration into coarse-grain column (Chen et al, 2025).</figcaption></figure>

## EXPERIMENTAL AND NUMERICAL SETUPS

### - Fine Infiltration Test

In our experiments, Hostun sand NH1/2.5 with sub-angular grains shape was selected as the erodable base soil. It was prepared at a relative density of 20 %, with its particle size distribution (PSD) given in Figure 3(a). For the injected fine particles, Fontainebleau sand NE34 with sub-rounded grains shape was used. To achieve various values of R ($D_{15}/d_{85}$), this Fontainebleau sand was further sieved into different size fractions, as illustrated in Figure 3(b). Suffusion permeameter device (SEPT) (Nguyen et al, 2019) was used after being modified and upgraded to allow for the injection of fine grains suspension into a coarse column (see Figure 2(a)). A magnetic stirrer is put inside an air-tight reservoir to uniformly disperse the fines in the water and prevent their sedimentation. Then, it is connected to the suffusion permeameter to introduce the fine suspension into the coarse-sand column. Depending on the interaction between the fines and the granular skeleton, the injected fine suspension would be either retained inside or percolate through the column.

In parallel, a series of coupled PFV-DEM simulations were conducted. Numerical sand columns were generated by adopting the same Hostun sand PSD as in the experiments considering two extreme densities (loosest and densest states). The size ratio R ($D_{15}/d$) was varied from 4 to 9.6. The fine particles batch was randomly positioned above the coarse column and then released either under gravity or hydraulic force resulting from an internal fluid flow (Figure 2(b)).
- Undrained Triaxial Test

Based on the injection experimental results conducted above, an appropriate size ratio R of 11.3 is selected to achieve a satisfactory homogeneity of fine distribution within soil matrix, without causing the formation of filter cakes. After an additional 4 rounds of injection, homogeneous fines content in all the samples reached a threshold of roughly 4%, at which point filter cakes began to form once again whenever injection was stopped. Eventually, samples with R = 11.3 and injected fc around 4% were chosen to perform the triaxial tests and compared with the reconstituted fine-coarse mixture sample mixed with the same fc.

<figure><img src="image_4.png" /><figcaption>Figure 3. (a) PSD of Hostun HN1/2.5 and Fontainebleau NE34 sand and (b) sieved Fontainebleau NE34 sand to obtain different fine sand sizes for experimental injection.</figcaption></figure>

Meanwhile, DEM simulations of a series of idealized loose skeleton at REV scale were also conducted to reproduce the constant-volume triaxial shearing and observe the occurrence of static liquefaction. The original skeletons adopted the same PSD as Hostun HN1/2.5 sand but with varied initial void ratio ranging from 0,67 to 0,73, as shown in Figure 6(b)) to reproduce a series of gradually-loosening skeletons. Then, an extra fine content (fc=5%) was added to densify the original loose coarse skeletons using two approaches: one by randomly filling the fine sand in the void neglecting the effect of gravity, and the second by placing them at smaller constrictions to predispose the clogged aggregations (fc=1%, 2%, and 5%) considering the effect of gravity on fine grains. These two approaches allowed the generation of mixtures with controlled fine sand micro-configurations. The constant-volume triaxial shearing were then conducted on the densified samples to examine their respective effect on the liquefaction resistance of the soil mixture.

RESULTS AND DISCUSSIONS

- Fine Sand Retention Characteristics
<figure><img src="image_5.png" /><figcaption>Figure 4. Passing fraction profiles of fine sand along normalized column depth (/D50) based on infiltration experiments in loose column (a); and PFV-DEM infiltration simulations in loose (b) and dense (c) columns.</figcaption></figure>

Figure 4 provides both the numerical and experimental results of passing fraction of fine sand along normalized column depth. It is found that the fine sand retention can be well-fitted by an exponential decaying function along infiltration depth. The fitting results allow for the determination of a key measure - the characteristic infiltration depth of fines L₀-which can be used to quantitatively compare and describe the efficiency of filters for different parameters of concern, such as relative density, size ratio R, etc. The fitting results from experimental and numerical analyses were found quite consistent as indicated in Figure 5(a). Then, we propose a simple probabilistic model based on the size of the fine particles and the pore-constriction size statistics of the filter, to predict the average penetration distance of the fines within the coarse skeleton. P_d that a fine particle of diameter d can pass through a constriction in the coarse matrix. Under assumptions of a homogeneity and equal spacing of constrictions, the distance ΔZ between two successive constrictions is related to the mode value of the pore size distribution D_{p50} as shown in Figure 5(b). To obtain the pore/constriction size distribution, a modified Rayleigh distribution function is introduced based on the PSD of Hostun Sand. Finally, the model is described as below:

$$L_0 = \frac{-1}{(3 - \sqrt{6})} \frac{D_{p50}}{\ln(P_d)}$$

This probability model is then used to predict L₀ given the packing density and size ratio R. Interestingly, this simple probabilistic model is found to predict quite well the L₀, with a linear trend for log(L₀) versus R in both dense and loose columns (Figure 5(a)). This consistency supports the idea that the average probability P_d of passing through constrictions in soil with a homogeneous pore-constriction network captures the fundamental physics governing the infiltration process.
<figure><img src="image_6.png" /><figcaption>**Figure 5.** Probabilistic model predicting decaying length: (a) micro-mechanical configuration to calculate ΔZ via Dp50; (b) predicted L₀ compared with experimental and numerical results.</figcaption></figure>

- Microstructure-Altering Liquefaction Resistance

Undrained triaxial experiments were conducted on the non-injected, injected and reconstituted samples at the same effective stress of 300 kPa with the results presented in Figure 6. As shown, there is a marked difference in the mechanical behavior between the three types of some previous studies showing larger strain softening with addition of fines (Thevanayagam et al, 2002). A possible assumption could be that injected fine grains inside the sand matrix creates a unique fabric which is different from simply mixing the fine grains with sand, whereas some inherent experimental limitations do not allow a direct observation or interpretation for this conceptual micromechanical notion.

<figure><img src="image_7.png" /><figcaption>**Figure 6.** Experimental undrained triaxial test comparison of coarse skeletons with injected and reconstituted samples.</figcaption></figure>

Consequently, a series of DEM undrained tests were conducted to investigate the above remediative effect of injected sample with microgeometry control. The results obtained with original loose skeletons (represented by grey lines) of varying e₀ from 0,67 to 0,73 with their corresponding injected samples by random filling (by blue curves) and predisposed clogged aggregates (represented by red curves) are compared in Figure 7. The results from Figure 7(a) confirm the previous knowledge that fine sand effect is negligible in underfilled mixture condition as the particles floating within the void space.
<figure><img src="image_8.png" /><figcaption>Figure 7. DEM undrained triaxial test of original coarse skeletons (gray) compared with injected samples using (a) random filling (blue) and (b) clogged aggregates (red) approaches.</figcaption></figure>

CONCLUSION

This study presents a comprehensive investigation into the remediation of suffusion-affected
soils through fine particles injection, combining experimental and numerical methodologies to
assess both infiltration behavior and mechanical improvement. The results demonstrate that the
infiltration of fine particles into a coarse sand matrix follows an exponential retention profile,
enabling a quantifiable measure of infiltration depth (L₀). A micro-mechanically inspired
probabilistic model was proposed and successfully validated to predict L₀ based on pore
constriction statistics. This model offers a practical framework to assess filter effectiveness under
varying size ratio R and densities.

Mechanically, undrained triaxial experiments revealed that the micro-configuration of fines within the coarse skeleton plays a crucial role in influencing the soil's liquefaction resistance. Specifically, injected fine grains tend to reinforce the existing force chains and enhancing overall stability. In contrast, reconstituted mixtures with the same fine content showed increased compressibility and higher susceptibility to static liquefaction. Conducted to examine that the impact of fines is highly dependent on their microconfiguration-randomly filling approach has limited effect, whereas targeted clogging arrangements could significantly enhanced resistance to liquefaction.
Overall, this research highlights the importance of fine particle microstructure—even with a small —in controlling the macroscopic behavior of granular soils prone to suffusion and liquefaction. The findings offer valuable insights for developing targeted remediation strategies that leverage fine injection techniques to restore the mechanical integrity of vulnerable hydraulic structures. These results underscore the potential of microstructure-informed interventions in soil stabilization, especially in cases where conventional concepts may fall short.

## REFERENCES

Benahmed N., Canou J., Dupla J.C., « Initial structure and static liquefaction properties of sand », Comptes-Rendus Mécanique de l'Académie des Sciences, vol. 332, p. 887-894.

Bonelli, S., (2013). "Erosion in geomechanics applied to dams and levees". John Wiley & Sons.

Chen, F., Wautier, A., Philippe, P. et al. (2025). "Micromechanics of fine-grain infiltration in coarse grain sands". Acta Geotech. 20, 1533-1548.

Foster, M., Fell, R., Spannagle, M. (2000). "The statistics of embankment dam failures and accidents". Canadian Geotechnical Journal, 37(5), 1000-1024.

Thevanayagam, S., Shenthan, T., Mohan, S., Liang, J., (2002). "Undrained fragility of clean sands, silty sands, and sandy silts". Journal of Geotechnical and Geoenvironmental Engineering 128, 849-859.

Nguyen, C.D., Benahmed, N., Ando, E., et al. (2019). "Experimental investigation of microstructural changes in soils eroded by suffusion using x-ray tomography". Acta Geotechnica 14, 749-765. doi:10.1007/s11440-019-00787-w.

Wautier, A., Bonelli, S., Nicot, F., (2017). "Scale separation between grain detachment and grain transport in granular media subjected to an internal flow". Granular Matter 19, 22.

Wautier, A., Bonelli, S., Nicot, F., (2019a). "DEM investigations of internal erosion: Grain transport in the light of micromechanics". International Journal for Numerical and Analytical Methods in Geomechanics 43, 339-352.

Wautier, A., Bonelli, S., Nicot, F., (2019b). "Rattlers' contribution to granular plasticity and mechanical stability". International Journal of Plasticity 112, 172-193.
INTERNATIONAL SOCIETY FOR
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The paper was published in the proceedings of the 12th
International Conference on Scour and Erosion and was edited
by Shinji Sassa as the Chair of the TC213 on Scour and Erosion.
The conference was held in Chongqing, China from November
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