Use of Lignin, Waste Tire Rubber, and Waste Glass for Soil Stabilization

Abstract

The complex interactions between soil and additives such as lignin, glass powder, and rubber tires were investigated using principles of material and soil mechanics. Previous research has mainly focused on individual additives in clay soils. In contrast, this study investigates soil improvement with two different types of waste materials simultaneously. The improvement of soil properties by hybrid waste materials was evaluated using several laboratory tests, including the standard Proctor test, the unconfined compressive strength test, the California Bearing Ratio (CBR) test, and cyclic triaxial tests. The aim of this research is to identify key parameters for the design and construction of road pavements and to demonstrate that improving the subgrade with hybrid waste materials contributes significantly to the sustainability of road construction. The mechanical and physical properties were evaluated in detail to determine the optimal mixtures. The results show that the most effective mixture for the combination of waste glass powder and rubber tires contains 20% glass powder and 3% rubber tires, based on the dry weight of the soil. For the combination of waste glass powder and lignin, the optimum mixture consists of 15% glass powder and 15% lignin, based on the dry weight of the soil. These results provide valuable insights into the sustainable use of waste materials for soil stabilization in road construction projects.

1. Introduction

In recent years, the recycling of waste materials has become increasingly important due to its sustainability and environmental benefits. For this reason, the use of waste materials in ground improvement applications has been put on the agenda [1]. The use of waste materials in construction works not only saves money by reducing the amount of construction materials used but also reduces the demand for natural construction materials [2]. Therefore, researchers have been trying to reuse granite waste in construction and protect natural resources for several years [3]. Industrial waste was used as aggregates or cement for concrete production. Waste materials are also used in geotechnical engineering for soil improvement [4,5].

In roadway construction, the traditional options for improving the subgrade are lime/cement-stabilized subgrade and geosynthetically reinforced subgrade. Stabilization is a method by which the properties of the soil are altered to improve its performance [6]. To increase the bearing capacity and the shear strength of the soil, the soil properties are either physically altered (by vibration) or a more resistant material is added or incorporated. In recent decades, intensive efforts have been made to develop economical, environmentally friendly, and durable soil stabilizers such as lime, cement, fly ash, and others [7,8,9,10,11,12,13]. Conventional stabilizers such as soil stabilizers treated with lime and cement have been investigated for their stabilizing effect and their suitability as geomaterials for road construction [14,15]. Although these stabilizers have shown significant improvement in soil capacity and durability, their high cost, potential shrinkage, and high alkalinity are still critical issues that prompt geotechnical engineers to look for other alternatives. In recent years, there has been a great deal of interest in the use of non-traditional stabilizers for this purpose [16]. With the pursuit of sustainable development, there is an increasing need for environmentally friendly building materials with improved properties, which has drawn the attention of researchers to the use of used tires, glass powder, lignin, etc., and has led to increased research on this topic [6,17,18,19]. Due to the increase in waste production, researchers have started to use waste materials in different areas to reduce the amount of industrial waste thrown into nature. Waste materials can be used in various areas of construction.

Lignin as a waste material can be used effectively in subgrade improvement to increase the strength and stability of the soil, and it offers a sustainable and environmentally friendly solution for construction applications. In recent years, studies on the use of lignin in soils with poor engineering properties have increased [20,21]. Various researchers have reported that the use of lignin has achieved positive results in soil parameters, especially in problematic soils with a low bearing capacity [22,23,24,25,26]. Zhang et al. [27] conducted the shear wave test and unconfined compression test to evaluate the shear modulus and unconfined compressive strength of lignin-stabilized silty soil. They found that the shear modulus and compressive strength of the lignin-treated soil increased logarithmically with the curing time. In addition, they pointed out that the interaction mechanism between lignin and soil and the durability of the soil still needs to be investigated and confirmed by additional laboratory tests. Lignin may initially increase the strength of the soil by coating the particles with an adhesive film, but long-term effects may include a reduction in strength due to the dispersion of clay particles caused by the negative surface charge of the lignin [16,28].

Every year, around 1.5 billion tires are thrown away at the end of their useful life, and this figure is expected to rise to 5 billion by the end of 2030. This leads to various environmental problems such as soil and air pollution. Therefore, it is very important to find an environmentally friendly way to dispose of used tires [29,30]. They showed that recycled tire waste mixed with soil can be used as a lightweight fill material for embankments, road and pavement bases, and retaining walls to increase strength. Ravichandran et al. [31] used waste tires as additives to increase the strength of soft soils. Using CBR tests, they added less than 10% rubber powder to stable soils to investigate their strength. Abbaspour et al. [32] used six different ratios of waste tires on clay soils. They found that this had a positive effect on the density, the direct shear strength, the uniaxial compressive strength, and the CBR value. The proportion of used tires increases the CBR value and contributes positively to the results of shear strength and uniaxial compressive strength. Yarbaşı [33] investigated the strength properties of clay soils with low plasticity improved with marble dust and rubber waste. In his study, he obtained the highest unconfined compressive strength (UCS) values at the end of 28 days of curing in a mixture of clay and three different proportions of waste tires. Gao et al. [34] found that the addition of 3% waste tires to clay soil changed the distribution of pore volume, increased the pore size, and improved the mechanical properties of the soil by making the structure of the soil more complex.

Less than 30% of the used glass produced is recycled, which means that a large proportion of glass waste is generated. This shows that glass waste management plans in particular need to be improved [35]. Çanakçı et al. [36] investigated the stabilization of low-plasticity clay using waste glass powder. For the mixtures, the soil was mixed with glass powder in an amount of 3, 6, 9, and 12% of its dry weight. As a result of the mechanical tests, it was found that the mechanical properties improved with increasing glass percentage increased. Khan et al. [37] found an increase in CBR, Proctor density, and shear strength with the addition of different fine glass powders to the clay soil sample. Blayi et al. [38] added different percentages of waste glass powder to the dry weight of the soil and conducted experiments (2.5%, 5%, 10%, 15%, and 25%). It was found that the percentage of waste glass powder at 25% made a positive contribution to the Atterberg limits such as plasticity limit, liquid limit, plasticity index, and shrinkage. It was found that the proportion of 15% glass powder increased the CBR value by 171% and then decreased. Lodha et al. [39] conducted experiments with different proportions of waste glass powder (0%, 10%, 20%, and 30%) in relation to the dry weight of the clay soil. They found that the values for the plasticity limit, plasticity index, and liquid limit decreased as the proportion of waste glass powder increased. Based on their results, Perera et al. [40] concluded that the optimum addition of glass powder that best improves the overall mechanical and physical properties is an addition of 15%. They found that the CBR contributes positively to improving the elastic modulus and properties of clay with low plasticity.

Waste glass, car tires, and lignin as forest waste, which accumulate in large quantities due to human activities, are a major problem for many countries. This type of waste causes environmental problems such as air pollution in the case of incineration, the need for storage areas, soil pollution, and the pollution of surface water and groundwater and has indirect negative effects on human health [41,42,43,44,45,46]. For example, rainwater collected in used tires is an ideal breeding ground for mosquitoes. This situation leads to an increase in mosquito-borne diseases such as malaria and deaths [47]. Glass waste increases the rigidity and compressive strength of the soil. Rubber tires ensure a more even distribution of loads due to their elasticity. Although lignin primarily improves the organic content and water retention capacity, it also contributes to the binding of soil particles and, thus, to overall stability. This combination leads to a synergistic effect that improves the load-bearing capacity of the soil while maintaining its resilience.

As outlined above, several research studies have been conducted to define the mechanical and physical properties of soils, especially low-plasticity clay, by adding different types of waste materials such as lignin, waste tires, and glass powder, with each study focusing on the addition of only one type of waste. In this article, however, the properties of a modified soil with the simultaneous addition of two different types of waste are investigated. This approach aims to improve several soil properties at the same time. As the aim is to improve the subgrade for pavement structures, the main mechanical properties that are important for pavement design are investigated. In addition, the deformation properties are evaluated using cyclic triaxial tests to determine the resilient modulus. The aim of this research is to determine the most suitable combination of hybrid wastes to improve the subgrade of road pavements. The results of this experimental program provide important information for the design of road pavements in which the subgrade is improved with hybrid wastes such as lignosulfonate, glass powder, and rubber tires. In addition, the most effective mixtures of hybrid wastes are proposed to achieve the most subgrade improvements.

2. Materials and Methods

2.1. Soil Properties

The clay soil utilized in this investigation was sourced from a location in the center of Afyonkarahisar, Turkey. The soil was excavated from a depth of approximately 1 to 1.5 m, resulting in disturbed samples. Analysis revealed that 86.2% of the soil particles passed through the US No. 200 sieve (<0.075 mm), classifying the material as low-plasticity clay (CL) under the Unified Soil Classification System (USCS). The clay exhibited a specific gravity of 2.62, a unit weight of 16.5 kN/m³, a shrinkage limit of 5.8%, a maximum dry density of 1.58 g/cm³, an optimum moisture content of 21.9%, a liquid limit of 48.6%, and a plastic limit of 23%. Soil characterization tests were performed in accordance with TS 1900-1 [48], ASTM D 854 [49], ASTM D 422 [50], and ASTM D 4318 [51] standards. The particle size distribution, illustrated in Figure 1, was determined using both mechanical sieving and hydrometer analysis. Additionally,

Table 1. XRF data for the low-plasticity clay sample.

Elements	Clay (%)	Oxide	Clay (%)
Na	1.6	Na2O	2.4
Mg	1.3	Mg	1.4
Al	9.0	Al2O3	19.9
Si	24.4	SiO2	60.7
K	2.4	K2O	3.4
Ca	3.1	CaO	5.0
Fe	1.9	Fe2O3	7.2
O	68.8	O	17.9

details the elemental composition of the low-plasticity clay, as identified through X-ray fluorescence (XRF) analysis conducted with the Bruker D8 Advance. It should be noted that the total percentage exceeding 100% in XRF results is usually due to the combined effects of the conversion of elemental compositions to oxides, matrix effects, and potential inaccuracies in calibration or detection.

Figure 2 shows the different phases of the mineralogy of each sample as identified by scanning electron microscopy and energy-dispersive X-ray analysis (SEM-EDX). The clay used in the study contains high amounts of silica, as shown in Figure 2.

2.2. Lignosulfonate, Glass Powder, and Rubber Tire Waste Samples

The sodium lignosulfonate used in the experiments was supplied by Aker Chemical Co, Ltd. (İstanbul, Turkey). Sodium lignosulfonate is a brownish powder that is easily soluble in water and has good stability. It has good dispersibility, cohesiveness, and chelation. The chemical properties of sodium lignosulfonate are known—Ph value of 4–5, moisture of 3–6%, ash content of 17–19%, density of 0.5 g/cm³, and water-insoluble matter ratio max of 0.2%. It can be used as a water reducer in engineering applications, as shown in Figure 3a.

The glass powder used in the experimental study came from Kırıkcam Recycling Plant Co, Ltd. (Eskişehir, Turkey). The company collected waste glass from the city and then crushed, pulverized, and distributed it in the laboratory. The additive considered in the study was waste glass powder with a maximum size of 2 mm. The glass powder is processed at a local recycling plant, where glass bottles and other recyclable glass items are crushed to achieve a particle size distribution of ~2 mm. The specific gravity of the glass powder is 2.66. The oxide contents of the glass powder are also known: SiO₂ at 67.5%, CaO at 14.15%, Na₂O at 12.51%, and Al₂O₃ at 2.18%. As can be seen in Figure 3b, the glass powder consists mainly of sand-like particles, although a limited number of silt-like particles is also present.

The rubber tire samples were collected by Mirkacev Environmental Technologies Co (Mersin, Turkey). The rubber was obtained from rubber tire waste containing 5 mm thick steel wires. The rubber crumb sample was sieved and collected with a sieve size of 425 μm. This size was chosen to prevent the size of the rubber particles in the rubber–clay matrix from being random. This allows the rubber crumbs to blend well into the mixture. The crumb rubber has a spherical shape and a rough surface, as can be seen in Figure 3c. In addition, rubber has properties such as a low specific unit weight, good durability, and high shear and tensile strength, which makes it a suitable material for ground improvement.

It is expected that the selected waste materials will have various positive effects on the weak soil due to their structural properties. It is expected that rubber tires will improve the flexibility of the soil, glass powder will increase the mechanical strength, and lignin will contribute positively to the bearing capacity of the soil due to its binding properties.

3. Specimen Preparation and Test Methods

The complete experimental program of this study is shown in Figure 4. The low-plasticity clay, the glass powder, and the lignin were placed in an oven at 105 °C for 24 h to remove the moisture and dry completely. A standard compaction test was conducted to determine the optimum water content and maximum dry unit weight of all predetermined mix ratios for experimental studies. The standard compaction test was performed according to ASTM D 2216 [52] and Turkish standard TS 1900-1 [48] for different mix ratios.

In the standard compaction test, water was gradually added to the soil samples. Each moistened sample was then placed in a standard compaction mold with a diameter of 105 mm and a height of 115.5 mm. The soil was compacted in three layers, with each layer having approximately the same mass. Each layer was compacted by 25 blows with a 2.5 kg hammer dropped from a height of 305 mm above the sample. This procedure was repeated three to five times, adding more and more water to the soil. The relationship between dry density and water content observed in these tests allowed the maximum dry density and optimum water content to be determined.

For sample preparation, the clay soil was mixed with lignosulfonate, glass powder, and rubber tires in different ratios (three different mixtures for lignosulfonate and five different mixtures for rubber tires), using the dry weights of the materials for the mixtures. The mixing ratio of the additives was chosen on the basis of previous studies [53,54,55,56] in which each type of waste was tested in different ratios.

All specimens were compacted at standard Proctor compaction energy and optimum water content. The specimens were also prepared at optimum water content for uniaxial compressive strength, California Bearing Ratio, and dynamic triaxial testing. The compositions of the specimens with the different mixtures are shown in

Table 2. Abbreviations and compositions of mixtures.

Sample	Clay (%)	GP (%) 1	LG (%) 2	RT (%) 3
S	100	-	-	-
S1	90	5	5	-
S2	80	10	10	-
S3	70	15	15	-
S4	94.5	5	-	0.5
S5	89	10	-	1
S6	83	15	-	2
S7	77	20	-	3
S8	71	25	-	4

¹ GP, Glass Powder; ² LG, Lignosulfonate; ³ RT, Rubber Tire.

.

To assess geotechnical properties, unconfined compressive strength (UCS) tests were conducted on soil samples stabilized with various waste additives. These samples, prepared at optimum moisture content, were molded into specific dimensions using sample molds. The UCS test is used to evaluate the strength of soil stabilization mixtures and to understand the impact of additives on soil strength. The test followed the procedures outlined in TS 1900-2 [57] and ASTM D 698 [58] with a strain rate of 1% per minute maintained throughout. The soil mixtures containing glass powder, rubber tire crumbs, and lignosulfonate were compacted using a standard Proctor mold. After compaction, the specimens were extracted from the mold with a sharpened brass tube having an inner diameter of 38 mm and a height of 76 mm. Three specimens were prepared for each mixing ratio. These test specimens were then wrapped in plastic foil and covered with a damp cloth to maintain moisture, and the cloth was moistened daily. The specimens were cured for periods of 7 and 28 days. The UCS results reported are the average values obtained from the three tests conducted on each mixture.

In determining the proportions of waste materials added to the mixtures, an approach has been adopted that aims to reuse the maximum amount of waste possible, taking into account both the positive contribution of the waste materials used to engineer the properties of the soils and sustainability. Curing times longer than 7 and 28 days are expected to increase the strength properties of the mixtures. However, in this study, tests were carried out with the most common curing times (7 and 28 days) and the results evaluated.

The California Bearing Ratio (CBR) value is a critical factor in road pavement design, as it correlates with the required pavement thickness. A lower CBR value necessitates a thicker pavement, whereas a higher CBR value allows for a thinner pavement. The CBR test follows the standards set by ASTM D 1883 [59] and TS 1900-2 [57]. Test specimens are prepared similarly to those used in unconfined compressive strength (UCS) tests, compacted in a CBR mold at optimum moisture content using standard Proctor compaction methods. The specimens are compacted in three layers, with each layer receiving 61 blows from a 2.5 kg hammer. Testing is conducted on both the top and bottom surfaces of the specimens, and the average of these values is reported for all mixtures. The test involves a plunger with a surface area of 19.35 cm², penetrating at a rate of 1.27 mm/min. The CBR values are measured both at optimum moisture content (Dry CBR) and after soaking the specimens in water for 4 days (Wet CBR).

For the cyclic triaxial test, cylindrical specimens with a diameter of 100 mm and a height of 200 mm were compacted to achieve the maximum dry unit weights and optimum moisture content, as determined by the standard compaction test. The preparation process mirrors that of the UCS tests to ensure the target dry unit weight is achieved. The specimen is placed in a split mold lined with a 0.4 mm thick rubber membrane and held under vacuum. The specimens are double-wrapped in plastic and cured in a controlled environment with specific humidity and temperature conditions for a designated period. The resilient modulus (M_r) tests are conducted according to the AASHTO T-307-99 [60] standard procedure. After conditioning, the resilient modulus is determined using five different deviatoric stresses and three confining stresses (13.8 kPa, 27.6 kPa, and 41.4 kPa), yielding 15 M_r values per specimen. The cyclic loading process applies a haversine deviatoric stress pulse for 0.1 s, followed by a 0.9 s rest interval before the next pulse. The resilient modulus values reported in this study are the averages obtained from two tested samples for each sample mixture (S, S1,…, S8).

4. Results and Discussion

4.1. Results of the Standard Compaction Tests

Table 3. Optimum water content and maximum dry unit weight of the tested samples.

Sample	Mixture	γdmax (g/m3)	wopt (%)
S	Soil (clay, CL)	1.47	24.1
S1	S + %5GP + %5LG	1.57	22.3
S2	S + %10GP + %10LG	1.59	19.6
S3	S + %15GP + %15LG	1.63	16.1
S4	S + %5GP + %0.5RT	1.56	23.3
S5	S + %10GP + %1RT	1.59	22.1
S6	S + %15GP + %2RT	1.62	20.6
S7	S + %20GP + %3RT	1.65	19.1
S8	S + %25GP + %4RT	1.66	17.8

GP—Glass Powder; LG—Lignosulfonate; RT—Rubber Tire.

shows how different mixtures of additives affect the optimum moisture content (w_opt) and the maximum dry unit weight (γ_dmax) of low-plasticity clay soil (CL). The pure clay soil (S) has a γ_dmax of 1.47 g/cm³ and a w_opt of 24.1%. With the addition of 5% glass powder (GP) and 5% lignin (LG) (S1), γ_dmax increases to 1.57 g/cm³ while w_opt decreases to 22.3%. If the proportions of GP and LG are increased to 10% (S2) and 15% (S3), respectively, γ_dmax increases to 1.59 g/cm³ and 1.63 g/cm³ while w_opt falls to 19.6% and 16.1%. Similarly, adding 5% GP and 0.5% recycled tire rubber (RT) (S4) increases γ_dmax to 1.56 g/cm³ and reduces w_opt to 23.3%. If the GP and RT content is increased to 10% GP and 1% RT (S5), γ_dmax increases to 1.59 g/cm³ and w_opt drops to 22.1%. Further increases to 15% GP and 2% RT (S6), 20% GP and 3% RT (S7), and 25% GP and 4% RT (S8) lead to γ_dmax values of 1.62, 1.65, and 1.66 g/cm³, respectively, with corresponding w_opt values of 20.6%, 19.1%, and 17.8%. It was found that as a result of the homogeneous distribution of the grains in the stabilized clay soil, the microstructure of the soil changed positively so that the void ratio and optimum water content of the soil decreased while the maximum dry unit weight increased (see Figure 5).

Since lignin has a water-reducing effect due to its chemical properties, the water content of the samples mixed with glass powder and lignin was low. It is assumed that the reason why the water content of the samples with glass powder + rubber tire additive is higher than that of the samples with glass powder + lignin additive is due to the shape of the rubber tire particles. The main reason why the maximum dry unit weight values of all samples are close to each other is probably the glass powder. Since the specific density of glass powder is higher than that of other waste materials, the water requirement decreases with increasing density [40,61,62,63,64].

As can be seen from Table 3, the water content of the GP + LG soil mixtures is slightly lower than the water content of the GP + RT soil mixtures. This is due to the hydrophobic properties of lignin and the porous voids of the rubber tire. Since glass powder does not absorb water, the maximum dry unit volume weight increases with increasing glass powder content in the mixtures.

4.2. Results of the Unconfined Compressive Strength

The results of the uniaxial compressive strength test are shown in Figure 6. The uniaxial compressive strength (q_u) increases with the addition of glass powder and rubber tires. The increase in strength was also observed with increasing curing time. At the end of the 7- and 28-day curing time, the highest strength values for the samples with glass powder and rubber tire additive were 6.07 kg/cm² and 6.81 kg/cm² for the sample (S7) with 20% GP + 3% RT additive. At the end of the 28-day curing period, a positive increase of 342% was achieved compared to the control sample. Although the strength value of the sample (S8) with 25% GP + 4% RT additive decreased compared to the sample (S7) with 20% GP + 3% RT additive, a strength increase of 333% was achieved compared to the control sample.

The samples with added glass powder and lignin also showed an increase in strength compared to the samples without additives. However, this was not as strong as with the RT additives. The highest strength value was achieved in the sample (S1) with 5%GP + 5%LG additive. At the end of the 28-day curing period, a positive increase of 150% was achieved compared to the control sample. Although the lowest strength value among the samples with additives was achieved in the sample with 15%GP + 15%LG additive (S3), a 38% increase over the control sample was still achieved at the end of 28 days.

According to Zorluer et al. [53], industrial waste can be used effectively for soil stabilization. In some cases, this can also be used instead of conventional binders to improve soil properties. The use of industrial waste as soil stabilizers can contribute to significant cost savings and environmental protection. It has been shown that industrial waste (fly ash, marble dust, boron dust, granite dust, etc.) used in varying proportions has a positive effect on the physical and mechanical properties of clay soils. It can be seen that the results of tests such as CBR and UCS are consistent [54]. In some studies, although the strength values were high at optimum additive ratios, a decrease in strength values was observed with an increasing additive ratio [55,56].

4.3. Results of the California Bearing Ratio (CBR) Tests

The CBR value at optimum moisture content (Dry CBR) and the soaked CBR value (Wet CBR) of the test specimens are shown in Figure 7. The addition of GP and LG in equal percentages (S1, S2, S3) shows that the Dry CBR values are relatively low, but there is a significant increase in the Wet CBR values as the percentage of GP and LG increases. The samples with GP and RT show a significant increase in both Dry and Wet CBR values as the percentage of GP and RT increases. Sample S7 (20% GP + 3% RT) has the highest Dry and Wet CBR values, indicating the highest strength among the tested samples.

The test results show that the addition of GP and LG generally decreases the Dry CBR values by up to 47% (S1) but slightly increases the Wet CBR values by up to 47% (S3), especially at higher proportions. In contrast, the addition of GP and RT significantly increases both Dry and Wet CBR values, with Dry CBR increasing by up to 209% and Wet CBR by up to 190%. The most significant improvements are observed in samples with higher proportions of GP and RT. The reason for this is that the steel fibers in the tire rubber have a positive effect [65,66,67].

The CBR value of soils is one of the most important parameters in the pavement design. The higher the CBR value, the lower the thickness of the pavement that needs to be produced on the subgrade for a given level of traffic. This not only reduces the budget required for road construction but also contributes to sustainability, as fewer natural materials need to be used for road construction.

4.4. Results of the Resilient Modulus

Data obtained from standard Proctor, CBR, and uniaxial compression tests for road subgrade do not provide information on the behavior of the material under dynamic loading. For this reason, resilient modulus tests were performed on the samples to determine the behavior of the prepared mixtures under cyclic loading. In the cyclic triaxial tests, the largest values are generally obtained for the smallest bulk stress under a 41.4 kPa confining pressure, while the smallest resilient modulus values are obtained for the largest bulk stress value under a 13.8 kPa confining pressure. For all mixtures of waste glass powder, lignin, and waste tire rubber, an increase in the resilient modulus was achieved with an increasing proportion of additives compared to the control sample. Among the samples with additive, the lowest value of the resilient modulus was obtained in the sample 5%GP + 0.5%RT (S4) under a 13.8 kPa confining pressure, while the highest value of the resilient modulus value was obtained in the sample 20%GP + 3%RT (S7). Compared to the control sample, the greatest increase in the samples made with waste glass powder and waste tires was achieved in the sample with 20%GP + 3%RT (S4) additive, where an improvement of 110% was achieved under a 13.8 kPa confining pressure. For the samples made from waste glass powder and lignin, the sample with 15%GP + 15%LG (S3) was the one with the highest improvement, achieving an increase of 85% (see Figure 8).

Compared to the control sample, the greatest increase among the samples produced with waste glass powder and waste tires was achieved in the sample with 20%GP + 3%RT (S7) additive, where an improvement of 113% was achieved under a 27.6 kPa confining pressure. For the samples made with waste glass powder and lignin, the sample with 15%GP + 15%LG (S3) was the one with the highest improvement, achieving an increase of 97% (see Figure 9).

Since the subgrade materials are located below the pavement layers, the resilient modulus value under a confining pressure of 41.4 kPa, which represents the stress in the deep layers, is of greater importance. Compared to the control sample, the greatest increase in the waste glass powder and waste tire samples was achieved in the sample with 20%GP + 3%RT (S7) additive, where an improvement of 178% was achieved under a 41.4 kPa confining pressure. For the waste glass powder and lignin samples, the sample with 15%GP + 15%LG (S3) was the one with the highest improvement, showing an increase of 147% (see Figure 10).

As can be seen from Figure 8, Figure 9 and Figure 10, the resilient modulus values generally decrease as expected as the bulk stress increases for each confining pressure. According to the results of the resilient modulus tests, the specimens made with waste glass powder and waste tires generally perform better than the specimens made with waste glass powder and lignin. In general, the void ratio can be significantly changed by adding recycled rubber and crushed glass to the soil or by applying pressure. This is due to the fact that the addition of rubber tires and glass powder changes the size, shape, and connection of the soil pores. It can, therefore, be concluded that the addition of waste tires and glass powder plays an important role in the pore size and strength parameters. When the addition of glass powder and waste tires began, large aggregate pores formed and caused the formation of smaller and more interconnected pores [63,68,69,70].

The evaluation of all resilient modulus test results taken together shows that the stabilization in this study makes a positive contribution to the resilient modulus of the soil. This means that under the actual loading conditions, the compressive strain effect on the stabilized subgrade is reduced and, thus, the rutting resistance of the stabilized subgrade is increased.

5. Conclusions

The complex interaction between soil and additives such as lignin, glass powder, and rubber tires was investigated using principles of material and soil mechanics. The aim of this research is to determine the most suitable combination of hybrid wastes to improve the subgrade of road pavements. The results of this experimental program will provide important information for the design of road pavements where the subgrade is improved with hybrid wastes such as lignosulfonate, glass powder, and rubber tires. In addition, the most effective mixtures of hybrid wastes are proposed to achieve the most subgrade improvements. The results of the laboratory tests allow engineers to determine the most important parameters for the design of road pavements that include subgrade stabilization with hybrid wastes. The most important observations resulting from the test program are the following:

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The homogeneous distribution of grains in the stabilized clay soil has caused the microstructure of the soil to change positively so that the void content and optimum water content of the soil have decreased while the maximum dry unit weight has increased.

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Since lignin is water-repellent due to its structure, it is thought that the water content of lignin-added samples is lower. Waste tire rubber-added samples absorbed water more than lignin-added samples. It was found that glass powder had a positive influence on the unit weight values of all samples tested.

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Increasing the amount of waste glass powder and lignin additives as well as the curing time had a positive effect on strength, which is consistent with several research papers [24,71,72,73,74].

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Regardless of the amount of additives, an increase in the curing time increased the strength of the samples at all mixing ratios.

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Based on these results, it is assumed that waste vehicle tires have a positive effect on the load-bearing capacity of the control sample due to their steel fiber content, as also reported in various research articles [65,66,67].

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The soaked CBR value of the samples with lignin admixture is more pronounced compared to the samples with rubber tires.

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The results show that the most effective mixture for the combination of waste glass powder and rubber tires contains 20% glass powder and 3% rubber tires, based on the dry weight of the soil. For the combination of waste glass powder and lignin, the optimum mixture consists of 15% glass powder and 15% lignin, based on the dry weight of the soil. These results provide valuable insights into the sustainable use of waste materials for soil stabilization in road construction projects.

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This study focuses on the stabilization of a single soil type with waste materials. This limits the generalizability of the results of this study to the potential to improve different soil types with waste additives. In addition, the prepared mixtures can be conditioned for longer curing times, and the changes in their strength can be observed. Future studies can focus on the potential of using the wastes used in this study as additives for soils with different Atterberg limits, particle size distributions, and shear strength. In this way, the results can be evaluated with a statistical approach and a general conclusion can be drawn about the use of waste as a stabilizing material.

Further research is needed to investigate the rubber tire crumb in terms of physical properties and angular and rough particles on mechanical properties. In addition, to ensure the adequate CBR of the subgrade, the appropriate thickness of the layer to be subjected to soil improvement needs to be further determined. Moisture content is a critical factor affecting the resilient modulus, i.e., increasing the moisture content beyond the optimum moisture content will result in a decrease in the resilient modulus due to a decrease in effective stress.