# Reinforcement of Eroded Granular Materials Through Fine Grains Injection

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

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Abhijit Hegde, Fan Chen, Nadia Benahmed, Antoine Wautier, Pierre Philippe. Reinforcement of Eroded Granular Materials Through Fine Grains Injection. IS Grenoble 2024, Sep 2024, Grenoble, France. IOP Conference Series: Earth and Environmental Science, 1480 (1), pp.012124, 2025, 10.1088/1755-1315/1480/1/012124. [https://doi.org/10.5457/881](https://doi.org/10.5457/881)

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# Reinforcement of Eroded Granular Materials Through Fine Grains Injection

To cite this article: Abhijit Hegde et al 2025 IOP Conf. Ser.: Earth Environ. Sci. 1480 012124

View the [article online](#) for updates and enhancements.

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# REINFORCEMENT OF ERODED GRANULAR MATERIALS THROUGH FINE GRAINS INJECTION

Abhijit Hegde¹✉, Fan Chen¹, Nadia Benahmed¹, Antoine Wautier¹ and Pierre Philippe¹

¹ INRAE, Aix-Marseille Université, RECOVER, Aix-en-Provence, France

✉ abhijit.hegde@inrae.fr

**Abstract:** In-order to counter the loss of fine particles during suffusion, a novel method of reinforcement is proposed which involves the infiltration of fine grains inside the eroded soil mass. We investigate the impact of this technique on the mechanical properties of the infiltrated material. A loose soil column consisting of only coarse particles is prepared inside a Suffusion Permeameter device and injected with fluidized fine grains until reaching a steady profile of fines concentration. Then, the injected samples are subjected to drained triaxial tests to study their mechanical behavior. Reconstituted samples (with the same fines concentration profile as the injected samples), as well as non-injected ones (with only coarse particles) are also tested for comparison. In-order to obtain the microscopic perspective of the interaction between the fine particles and coarse grains during injection, numerical simulations are performed using the DEM-PFV coupling scheme with YADE.

## 1. Introduction

Internal erosion processes such as suffusion occur due to the selective removal of fine grains from the soil skeleton under the influence of seepage forces. Even if fine grains are not part of the load bearing skeleton, their removal increases the risk of static liquefaction. One way of mitigating the onset of such mechanical instabilities would be to re-introduce fines into the eroded soil. Indeed, the addition of a small amount of fines has been demonstrated to have a stabilizing effect on the granular medium by providing lateral support to the existing force chains [1]. In this work, we combine injection tests with mechanical ones to assess whether the particular microstructure resulting from fine grain infiltration has a positive impact on the constitutive behavior and on the risk of static liquefaction.

## 2. Experimental Program

### 2.1. Injection experiments

The first step of the experimental program is the injection of fines grains into the coarse granular material. The latter is prepared inside the Suffusion Permeameter device by moist tamping technique at a target relative density, in 7 layers, each 2 cm thick, with the final dimensions of the sample being 14 cm of height by 7 cm of diameter, and fits on the triaxial setup. Once prepared, the samples are first flushed with CO₂ gas for about 20 minutes in-order to facilitate saturation, and then with water by applying a low flow rate of 50 ml/min in upward direction. Once the saturation is completed, an air-tight reservoir filled with water and a predetermined amount of fines, is connected to the suffusion permeameter. A magnetic agitator is put inside the reservoir to fluidize the fines. Injection is then performed by applying a downward, constant flow rate of 2500 ml/min. The entire experimental setup is illustrated in Figure 1. Hostun sand 1/2.5 is used as the coarse material. It has sub-angular grains, a mean diameter *D*₅₀ of 1.7 mm, and a coefficient of uniformity *C*<sub>u</sub> of 1.4, and has been used previously in suffusion studies [2]. The infiltrated material corresponds to Fontainebleau sand, with sub-rounded grains, a mean diameter *d*₅₀ of 0.21 mm, and a *C*<sub>u</sub> of 1.52. This combination gives a size ratio of *D*₅₀/*d*₅₀ ~ 8, which is expected to be sufficiently large to allow infiltration according to standard filter rules [3].

<figure><img src="image_3.png" /></figure>

Content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
<figure><img src="image_4.png" /><figcaption>Figure 1. Photo (a) & schematic (b), of the experimental setup used for injection of fines.</figcaption></figure>

After injection, fine grain infiltration profiles are obtained in a destructive manner by sieving 20 mm thick sections along the height of the specimen.

## 2.2. Triaxial experiments

To investigate the effect of injected fines on the mechanical behavior of the coarse sample, drained compression triaxial tests are performed on three sets of samples: (i) injected coarse samples; (ii) non-injected coarse samples; (iii) reconstituted coarse samples with fines concentration in different layers equal to those obtained during injection tests. The injected samples are first desaturated (water content ~ 6 %), and then frozen in the suffusion permeameter, before being transferred to the triaxial cell for performing the tests. After letting the sample thaw under a confining pressure of 100 kPa, a drained compression test is applied under fully saturated conditions (B value of 96 %). The lateral effective stress is kept to 100 kPa and the vertical strain rate is set to 1 %/min. The non-injected, reconstituted and injected samples all have the same initial inter-granular void ratio of 0.78 [2].

## 3. Experimental results

The vertical profile of fines concentration after the injection test is presented in Figure 2 along with the evolution of hydraulic conductivity. It can be seen that the concentration of fines falls off exponentially as we move deeper into the coarse material ($C(z) = C_0e^{-z/L_0}$), indicating that the infiltration process leads to the loss of homogeneity at the sample scale. The characteristic penetration depth $L_0$ is found to be around $15D_{50}$. The hydraulic conductivity of the sample decreases following two distinct regimes as shown in Figure 2 (b), until reaching a steady value that suggests the completion of the injection process. We can underline that the second regime coincides with the formation of a filter cake at the top of the sample, which progressively prevents more fines to infiltrate.

The results of triaxial tests carried out on non-injected, injected and reconstituted samples are depicted in Figure 3. All the tests were conducted up to 30 % axial strain to reach a constant deviatoric stress. The comparison between the injected and non-injected samples (with and without fines) shows that the presence of clogged fines do not impact much the shear strength and the volumetric behavior. However, the behavior of the reconstituted samples is significantly different, especially in terms of volumetric strains. They exhibit more contraction at small and medium strains and a slightly lower shear strength. These results are in accordance to that of Nguyen et al. [2] where the eroded samples showed less contraction than reconstituted ones. The larger contractancy for reconstituted samples
<figure><img src="image_5.png" /><figcaption>Figure 2. (a) Concentration profile of fines in the injected sample. (b) Evolution of hydraulic conductivity during injection.</figcaption></figure>

might be attributed to the moist tamping preparation method. Indeed, this method permit to reach low relative density thanks to capillary effects. Since capillary cohesion increases as grains get smaller, the use of moist tamping for samples with fine grains generate special microstructures with groups of jammed fine grains in between coarse grains. Consequently, reconstituted samples have more macropores resulting in a more contractive behavior. It is also important to remember that fines concentration within the sample is non-homogeneous, which complicates the interpretation of tests results from a material point perspective. Complementary investigations relying for instance on discrete element simulations (DEM) can help better understand the impact of the gradient of fine concentration.

<figure><img src="image_6.png" /><figcaption>Figure 3. Effect of fines on drained mechanical behavior of soil samples depicted in (a) q-ε<sub>a</sub> plane. (b) ε<sub>v</sub>-ε<sub>a</sub> plane.</figcaption></figure>

To complement the mechanical analysis for loading conditions more likely to mobilize the fine grains present in the pore space, undrained triaxial tests are currently being conducted.

4. Perspectives: building a digital twin model with DEM

To obtain a micro-mechanical perspective of the injection process, DEM simulations can be used, with particle size distributions matching that of Hostun sand 1/2.5. In DEM the impact of sample density can be investigated by tuning the friction angle during the sample preparation step (gravity
deposition). Dense samples are obtained by selecting a contact friction of 0°, whereas loose samples are obtained when using the material nominal friction of 30°, and the corresponding void ratios of the samples are 0.61 and 0.85 respectively, in good agreement with the experimentally determined $e_{min}$ and $e_{max}$ values [2]. After the preparation of coarse column (with dimensions ~ 25$D_{50}$ in each direction as shown in Figure 4 (a)), a few fine grains (to avoid collective clogging) are positioned randomly near the center of the column and injected into the sample by imposing hydraulic forces using the DEM-PFV coupling method [4]. This process is repeated several times to obtain penetration statistics over 500 grains, which is presented in Figure 4 (b). The population of fines within the sample is found to follow an exponential decay curve, in accordance with the experimental results presented in Figure 2. The characteristic penetration depths vary from 6$D_{50}$ to 16$D_{50}$ for the dense and loose cases respectively, which is comparable to the value of 15$D_{50}$ obtained experimentally on loose material.

<figure><img src="image_7.png" /><figcaption>Figure 4. DEM simulation of fines injection: (a) Sample after injection of fines. (b) Distribution of injected fines within the sample as a function of penetration distance (blue-dense; red-loose).</figcaption></figure>

Building upon these results, injection experiments are currently being performed with a large number of fine grains to reach a fines concentration similar to what is observed in experiments. Then, the mechanical behavior will be assessed through standard drained and undrained compression test and stability analyses will be conducted for different stress states using the second order work criterion [1]. Such an exhaustive stability analysis remains out of reach of experimental campaigns

References

[1] Wang, T., Wautier, A., Liu, S., and Nicot, F. (2021). How fines content affects granular plasticity of under-filled binary mixtures. *Acta Geotechnica*, pp. 1–15.

[2] Nguyen, C. D., Benahmed, N., Andò, E., Sibille, L., and Philippe, P. (2019). Experimental investigation of microstructural changes in soils eroded by suffusion using x-ray tomography. *Acta Geotechnica*, **14**, 749–765.

[3] Seblany, F., Vincens, E., and Picault, C. (2021). Determination of the opening size of granular filters. *International Journal for Numerical and Analytical Methods in Geomechanics*, **45**, 1195–1211.

[4] Catalano, E., Chareyre, B., and Barthélemy, E. (2014). Pore-scale modeling of fluid-particles interaction and emerging poromechanical effects. *International Journal for Numerical and Analytical Methods in Geomechanics*, **38**, 51–71.