Investigation of the Effects of Almond Husk Ash on the Engineering Properties of Expansive Soil

Abstract

In recent years, the use of waste materials for soil improvement has gained increasing importance due to sustainability concerns and the need for effective waste disposal. Almond husk ash (AHA), though considered a major environmental pollutant, is classified as a non-hazardous and noninert waste. One of the primary challenges associated with such industrial wastes is their storage; therefore, environmentally safe disposal methods are essential. This study aimed to investigate the potential of AHA in improving expansive soil (ES). The findings revealed that ES can be effectively stabilized using AHA and geogrids, both individually and in combination. The optimal conditions for soil improvement were identified as follows: 25% AHA content, a zone depth of 1.5 units, and three layers of geogrids. The bearing capacity ratios showed significant improvement under various conditions: a 2.56-fold increase with AHA alone, a 2.87-fold increase with geogrids alone, and a 5.60-fold increase when both AHA and geogrids were used together. The greatest enhancement was achieved through the combined application of AHA and geogrids. AHA was thus demonstrated to be an effective, economical, and environmentally sustainable additive for the stabilization of expansive soils. Furthermore, microstructural analyses using scanning electron microscopy (SEM), X-ray fluorescence (XRF), and X-ray diffraction (XRD) supported the improvements observed in the experimental results.

1. Introduction

Soil improvement is used widely to prevent excessive settlement and construction impairment in roads, airports, bridges, and other foundations [1]. The following conditions should be ensured in soil improvement.

• If the soil becomes liquid, structures may get damaged, so the density and hardness of the soil should be increased to prevent this situation [2].
• The soil should be improved to increase the bearing capacity and shear strength and prevent slip failure [3].
• Soil improvement is used to immobilize or stabilize contaminants in dredged soil to mitigate and preferably eliminate environmental impacts [4].

Expansive soils are unsafe because as the moisture content increases, these soils have a tendency to swell [5,6]. This is an important problem for engineering structures, such as roads, bridges, buildings, and airport runways. The evaluation of industrial waste materials for the improvement of soils is both an efficient and an environmentally friendly method. Many researchers have evaluated the impact of industrial wastes on soil improvement and different application areas [7,8,9]. Jack and Pandian [7] investigated chemical soil stabilization methods, highlighting that cement works well for low-to-moderately plastic soils, while lime is preferred for plastic clays but has limitations in sulfate-rich soils and harsh conditions. To address these issues, their research focused on using solid wastes as additives or replacements for traditional stabilizers. Their study emphasizes that combining industrial wastes with lime or cement improves soil stabilization and promotes sustainable waste management, providing a foundation for future research in this area. Mistry et al. [8] reviewed the use of waste tires to improve clayey soil properties. Their study highlights that waste tires can enhance soil strength and durability, offering a cost-effective and sustainable solution for construction. They also stressed the need for further research to confirm these benefits for applications like highways and railways. Zhang et al. [9] studied the impact of different fertilization treatments on soil respiration and temperature sensitivity (Q₁₀) in winter wheat–summer maize rotation in the North China Plain. They found that fertilization increased soil respiration and crop yields, with organic plus mineral fertilizer (OMNPK) showing the best results for crop productivity and carbon sequestration. Their work highlights that combining organic and mineral fertilizers is an effective strategy for sustainable agriculture and carbon management.

Several researchers have investigated the use of various industrial ashes as soil improvement additives. Cokca [10] conducted experiments to examine the effects of fly ash on clay soils. The results demonstrated that the swelling pressure decreased by 75% after 7 days of curing and by 79% after 28 days when 20% fly ash was incorporated. Class C fly ash was found to be particularly effective in reducing both the swell potential and the plasticity index of clay soils. Lin et al. [11] studied the use of sewage sludge ash and fly ash for enhancing soft cohesive subgrades. Test specimens with varying proportions of these ashes were subjected to compression, pH, California Bearing Ratio (CBR), unconfined compressive strength (UCS), and triaxial compression tests. The results indicated that sewage sludge ash could serve as a viable alternative to fly ash in the stabilization of clay soils. Keerthi et al. [12] evaluated the use of cement kiln dust for soil stabilization and found that it significantly improved certain geotechnical properties of soils, especially compressive strength. Similarly, Hasan et al. [13] investigated the use of bagasse ash in the stabilization of road subgrades in Australia. Their study involved the use of bagasse ash, hydrated lime, and black soil samples collected from Queensland. These samples were tested using the free swell ratio (FSR), UCS, and CBR methods. The findings confirmed that expansive soils could be effectively stabilized using a combination of bagasse ash and hydrated lime. Brooks [14] also examined the stabilization of clay soils using both fly ash and corn waste ash. A series of laboratory experiments were performed on specimens containing different proportions of these additives to assess their effectiveness. Experimental results showed that the impact on the UCS of 15% fly ash and 6% corn waste ash with a 28-day curing period was 840 kPa. With 15% fly ash and 12% corn waste ash and a 28-day curing period, the UCS was 959 kPa and the CBR improved from 3.7 to 10.6% for 25% fly ash. Basha et al. [15] investigated the effect of cement and rice husk ash on the improvement of soils. Their study recommends that the optimum amount in terms of plasticity, compression and strength properties, and economy would be obtained by adding 6–8% cement and 10–15% rice husk ash. Ramakrishna and Pradeepkumar [16] used rice husk ash and cement for soil improvement. According to strength characteristics, they recommended 8% cement and 10% rice husk ash as an effective dosage for stabilization. In addition, in a study by Sharma et al. [17], the efficiency of expansive soil stabilized with lime, calcium chloride, and rice husk ash was investigated. Their effective dosage was 4% lime and 1% calcium chloride without the addition of rice husk ash. According to uniaxial compression strength and CBR test results, when the soil was mixed with lime or calcium chloride, a rice husk ash content of 12% was found to be effective. Similarly, Bagriacik et al. [18] suggested that clay soil and olive mill residual solid ash could also be used in clay soil improvement. The researchers revealed that this ash can provide 6.40 times’ improvement in the value of the bearing capacity.

Other researchers have also explored the use of various industrial wastes, such as marble and construction debris, for soil improvement. Ural et al. [19] evaluated the use of waste marble for the improvement of expansive soils. They investigated the physico-mechanic and physicochemical characteristics of expansive soils with marble waste dust additives. Their analyses indicated that some mechanical and physical improvement (bearing capacity, settlement, swelling potential, strength) occurred in the behavior of expansive soil. Gupta and Sharma [20] used marble dust for expansive soil improvement in their study. They found that 15% marble dust was sufficient to increase the CBR soaked value up to approximately 200%. Bagriacik and Güner [21] investigated the potential use of drinking water treatment plant sludge as an additive for soil improvement. Their study identified optimal mixing ratios, reinforcement thicknesses, consolidation settlements, and the reinforcement mechanisms involved in sludge–soil mixtures. The findings indicated that the addition of water treatment sludge led to a reduction in consolidation settlements by up to 62%. Similarly, Mahmutluoglu and Bagriacik [22] examined the applicability of glass manufacturing waste for soil reinforcement. Their results demonstrated that the inclusion of this waste material enhanced the bearing capacity of soil by a factor of 5.51. Abbaspour et al. [23] explored the use of waste tire textile fibers for soil stabilization and found that an optimal improvement in expansive soil properties was achieved with a 1% fiber addition. Furthermore, Latifi et al. [24] assessed the improvement of clay soils using calcium carbide residue. Their results suggested that this waste material was effective for clay soil stabilization, with the most significant strength gains observed at 9% and 12% inclusion levels.

The literature shows that there are many additives used for soil improvement [25,26]. However, the use of almond husk ash (AHA) for soil improvement has not been investigated, so the evaluation of this waste will be presented with an innovative perspective in this research. The purpose of this research was to examine the feasibility of using AHA, geogrids, and AHA–geogrid reinforcement to increase the load-bearing capacity of expansive soil (ES). Although numerous additives have been investigated for soil improvement in the existing literature [25,26,27,28,29,30], the potential use of almond husk ash (AHA) as a soil-stabilizing agent has not yet been explored. This study introduced a novel approach by evaluating AHA as a sustainable material for enhancing the engineering properties of expansive soils. The primary objective of the research was to assess the effectiveness of AHA, geogrid reinforcement, and their combined application (AHA–geogrid) in improving the load-bearing capacity of expansive soils.

2. Materials and Methods

2.1. Materials

ES and AHA were obtained from Adana and Mersin, respectively (Figure 1). In the experiments, low-resistivity and expansive soil finer than a 0.074 mm sieve was used. The AHA used was preferred in a smaller size of 2 mm due to the creation of a high surface area and the belief that this formation would provide better adhesion.

The almond (Prunus dulcis) is a tree species whose fruit consists of four main parts: the kernel, the middle husk, the almond husk, and the seed husk. The commercial value of the almond fruit lies primarily in its kernel, while the other components—such as the husk and shell—are typically used as livestock feed or combusted as fuel. In Turkey, the annual almond production is approximately 85,000 metric tons, of which only about 40% is consumable; the remaining 60% constitutes agricultural waste [31]. The outer husk of the almond is commonly used as a low-cost fuel in industrial settings, such as direct-fired ovens, barbecue restaurants, and poultry farms. This practice helps reduce wood consumption and is therefore considered environmentally friendly. Upon combustion, almond husk generates approximately 3.2% ash. Almond husk ash (AHA), classified as a combustion by-product, is regarded as a significant environmental concern due to storage challenges. Currently, AHA has no recognized industrial application and remains an underused waste material.

In the experiments, expansive soil with low bearing capacity was selected below the 74 µ sieve range. A set of tests were carried out in the laboratory in order to determine the engineering properties of the expansive soil in question. The liquid limit value of the expansive soil was around 42%, and the plastic limit value was around 24%. The class of soil was identified as medium-plasticity ES according to ASTM D2487 [32]. The chemical contents of the ES and AHA were detected using XRF (Minipal 4) (

Table 1. Chemical contents of ES and AHA [33].

(%)	MgO	Al2O3	SiO2	P2O5	K2O	CaO	MnO	Fe2O3	Na2O	TiO2	SO2	LOI
ES	6.10	18.4	50.60	0.65	3.10	3.20	3.10	8.70	2.50	1.65	-	3.15
AHA	3.39	1.01	12.30	2.70	48.3	20.5	0.05	2.35	1.60	0.27	0.80	6.73

).

Table 1 reports that almond husk ash (AHA) contains 12.3% SiO₂; however, the specific form of silica—whether amorphous (reactive) or crystalline (non-reactive)—is not delineated. The pozzolanic activity of silica is largely dependent on its structural form, as amorphous silica readily reacts with calcium hydroxide to form calcium silicate hydrate (C–S–H) phases, thereby enhancing the cementation process and improving the mechanical properties of stabilized soils and cementitious composites.

X-ray diffraction (XRD) peaks of the samples were obtained with Rigaku Miniflex XRD equipment (Suzhou, China) (Cu Ka radiation in the 2ø range of 5–85°), and the minerals were defined by using PDXL software (Version 1.8. 0.3) with the current database. The crystalline phases in ES were determined to be montmorillonite, quartz, and hematite minerals (Figure 2), while an amorphous structure was detected in AHA (Figure 3).

The morphological structures of the samples were analyzed with a Quanta FEG 650 scanning electron microscopy (SEM) (Figure 4). Figure 4a shows that the microscopic structure of ES was comparatively flaccid, with porosity and a small grain size. SEM micrographs of AHA presented a characteristic porous form with broken edges, as shown in Figure 4b.

2.2. Method

A standard compaction test [34] was conducted to determine the optimum moisture content and maximum dry unit weight of the soil specimens. For model tests, a circular test tank with a diameter of 60 cm was used. The tank was constructed from 0.5 cm thick steel profiles, and its base consisted of a 2 cm thick steel plate (Figure 5). A circular steel plate, 10 cm in diameter and 1 cm thick, was used as the model footing during the experiments.

To measure the applied load, an electronic load cell manufactured by ESIT was used, while vertical displacements at the base of the footing were recorded using electronic displacement transducers produced by ELE. Displacement measurements were taken at two points aligned along the centerline of the footing, and the average of these readings was considered as the settlement value corresponding to each load increment.

Both the load values from the load cell and the displacement readings from the transducers were transmitted to a 32-channel data acquisition system (Autonomous Data Acquisition Unit—ADU). The acquired data were subsequently processed and converted into numerical values using DS7 software (DataSystem 7.3) as shown in Figure 5.

During the sample preparation phase, soil specimens collected from the field were oven-dried for 24 h at a constant temperature of 105 ± 5 °C. Once dried, the samples were pulverized, and standard geotechnical laboratory tests were performed to determine their physical and mechanical properties. Following this characterization, the soil was reconstituted at its natural moisture content for use in laboratory-scale model experiments. The pulverized soil was mixed with water to achieve an average field moisture content of 18% and thoroughly kneaded to ensure uniform moisture distribution.

The prepared soil was then divided into equal portions of 5 kg, sealed in plastic bags, and stored in a curing room for 24 h to prevent moisture loss. For model testing, the expansive soil was placed into the test box in layers, each 2 cm thick. Each layer was compacted using a specially designed rammer, applying standard compaction energy, with approximately 80 blows per layer, as recommended by standard procedures [35].

AHA was incorporated into the expansive soil at varying proportions by dry weight of the soil in order to assess its influence on the geotechnical behavior of the treated specimens. Once the placement of the expansive soil was completed, the test model was centered within the base frame. A measurement system, consisting of two vertical displacement gauges and a load cell, was installed. Prior to testing, all instruments were zeroed, and the setup was checked for stability and proper alignment. Vertical static loading was applied to the center of the footing plate at a constant rate (experiments were carried out using a motor set to load at a speed of 0.5 mm/min). At each loading stage, displacements were recorded and monitored until they stabilized. Upon completion of the tests, the load and displacement data were transferred to the ADU device and converted into numerical form using DS7 software (DataSystem 7.3). From the obtained data, load–settlement curves were plotted, and the bearing capacity was determined.

To quantify the improvement in bearing capacity due to the stabilized backfill additive, the bearing capacity ratio (BCR) was calculated using the following equation [36]:

$$\mathbf{B}\mathbf{C}\mathbf{R}={\displaystyle \frac{\mathbf{B}\mathbf{e}\mathbf{a}\mathbf{r}\mathbf{i}\mathbf{n}\mathbf{g}\mathbf{c}\mathbf{a}\mathbf{p}\mathbf{a}\mathbf{c}\mathbf{i}\mathbf{t}\mathbf{y}\mathbf{f}\mathbf{o}\mathbf{r}\mathbf{e}\mathbf{x}\mathbf{p}\mathbf{a}\mathbf{n}\mathbf{s}\mathbf{i}\mathbf{v}\mathbf{e}\mathbf{s}\mathbf{o}\mathbf{i}\mathbf{l}\mathbf{w}\mathbf{i}\mathbf{t}\mathbf{h}\mathbf{a}\mathbf{d}\mathbf{d}\mathbf{i}\mathbf{t}\mathbf{i}\mathbf{v}\mathbf{e}\left({\mathbf{q}}_{\mathbf{r}}\right)}{\mathbf{B}\mathbf{e}\mathbf{a}\mathbf{r}\mathbf{i}\mathbf{n}\mathbf{g}\mathbf{c}\mathbf{a}\mathbf{p}\mathbf{a}\mathbf{c}\mathbf{i}\mathbf{t}\mathbf{y}\mathbf{f}\mathbf{o}\mathbf{r}\mathbf{o}\mathbf{n}\mathbf{l}\mathbf{y}\mathbf{e}\mathbf{x}\mathbf{p}\mathbf{a}\mathbf{n}\mathbf{s}\mathbf{i}\mathbf{v}\mathbf{e}\mathbf{s}\mathbf{o}\mathbf{i}\mathbf{l}\left({\mathbf{q}}_0\right)}}$$

(1)

3. Results

The primary objective of this study was to systematically evaluate the efficacy of (AHA) as a soil-stabilizing agent. The research specifically aimed to determine the optimal AHA dosage for the maximum soil performance, identify the most effective depth of application within the soil profile based on the mechanical response, and assess the ideal number of geogrid reinforcement layers required to achieve enhanced strength and stiffness characteristics in treated soils. Findings are presented next.

3.1. Effect of Water Content

The variation in the dry unit weight with respect to the water content for different mix proportions is illustrated in Figure 6. Additionally, the optimum water content and the corresponding maximum dry unit weight values for each ash–soil mixture (AHA) proportion are presented in Figure 7. The results indicated that increasing the AHA content up to 25% led to a rise in the maximum dry unit weight. However, beyond this threshold, a slight decrease in the maximum dry unit weight was observed. Furthermore, an increase in the AHA content resulted in a higher water demand to achieve optimal compaction. This trend is attributed to the water-absorbing properties of certain components present in AHA, in accordance with ASTM D698 [37].

When water is added to the soil, the friction between particles decreases up to a certain moisture level, allowing soil grains to pack more closely. This results in an increase in the dry unit weight. As the water content continues to rise, the compressibility of the soil increases until it reaches a peak. In this study, the optimum water content was determined to be 19.76%, at which point the maximum dry unit weight of 18.91 kN/m³ was achieved. Beyond this water content, excess moisture begins to accumulate between soil particles, reducing interparticle contact and hindering further compaction. As a result, both compressibility and dry unit weight begin to decline.

3.2. Effect of AHA on the BCR

Figure 8a–c presents the load–settlement behavior, bearing capacity, and bearing capacity ratio (BCR) for various mixing ratios. Analysis of the test results indicated that the BCR increased by up to 2.56 times as the proportion of AHA in the expansive soil (ES) increased. This suggests that AHA can be effectively used as a soil improvement additive. The maximum BCR was observed at a 25% AHA content. Beyond this percentage, no significant additional improvement was noted. Therefore, a 25% AHA content was determined to be the optimum dosage for enhancing the engineering properties of the soil using this additive. Exceeding this proportion not only offers negligible improvement but also results in inefficient use of waste resources.

The improvement in soil strength with increasing AHA content can be attributed to its chemical composition. The potassium present in AHA helps reduce the swelling potential of clay soils by limiting water absorption, thereby enhancing the strength of the soil matrix [38,39]. Additionally, the calcium content in AHA contributes to increased soil strength by promoting cementitious reactions within the soil.

3.3. Effect of the Depth of the AHA Zone

To evaluate the influence of the reinforcement depth on soil improvement, a series of experiments were conducted using the optimal AHA–soil mixture at varying reinforcement depths (Rd). The corresponding bearing capacity ratio (BCR) values are presented in Figure 9a–c. The selected range of improvement depths was based on prior studies reported in the literature [22,40,41,42,43].

According to the test results, an increase in the BCR was observed at all reinforcement depths in the untreated expansive soil (ES). When reinforced with the AHA mixture, the BCR increased significantly up to 2.41 times up to a relative reinforcement depth of Rd = 1.5. Beyond this point, further increases in depth had a negligible impact on the BCR, which remained nearly constant. Therefore, Rd = 1.5 was identified as the optimum reinforcement depth.

In practical applications of soil stabilization using AHA, reinforcing only up to a relative depth of 1.5 is sufficient, eliminating the need to treat the entire expansive soil layer (e.g., Rd = 6.0). This selective treatment approach not only reduces the required quantity of stabilization mixture and compaction energy but also promotes more efficient use of industrial waste materials.

3.4. Effect of the Geogrid Layer

In order to determine the effects of the geogrid number on the bearing capacity of ES, experiments were carried out using different numbers of geogrids. Figure 10a–c shows the load–settlement ratio, bearing capacity, and BCR for the different numbers of geogrid layers using the appropriate geogrid ranges determined in the literature. In accordance with the experimental results, there was an increase in the BCR when it was just expansive soil for all geogrid numbers placed in the expansive soil. In addition, the bearing capacity up to N = 3 increased by 2.87 times to reach a high BCR. Using more than N = 3 geogrids did not provide an extra improvement in the bearing capacity.

Numerous studies in the relevant literature have examined the use of geogrids in varying quantities to enhance the engineering behavior of soils. A review of these studies reveals supporting evidence for the effectiveness of using three geogrid layers (N = 3)—a finding that aligns with the results obtained in this research [44,45,46,47,48,49].

In experimental models where geogrids were applied, the positioning of the geogrid layers was determined based on established findings from earlier studies [44,45,46,47,48,49]. Specifically, the depth from the soil surface to the first geogrid layer (u), the vertical distance between successive layers (h), and the length of each geogrid layer (b) were selected in accordance with these references. The total depth of reinforcement (d) beneath the model footing, which includes N geogrid layers and the mentioned parameters, can be calculated through a defined formula [42]:

d = u + (N − 1) × h

(2)

The bearing capacity ratio (BCR) of geogrid-reinforced soil systems is influenced by several dimensionless parameters, such as b/D, h/D, u/D, and d/D, where D is the diameter of the model foundation. Prior research has shown that when multiple geogrid layers are placed according to specific values of b/D, h/D, and d/D, the BCR of untreated soil tends to increase with rising u/D—up to a certain threshold—after which further increases result in diminishing improvements [44,45,46,47,48,49].

According to Shin et al. [47], the optimal h/D ratio for strip footings typically lies between 0.25 and 0.5. Similarly, the ideal b/D ratio for maximizing the bearing capacity is generally found within the range of 6 to 8 [18,49,50]. Based on insights from these prior investigations, this study adopted the following values for the parameters: u/D = 0.33, 0.67, h/D = 0.33, b/D = 6.444, and number of geogrid layers (N) = 0, 1, 2, 3, 4.

3.5. Effect of the Combination of Geogrids and AHA

In this study, experimental investigations were conducted to evaluate the changes in soil’s bearing capacity resulting from the use of both geogrids and AHA material. The results indicated a significant increase in the bearing capacity across all mixture alternatives, as shown in Figure 11 and Figure 12.

The bearing capacity of the expansive soil (ES) increased by a factor of 2.56 with the addition of AHA alone, by 2.87 with the application of geogrids alone, and by 5.60 when both materials were used in combination, as illustrated in Figure 13. These results clearly indicate that the most substantial improvement in the bearing capacity was achieved through the combined use of AHA and geogrids.

This enhancement is attributed to the synergistic interaction between the chemical and mechanical reinforcement mechanisms provided by AHA and geogrids, respectively. AHA contributes to improved soil performance by adsorbing excess water, thereby reducing the swelling potential of clay, and by forming chemical bonds that enhance the soil matrix. Concurrently, the inclusion of geogrid layers provides mechanical interlocking and confinement, which increases load distribution and stability. Together, these mechanisms lead to a more robust and load-resistant subgrade system.

Optimum results were achieved using a mixture containing 25% AHA and expansive soil (ES), and therefore, this proportion was selected for detailed analysis using scanning electron microscopy (SEM). It is hypothesized that the enhanced binding observed between AHA and ES is largely due to the high potassium and calcium content in AHA. This increased chemical connectivity contributes to the improvement in the bearing capacity of expansive soils. SEM images revealed that AHA possesses a porous structure with fractured edges, while expansive soil exhibits a loose texture characterized by high porosity and a fine particle size. These morphological features enable AHA and ES particles to interlock and fill voids by forming a more cohesive matrix, as illustrated in Figure 14.

The role of calcium and potassium in pozzolanic activity varies significantly in terms of their mechanism and effectiveness for expansive soil improvement [51]. Biomass ash (almond shell, nut shell, coconut husk, rice husk, etc.), when thermally activated, is rich in amorphous silica (SiO₂) and alumina (Al₂O₃), giving it pozzolanic properties that enable it to react with calcium hydroxide (Ca(OH)₂) in alkaline pore water [52,53,54]. Calcium ions (Ca²⁺) are essential for the initiation and progression of pozzolanic reactions. They not only provide the necessary alkaline environment through calcium hydroxide (Ca(OH)₂) but also directly participate in the formation of secondary cementitious products, such as calcium silicate hydrates (C-S-H) and calcium aluminate hydrates (C-A-H).

In contrast, potassium ions (K⁺) do not directly form binding compounds but contribute indirectly to pozzolanic activity. Through compounds like potassium hydroxide (KOH), potassium increases the pH of the pore solution, which promotes the dissolution of amorphous silica and alumina found in pozzolanic materials such as fly ash. This increased solubility accelerates the kinetics of the pozzolanic reaction, although potassium itself does not participate in the formation of C-S-H or C-A-H gels. Consequently, potassium acts as a chemical facilitator rather than a primary reactant within the pozzolanic reaction system.

AHA is a pozzolanic material that primarily consists of high contents of silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), and iron oxide (Fe₂O₃). These oxides react with calcium hydroxide (Ca(OH)₂) to produce secondary cementitious compounds, mainly calcium silicate hydrates (C-S-H) and calcium aluminate hydrates (C-A-H) [42]. Such pozzolanic reactions significantly enhance the mechanical and engineering properties of treated soils. Additionally, AHA predominantly contains amorphous (glassy) forms of silica and alumina, which exhibit higher reactivity compared to their crystalline counterparts. This increased reactivity accelerates the pozzolanic processes [27,50]. Consequently, due to its chemical composition and structure, AHA serves as an effective additive in soil stabilization, particularly when applied in combination with lime.

4. Conclusions and Discussions

This study provides a comprehensive evaluation of Almond husk ash (AHA) as a stabilizing agent for expansive soils (ES), which are characterized by low bearing capacity and high swelling potential. The key experimental findings are summarized as follows:

• The addition of AHA to the soil increased the optimum water content required for compaction, associated with an increase in the maximum dry density. As the AHA content rose, the water demand continuously increased, while the maximum dry density peaked at 25% AHA before declining at higher ratios. Therefore, 25% AHA was identified as the optimum content. The increased water requirement with higher AHA content is attributed to its fine-grained, porous structure and high moisture adsorption capacity.
• Incorporating 25% AHA into the soil resulted in a 2.56-fold increase in the bearing capacity. This enhancement is primarily attributed to improved chemical bonding and a reduction in soil swelling.
• Increasing the reinforcement depth up to a relative depth (Rd) of 1.5 led to a 2.41-fold improvement in the bearing capacity; however, no significant gains were observed beyond Rd = 1.5, indicating this as the optimum improvement depth for AHA stabilization.
• The installation of up to four layers of geogrid reinforcement yielded bearing capacity increases of up to 2.87 times. No notable improvement was observed beyond three layers, suggesting that three geogrid layers represent the optimal configuration. Considering both performance and cost-effectiveness, three layers are recommended as the most efficient reinforcement strategy.
• The combined application of 25% AHA and three geogrid layers produced the maximum bearing capacity improvement of up to 5.60 times. AHA contributes to soil stabilization by chemically reducing swelling and enhancing bonding, while geogrids provide mechanical reinforcement and improved load distribution. The optimum AHA content aligns with previous studies on agricultural waste ashes in soil stabilization, which similarly identified 20–30% as effective for optimizing pozzolanic activity and soil-binding characteristics [18,49,50]. The enhancement in bearing capacity is supported by the high silica and alumina contents in AHA, confirmed through XRF analysis, which facilitates the formation of cementitious compounds upon hydration.
• Microstructural analyses via SEM, XRF, and XRD elucidated the mechanisms underpinning soil improvement. SEM images revealed enhanced particle bonding, reduced porosity, and increased density in AHA-treated soils. Potassium in AHA was found to reduce water absorption, while calcium promoted pozzolanic reactions forming cementitious phases that strengthen the soil matrix. XRD patterns confirmed the emergence of new crystalline phases indicative of pozzolanic activity, corroborating the macro-scale improvements in strength and stability.
• Overall, the findings demonstrate that AHA effectively stabilizes expansive soils and significantly enhances their load-bearing capacity. Additionally, the use of AHA supports sustainable construction practices by recycling agricultural waste. This aligns with environmental sustainability objectives and engineering performance demands. As a non-hazardous agricultural byproduct, AHA offers a sustainable alternative to conventional stabilizers, such as lime or cement. Its use not only valorizes waste but also mitigates the environmental impact by reducing landfill disposal [28]. This study contributes to the expanding knowledge base on agro-industrial waste application in geotechnical engineering. Future research should investigate the long-term durability, behavior under cyclic loading, and field-scale validation of the proposed stabilization method.