Sustainable Soil Stabilization: Enhancing Expansive Soil with Natural Luffa Fiber

The soil sample used in this study is a grey expansive clay, collected from a depth of 1.2 meters below the ground surface in the Hammam N'Bail area, approximately 7 km from Guelma Province in northeastern Algeria. The test pit method was chosen for sampling due to its advantages, including direct visual inspection of soil layers, the ability to collect large and representative samples, and the opportunity to observe stratigraphy and in situ consistency. Excavation was carried out using a mechanical loader, and the extracted samples were carefully sealed in plastic bags to preserve their natural moisture content and structure during transport to the laboratory for comprehensive geotechnical characterization. The natural moisture content of the sample was found to be 19.01%.

Figure 1 shows shrinkage cracks on the surface of the clay soil in the Hammam N’Bail area, near Guelma (Algeria), a region known for its well-documented shrinkage issues.

Figure 1:

Surface shrinkage cracks in clay soil

The soil used in this study was sampled approximately 7 km northwest of Hammam N’Bail, along the main road to Guelma city, in northeastern Algeria (approximate coordinates: 36.3436° N, 7.6252° E). This semi-rural area experiences a semi-arid climate with marked seasonal variations in temperature and moisture.

Geologically, the region is part of the Neogene formations of northeastern Algeria, which are predominantly composed of high-plasticity clayey soils. These are classified as CH (high-plasticity clays) under the Unified Soil Classification System (USCS) and are characterized by expansive behavior, including notable shrink-swell potential.

Field observations at the sampling site revealed surface shrinkage cracks, seasonal ground heave, confirming the problematic nature of the soil. This makes the site particularly suitable for investigating sustainable stabilization techniques using natural materials.

Recently, numerous researchers have used luffa aegyptiaca fibers as reinforcement in the development of natural fiber composites to evaluate their performance in various matrix systems. In this study, these fibers were used to reinforce an expansive clay matrix. To obtain the fibers, commercially available luffa aegyptiaca fruits, commonly used for household purposes such as dishwashing or body scrubbing, were utilized. Figure 2 presents the mature luffa fruit and its fiber structure. Luffa fibers exhibit notable toughness, strength, and stiffness, comparable to those found in certain metals of similar density (Jino et al., 2017).

Figure 2:

Luffa and its internal structure

The fruits were procured and prepared for reinforcement applications. They were thoroughly washed and sundried to remove impurities. Once dried, the sponge-like structure was manually cut with scissors to extract fibers of varying lengths (ranging from 0.315 mm to 2 mm), with an average diameter of approximately 0.27 mm. Additionally, granules of various sizes, ranging from 0.25 mm to 0.063 mm in diameter, were also obtained.

The raw fibers were soaked in a 5% sodium hydroxide (NaOH) solution as part of an alkaline treatment to enhance their durability and reduce biodegradability. Chemical treatments with agents such as NaOH can significantly increase the surface roughness of luffa fibers, remove impurities, and reduce water absorption, thereby improving their overall performance characteristics. Following the chemical treatment, the luffa fibers were thoroughly washed and dried at 60 °C for 2 hours (Kamran et al., 2022). After testing and characterization, the treated luffa aegyptiaca fibers were used as reinforcement material in the expansive clay matrix. The resulting material, consisting of a mix of fibers is shown in Figure 3 below.

To ensure uniform distribution of luffa fibers within the soil matrix, the fibers were first cut into manageable lengths. A dry mixing process was used before moisture was added, and fiber portions were introduced incrementally to avoid clumping. To minimize the loss of luffa fibers due to their lightweight and tendency to become airborne, especially under windy conditions, the fibers were kept slightly moist rather than completely dried before cutting and mixing. This slight moisture helped reduce fiber dispersion and ensured better handling during the preparation process.

Mixing continued until a uniform blend was visually confirmed. For field applications, mechanical mixing is recommended to ensure even fiber distribution at larger scales.

Figure 3:

Luffa fiber used in this study

Tests were conducted on natural soil samples to determine their physical, chemical, and geotechnical properties. The samples were analyzed for physical and engineering characteristics, and the results are presented in Tables 1 and 2.

Sieve analysis and sedimentation tests were conducted to determine the particle size distribution of the soil. Figure 4 presents the grain size distribution curve of the soil sample, providing a visual representation of the results.

Figure 4:

Particle size distribution of the soil sample used in this study

The particle-size distribution indicates that the soil consists of 7.90% sand, 25.05% silt, and 66.47% clay. The liquid limit and plastic limit are 63.342% and 27.74%, respectively. Based on these values, the soil is classified as clay with intermediate plasticity. The one-dimensional free swell is approximately 10.32% for both natural and compacted specimens under a seating pressure of 6.9 kPa. Chemical properties were also evaluated following European standards NF EN 17542-3 and NF ISO 10390 (Association Française de Normalisation, 2022), and the results are summarized in Table 3 below.

X-Ray fluorescence (XRF) is a rapid, non-destructive analytical technique used to determine the elemental composition of materials. Key advantages include minimal sample preparation, high sensitivity, and the ability to analyze solid samples directly.

In this study, the chemical composition of the untreated soil, determined by XRF analysis, is presented in Table 4. The results indicate that the major components are silicon dioxide (SiO₂), calcium oxide (CaO), aluminum oxide (Al₂O₃), and iron oxide (Fe₂O₃).

It is worth noting that the predominant mineral in the soil, as determined by X-ray fluorescence (XRF) analysis, is quartz (SiO₂), with a content of approximately 45%. Additionally, the particle-size distribution indicates that the soil consists of 7.90% sand, 25.05% silt, and 66.47% clay. The liquid limit and plastic limit are 63.34% and 27.74%, respectively. Based on these properties, the soil is classified as an inorganic clay of low to medium plasticity.

Free swell, or volume change, is defined as the ratio of the increase in thickness to the initial thickness of a soil sample compacted at optimum moisture content, placed in a consolidation ring, and soaked under a seating surcharge of 6.9 kN/m² following XP P94-091 (Association Française de Normalisation, 1995). In contrast, the pressure exerted by expansive soil when it is not allowed to swell, or when volume change is restricted, is referred to as the swelling pressure.

Scanning Electron Microscopy (SEM) is a powerful imaging technique widely used in materials science to examine surface morphology and microstructural features at high magnification. It is particularly effective for analyzing fine-grained materials such as clays and silts, offering valuable insights into their physical behavior.

A key advantage of SEM is its ability to produce detailed, quasi-three-dimensional surface images using secondary electron (SE) detection, enabling high-resolution observation of particle shape, texture, cracks, and porosity (see Figure 5).

Figure 5:

A micrograph of the untreated expansive soil sample

In secondary electron (SE) mode, the SEM image reveals the surface topography of the clay sample. The microstructure is characterized by individual particles distributed across the surface. Voids or pores, visible as dark regions, are present between particles of varying sizes and exhibit no preferential orientation. This random distribution is typical of natural, unconsolidated clay materials and is further emphasized by the presence of finer grains, which highlight the material’s inherent porosity. Aggregates of various sizes are commonly observed, which is characteristic of natural clay systems. The surface texture ranges from rough to lamellar, indicative of clay minerals such as kaolinite, montmorillonite, or illite. At a magnification of 3000×, fine morphological features, including the shape and size of individual particles (typically around one micrometer or smaller), become distinguishable.

Particle edges may appear angular or rounded, depending on the clay’s origin and any mechanical or chemical treatments it has undergone. Brighter regions in the image correspond to areas with stronger secondary electron emission, such as edges or surface protrusions. Additionally, the presence of micro-cracks or fractures within particles is evident, likely resulting from dehydration processes or mechanical stress during sample preparation or environmental exposure.

As illustrated in Figure 6, the SEM micrograph of luffa after NaOH treatment reveals a rough and irregular surface with exposed fibrous structures (Ghali et al., 2009).

Figure 6:

SEM micrographs of a) untreated luffa fibers; b) treated luffa fibers

The morphology reveals partial disorganization due to the removal of substances such as lignin and hemicelluloses, resulting in a more porous texture. The visible pores indicate increased porosity, which may enhance the material’s adsorption capacity and its interaction with other substances.

X-ray diffraction (XRD) is a non-destructive technique used to identify the crystalline structure of materials by analyzing the interaction of X-rays with atomic planes. In soil analysis, XRD is employed to determine the mineralogical composition, particularly of clay minerals and oxides, with high precision. This information is essential for evaluating soil properties in agricultural, environmental, and geotechnical contexts. In the present study, the untreated soil was analyzed using X-ray diffraction (XRD). The analysis of the clay fraction revealed the presence of several minerals, including quartz (SiO₂), kaolinite (Al₂Si₂O₅(OH)₄), illite, and hematite (Fe₂O₃), as well as minor amounts of calcium- and magnesium-bearing phases. These results indicate the mineralogical heterogeneity of the clay sample, with quartz identified as the predominant phase. Figure 7 shows the XRD pattern of luffa fibers. The diffraction peaks observed at 2θ = 14.3°, 22.5°, and 34.5° correspond to the (110), (002), and (040) crystalline planes, respectively, indicating the presence of cellulose polymorph I as the dominant crystalline structure.

Figure 7:

XRD of luffa fiber

The Atterberg limits of both untreated and treated soils, incorporating various percentages of luffa fiber, were determined following the EN ISO 17892-12 (Association Française de Normalisation, 2022) standard, which specifies procedures for measuring the liquid limit, plastic limit, and plasticity index. The liquid limit was measured using the Casagrande apparatus after the sample was sieved through a No. 40 sieve (0.400 mm).

The primary objective of these tests was to determine the free swell and swelling pressure of both untreated and fiber-treated soils. Free swell tests were conducted following the XP P94-091 (Association Française de Normalisation, 1995), using a standard one-dimensional oedometer apparatus. For each test, soil samples, both untreated and treated, were mixed with water at their respective optimum moisture content (OMC) and thoroughly blended until a homogeneous mixture was obtained.

The oedometer ring used had a diameter of 50 mm and an initial height of 19 mm. Specimens were placed between two dry porous stones inside the oedometer cell and subjected to a seating pressure of 6.9 kPa. A dial gauge was mounted on the top cap to measure vertical displacement during the test.

The extensometer was calibrated, and the specimen was inundated from both the top and bottom to allow vertical swelling to occur. Deformation readings were recorded at time intervals of 0.1, 0.2, 0.5, 1.0, 2.0, 4.0, 8.0, 15.0, and 30.0 minutes, as well as at 1, 2, 4, 6, 24, 48, and 72 hours, until deformation stabilized following both the primary and secondary swelling phases.

The oedometric (oedometer) tests were performed at the optimum moisture content (OMC) determined for each fiber-reinforced mixture using the Standard Proctor compaction method.

The percentage of free swell was then calculated based on the specimen’s initial height. Swelling pressure tests followed a similar procedure; once primary swelling was complete, vertical pressure was incrementally applied to the top of the specimen until the initial void ratio was restored, that is, until no further vertical displacement was recorded by the extensometer.

The methylene blue test conducted in this study followed the French standard NF EN 17542-3 (Association Française de Normalisation, 2022). The test solution was prepared by dissolving 10 ± 0.1 g of methylene blue powder in one liter of distilled water, with continuous stirring at room temperature for one hour.

Methylene blue is a cationic dye with the chemical formula C₁₆H₁₈N₃SCl. When dissolved in water and mixed with a soil suspension, the chloride ions in the methylene blue solution are exchanged with cations present in clay minerals, allowing the dye to be adsorbed onto their surfaces. The amount of methylene blue adsorbed depends on the type and quantity of clay minerals, the cation exchange capacity (CEC), and the specific surface area of the soil. For each luffa fiber content, the methylene blue adsorption value was determined accordingly.

The primary objective of soil compaction is to produce a material that meets three fundamental requirements: reducing settlement under load, decreasing permeability to limit water-induced stresses, and improving shear strength and bearing capacity.

The optimum moisture content (OMC) and maximum dry unit weight (γ_dmax) are determined from the compaction curve. Standard Proctor tests were conducted following the NF P94-093-10 (Association Française de Normalisation, 2014) to evaluate these parameters for both untreated soil and soil treated with 0.5%, 1%, 1.5%, and 2% luffa fiber.

The pH values of both untreated and luffa fiber–reinforced soils were determined in accordance with the NF ISO 10390 (Association Française de Normalisation, 2022), which specifies procedures for determining soil pH in either water or a KCl solution.

In this study, soil samples were first air-dried and then sieved through a 2 mm mesh. A soil-to-water ratio of 1:2.5 was used; the mixture was stirred and allowed to equilibrate before the pH was measured using a calibrated pH meter. This test provides insight into the chemical reactivity of the soil and potential changes in its environment following fiber reinforcement.

The chemical composition of the untreated expansive soil (clay) and the soil treated with 1% luffa fiber was determined after 1, 7, and 28 days of air-drying. For the treated samples, the composition was evaluated as a function of curing time following reinforcement with luffa fiber. The results are presented in Table 7.

Silica content shows a slight variation, indicating limited pozzolanic reactivity. A similar trend is observed for alumina, where minor fluctuations may reflect initial interactions between luffa fibers and clay minerals, followed by stabilization. Iron oxide remains stable across all samples (4.69%–4.76%), suggesting it is unaffected by fiber treatment or curing and remains chemically inert. Calcium oxide content gradually decreases, reaching 17.15% at 28 days, possibly due to minor chemical interactions such as carbonation or bonding with reactive clay constituents.

Magnesium oxide remains relatively constant (2.2%–2.3%), confirming its chemical stability. Sulfate levels vary slightly (0.576%–0.673%) without a clear trend, likely due to transient ion exchange or measurement variability. Potassium and sodium oxide contents also remain stable, indicating no significant influence from luffa fiber treatment or curing.

Chloride content remains consistently low (0.018%–0.025%), well below the 0.03% threshold considered acceptable, indicating minimal risk of chloride-induced reactions or corrosion. This supports the long-term chemical stability of the treated soil.

In conclusion, X-ray fluorescence (XRF) analyses of clay treated with 1% luffa fiber at 1, 7, and 28 days confirm notable chemical stability. The consistency in major oxide contents (SiO₂, CaO, and Fe₂O₃) suggests no significant chemical reactivity between the fiber and the soil. These findings imply that the interactions are primarily physical, likely involving adsorption or dispersion of luffa particles, rather than chemical reactions.

The low and stable chloride (Cl⁻) levels further confirm the absence of soluble salt accumulation. Additionally, no heavy metals (e.g., Pb, Cr, and Cd) were detected, indicating that the treatment does not introduce toxicity.

Overall, these results demonstrate that incorporating luffa fibers into expansive soil does not cause chemical contamination or harmful mineralogical changes, reinforcing their environmentally friendly nature.

The XRD analysis of the matrix revealed intense diffraction peaks at 2θ = 26.6° and 29.6°, corresponding to quartz (SiO₂) and other mineral phases containing Fe, S, and K, respectively. Additional smaller peaks indicate the presence of various minerals characteristic of the matrix’s heterogeneous surface, including dolomite (CaMg(CO₃)₂), calcite (CaCO₃), albite (NaAlSi₃O₈), and kaolinite/illite (Al₂Si₂O₅(OH)₄).

The XRD analysis indicates that the mineralogical assemblage of the expansive soil remains unchanged after the incorporation of 1% luffa fibers. Quartz (SiO₂) continues to be the dominant crystalline phase, accompanied by aluminosilicate clay minerals (illite/kaolinite), carbonate phases (calcite/dolomite), and minor iron oxides (hematite). No new crystalline phases were identified in the treated sample, confirming the absence of chemical or mineralogical reactivity between the soil and the luffa fibers.

Two main modifications can be observed in the treated soil pattern:

• a slight reduction in the intensity of the major diffraction peaks, particularly those of quartz, suggesting partial masking of the crystalline signal by the organic matter of the fibers;

• a modest increase and broadening of the diffuse background in the 15–30° 2θ range, characteristic of an amorphous contribution related to the luffa.

These results demonstrate that luffa fibers act primarily as a physical and mechanical reinforcement rather than as a reactive mineralogical modifier.

Figure 16 presents a scanning electron microscopy (SEM) micrograph of soil treated with 1% luffa fiber, illustrating the morphological changes induced by the fiber addition.

Figure 16:

Scanning Electron Microscopy (SEM) micrograph of soil treated with 1% luffa fiber

The scanning electron microscopy (SEM) micrograph (Figure 16) reveals that the addition of 1% luffa fibers alters the morphology and structure of the treated soil, primarily through physical interactions. The presence of elongated, rough fibers indicates that the luffa does not dissolve but instead interacts with clay particles, leading to a less compact microstructure. This observation is further supported by the results of the Standard Proctor compaction test.