1 Introduction

The disposal of waste tires is a growing environmental challenge worldwide, leading to significant land use and health concerns (Ferronato and Torretta 2019; Akbarimehr and Hosseini 2022). Particularly in countries like Kuwait, millions of waste tires are disposed of each year, creating a substantial burden on landfills and disposal sites (Yaghoubi et al. 2018). The limited space available is increasingly inadequate to handle the escalating amounts of tire waste (Mmereki et al. 2019). In the hot summer months, the abundance of tires in landfills has led to numerous fires, resulting in safety risks and severe air pollution due to air contamination (Dabic-Miletic et al. 2021; Tajabadipour and Lajevardi 2022). These landfills, overwhelmed with millions of tires, are rapidly consuming large areas of land (Ferdous et al. 2021).

The severe environmental and health impacts of waste tires have catalyzed discussions about recycling rubber as a sustainable alternative to landfill dumping (Ismael and Shealy 2018; Mohajerani et al. 2020). The field of engineering has shown significant interest in tire rubber recycling due to its distinct properties, such as low density, high durability, and high frictional strength. Extensive research has investigated the use of shredded and ground rubber tires as additives in construction materials (Moussa and El Naggar 2024). Applications include integrating tire rubber into local sandy soils for various construction purposes: building embankments (Bosscher et al. 1992), serving as drainage layers in landfill sites (Reddy and Marella 2001), adding to hot mix asphalt pavements (Ganiron 2014), serving as a lightweight fill (Ravichandran and Huggins 2014; Abdullah et al. 2023) and enhancing the dynamic properties of sand (Feng and Sulter 2000) and granular soils (Mittal and Gill 2018). Other applications include using recycled rubber material as the upper layer of playgrounds to improve overall flexibility, compressibility, and damping capacity (Rao and Dutta 2006; Kosmela et al. 2024).

Earlier studies (Ismael et al. 1986) examined the properties of Kuwait's surface sands, which were found to be predominantly poorly graded, windblown fine to medium sand with little to no gravel content. While most research has focused on the use of shredded tire rubber to improve soil characteristics (Sheikh et al. 2013; Mashiri et al. 2015), interest in broader applications has been explored internationally. Ahmed et al. (2024) demonstrated significant stability improvements using crumb rubber in Pakistan, while Dimitris et al. (2024) discussed how regulatory and recycling practices in the USA and Japan have influenced the use of such materials in geotechnical engineering. A few have explored the use of ground tire rubber (GTR) for soil enhancement, mainly concentrating on its addition to bituminous mixtures (Goevert 2024). The Green Rubber Tire Recycling Plant is the only producer of GTR in Kuwait, generating particles ranging from 1 to 5 mm. However, the plant's output is currently limited due to the restricted applications, primarily in playgrounds and sports field tiling. To explore the potential of GTR as an additive to local surface sands in Kuwaiti construction projects, comprehensive geotechnical laboratory testing programs are necessary. These programs compare the properties of pure surface sand with sand-rubber mixtures (Christ and Park 2010; Ismael and Al-Sanad 2018; Eren et al. 2022).

This research aims to enhance local surface sand with two GTR aggregates as additives in different ratios, namely 0%, 10%, and 20%. The GTR used includes fine (0.5–0.6 mm) and coarse (1–3 mm) aggregates. Testing included basic properties, compaction, permeability, direct shear, and consolidation to determine the influence of GTR additives on the properties and behavior of the local surface sands and their potential applications in earthwork construction. Additionally, the study examined the impact of GTR size and the relative compaction of the samples, defined as the ratio of the dry unit weight to the maximum dry unit weight. Samples were compacted to both maximum dry density and optimum moisture content, and to 95% relative compaction to meet typical field specifications. The results are presented and discussed, with various practical applications recommended.

2 The Testing Program

The testing program incorporated GTR additives in proportions of 0%, 10%, and 20% by dry weight. Both fine and coarse rubber varieties were used. Figure 1 presents an image of the sand alongside the coarse and fine rubber aggregates utilized in this study, both of which are produced locally. Table 1 details the technical specifications of the Ground Tire Rubber (GTR) employed in the experiments, as provided by the Green Rubber Tire Recycling Plant in Kuwait (2013).

Fig. 1
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The sand, coarse rubber, and fine rubber aggregates used

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Table 1 Chemical analysis of the rubber aggregates
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The following tests were performed:

  1. 1.

    Mechanical analysis for determining grain size distribution curves according to ASTM D422 standard.

  2. 2.

    Modified Proctor Compaction tests on samples 101.6 mm diameter and 116.4 mm height according to ASTM 1557 standard.

  3. 3.

    Direct shear tests on samples prepared at maximum density and optimum moisture content, measuring 63 mm in diameter and 20 mm in thickness. The tests were carried out according to ASTM D3080 standard.

  4. 4.

    Falling head permeability tests on samples prepared at maximum dry density and optimum moisture content, sized at 100 mm diameter and 130 mm height.

  5. 5.

    Consolidation tests on samples compacted to maximum dry density and optimum moisture content, with dimensions of 75 mm diameter and 20 mm thickness. The tests were carried out according to ASTM D2435 standard.

  6. 6.

    Additional direct shear and consolidation tests as outlined in items three and five, but the samples were compacted to 95% of the maximum density or 95% relative compaction (RC = 95%). The relative compaction RC in percent is defined as the ratio of the compacted dry density to the maximum dry density obtained from the Modified Proctor Compaction Test multiplied by 100. This value was selected to match the most required engineering specifications for earth work construction.

For the direct shear, permeability, and consolidation tests, the samples were mixed manually and placed in four layers (5 mm each). Each layer was tamped carefully employing static compaction to achieve the required density. Using this method ensured homogeneity and uniformity of sample preparation which is essential for obtaining reliable and consistent test results.

3 Results

The grain size distribution curves for the clean sand used in the testing, as well as for the fine and coarse ground rubber aggregates, are displayed in Fig. 2. Table 2 shows the percentage of material passing through various sieves. According to the Unified Soil Classification System, the sand employed is classified as SP-SM (poorly graded sand with silt), while both the fine and coarse rubber aggregates are classified as SP (poorly graded sand).

Fig. 2
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Grain size distribution curves

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Table 2 Mechanical analysis of the sand and rubber samples
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Figure 3 illustrates the outcomes of the compaction test results. A notable trend is observed, as the percentage of rubber aggregates increases, both the maximum dry density and the optimum moisture content decrease. This reduction is more pronounced with the finer rubber. Specifically, the maximum dry density decreases by nearly 10% with the addition of 10% rubber, and by approximately 19% when the mixture includes 20% rubber additive. This significant decrease indicates the production of a considerably lighter weight material when rubber aggregates are added. Similar observations of a decrease in unit weight with rubber aggregate additives have been reported by other researchers (Christ and Park 2010).

Fig. 3
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Compaction characteristics curves

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Furthermore, from Fig. 3, it is evident that the optimum moisture content decreases significantly with the use of fine rubber aggregates—from 10.7% to approximately 6% when 10% or 20% fine rubber is added. In contrast, with coarser rubber, the reduction is less substantial. It falls by only 2.3%, from 10.7 to 8.4%, when using 20% rubber aggregate additive, and there is no noticeable reduction with 10% rubber. The reduction in the optimum moisture content is attributed to the adhesive nature of the rubber aggregates which are deposited at the particle contact causing welding or minor cementation and preventing lubrication and increase of density with added water.

Falling head permeability tests were conducted on samples compacted to their maximum unit weight and optimum moisture content. Figure 4 shows how the coefficient of permeability varies with the rubber content, and these results are further summarized in Table 3, which summarizes all the findings of this testing program. It is noticed that the permeability increases with the rising percentage of GTR. This increase is notably more significant when coarser rubber is used. For instance, with the addition of 10% rubber, the coefficient of permeability increased from 2.73 × 10–4 to 4.3 × 10–4 cm/s for fine rubber, and to 5.25 × 10–4 cm/s for coarse rubber. These changes represent increases in permeability of 57% and 92%, respectively. Similarly, with a 20% rubber additive, the permeability increases were 73% for fine rubber and an even more substantial 311% for coarse rubber. Such increases, although large in ratio, are small in magnitude and indicate a slow transition to soil with higher permeability. The increase in permeability is due to the deposition of rubber in the voids at particle contact thus preventing particle sliding, and substantial reduction of the void’s ratio upon compaction.

Fig. 4
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Variation of the coefficient of permeability with rubber content

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Table 3 Summary of all test results (RC = 100% and RC = 95%)
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The shear strength parameters were determined through drained direct shear tests on samples compacted to both 100% and 95% relative compaction (RC), with GTR additives of 0%, 10%, and 20%, and using both fine and coarse rubber aggregates. These parameters are summarized in Table 3. The results indicate that the cohesion 'c' exhibited very small values, slightly influenced by the GTR content. For samples with no rubber additive and compacted to RC = 100% and 95%, the angle of friction (∅) was recorded at 36.6° and 34°, respectively. These values remained nearly unchanged with the coarse GTR additive. The larger size of the rubber aggregates being 1–3 mm may have caused interlocking and friction and prevented significant reduction of the shear strength. However, the inclusion of fine GTR aggregates resulted in a reduction of ∅ by 3°–4° at both 100% and 95% relative compaction, indicating a decrease in strength. The presence of a minor cohesion of 14 kPa and 8 kPa for the clean sand at 100% and 95% relative compaction respectively is due to the presence of silt and moisture in the compacted samples.

Table 4 provides a summary of the shear displacement at failure under various normal stresses in the direct shear test for samples compacted at 100% and 95% relative compaction. The relationships between shear stress and shear displacement for samples with 0% and 20% fine and coarse GTR compacted to 100% RC are shown in Figs. 5 and 6. Generally, an increase in GTR content was associated with an increase in shear displacement at failure. This is since the mix becomes relatively more compressible with the addition of rubber aggregates. Analysis of Figs. 5 and 6 indicates a more pronounced compressibility and a softer response in samples with fine GTR additive, while little difference was observed with the coarse GTR additive. This is shown clearly in Fig. 6 where the shear stress vs. displacement curves are too close for the clean sand and the sand with 20% GTR additive, and the angle of friction is almost the same (Table 3). A similar trend was observed in samples compacted to 95% RC, but these exhibited lower maximum shear stresses. It is evident that the smaller size rubber aggregates induce softer and weaker soil structure with substantial influence on its behavior.

Table 4 Shear displacement at maximum shear stress for compacted samples
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Fig. 5
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Variation of shear stress with relative lateral displacement for clean sand and for sand with 20% fine rubber content at 100% relative compaction

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Fig. 6
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Variation of shear stress with relative lateral displacement for clean sand and for sand with 20% coarse rubber content at 100% relative compaction

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Consolidation tests were conducted on samples with GTR contents of 0%, 10%, and 20%, and at relative compactions of 100% and 95%. Both sizes of GTR were used for comparative purposes. While consolidation settlement typically characterizes cohesive soils and sandy deposits usually exhibit elastic settlement, the addition of GTR induces softening and adherence within the sandy soil structure. As a result, consolidation tests provide valuable insights into soil compressibility.

The relationship between void ratio and effective pressure (e-log σ curves) is illustrated in Figs. 7 and 8 for samples with coarse and fine rubber, respectively, at 100% relative compaction. Note that the slope of these curves is the compression index Cc. Figures 9 and 10 depict the e-log σ curves for samples with coarse and fine rubber compacted to 95% relative compaction. It is noted that the inclusion of rubber leads to a steeper slope of the e-log σ curves (the virgin compression line). Additionally, Fig. 11 and Table 3 show the variation of the compression index \({C}_{c}\) with GTR content across all mixtures. The value of this parameter increased with the addition of GTR as demonstrated by the steeper e- log σ curves, signifying increased compressibility, especially when fine rubber aggregate is used.

Fig. 7
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e-log σ plots of consolidation tests on coarse rubber samples with 100% relative compaction

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Fig. 8
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e-log σ plots of consolidation tests on fine rubber samples with 100% relative compaction

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Fig. 9
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e-log σ plots of consolidation tests on coarse rubber samples with 95% relative compaction

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Fig. 10
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e-log σ plots of consolidation tests on fine rubber samples with 95% relative compaction

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Fig. 11
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Variation of the compression index with rubber content

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A detailed analysis of Table 3 shows that for samples compacted to 100% relative compaction \({(C}_{c})\) increased from 0.0178 to 0.0676 and to 0.1095 with the addition of 20% coarse and fine rubber respectively. This indicates a 3.8-to-6.15-fold increase. The swell index also showed a significant increase, ranging from 2.4 to 3.4. For samples compacted to 95% relative compaction, \({C}_{c}\) increased from 0.0231 to 0.0743 and 0.1781 with the addition of 20% coarse and fine rubber, respectively. This represents a 3.2-to-7.7-fold increase, with the swell index ranging from 1.55 to 2.71. Although these values are relatively small, the proportional increase is substantial and highlights a significant increase in compressibility.

Table 3 also summarizes the variation in the average coefficient of consolidation (\({C}_{v}\)) for samples compacted to both 100% and 95% relative compaction. For samples compacted to 100% relative compaction, the average \({C}_{v}\) value decreased by 24% and 9% with the addition of 20% fine and coarse rubber, respectively. In samples compacted to 95% relative compaction, this decrease was 8.4% and 5%, as noted in Table 3. These findings suggest that sand mixed with fine GTR requires a longer time to achieve a certain degree of consolidation compared to coarse GTR, due to its increased compressibility. It is evident that the results are influenced by the compaction effort and the level of relative compaction achieved.

4 Comparative Analysis of Outcomes at 100% Versus 95% Relative Compaction

An examination of Table 3 indicates similar trends for samples compacted at 100% and 95% relative compaction. For both compaction levels, the incorporation of fine GTR resulted in increased compressibility and a reduction in strength. However, the addition of coarse rubber had minimal or no impact on the shear strength parameters (c and ∅), but the shear displacement at failure was generally larger compared to clean sand, as detailed in Table 4.

When comparing the parameters of samples at RC = 95% with those at RC = 100%, it is evident that the former has lower shear strength parameters (c and ∅), greater compressibility (as indicated by higher \({C}_{c}\) and \({C}_{s}\) values), and lower unit weight, being 95% of the maximum unit weight determined by the Modified Proctor Compaction Test. For example, with 20% fine GTR, the \({C}_{c}\) value increased from 0.1095 to 0.1781—a 63% increase—as relative compaction decreased from 100 to 95%. For 20% coarse rubber, the corresponding increase was from 0.0676 to 0.0743, or 10%, as shown in Table 3. Similarly, for 20% fine and coarse GTR additives, \({C}_{S}\) increased by 34% and 9%, respectively, for samples at 95% relative compaction as compared to 100% relative compaction. Figures 12 and 13 illustrate the comparison of shear stress versus displacement curves at 100% and 95% relative compaction for clean sand and sand with 20% fine rubber additive, respectively. As anticipated, a softer response with lower modulus and strength is observed at lower relative compaction.

Fig. 12
figure 12

Variation of shear stress with relative lateral displacement for clean sand at 100% and 95% relative compaction

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Fig. 13
figure 13

Variation of shear stress with relative lateral displacement for sand with 20% fine rubber content at 100% and 95% relative compaction

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5 Discussion and Practical Applications

Currently, the applications of GTR produced by the Green Rubber Recycling Company in Kuwait are confined to rubber tiles, running tracks, and as a minor additive (3 to 5%) in road asphalt bitumen mix. However, these applications are limited and require only small quantities of GTR, which is inadequate considering the Environmental Protection Authority of Kuwait has recently identified a need to dispose of or recycle at least 55 million waste tires.

An analysis of Fig. 3 and Table 3 indicates that incorporating GTR as an additive result in a lightweight soil. With 10–20% rubber content, densities are reduced by 10–20%, respectively. Such lightweight soil has potential use as embankment fill, reducing pressure on underlying soft or firm clay deposits and loose sands. The applied pressure (γH, where γ is the fill unit weight and H is the height) will be lower, thereby reducing both elastic and consolidation settlements and increasing the factor of safety against bearing capacity failure. However, the embankment itself will be lighter and more compressible, and with lower shear strength. Therefore, light structures or recreational facilities can be constructed on such embankments. It can also be used as backfill around retaining walls. The lateral earth pressure acting on these structures is ½\(\gamma {H}^{2}{K}_{H}\) where \({K}_{H}\) is the coefficient of lateral earth pressure and H is the height of the wall. This pressure will be reduced as \(\gamma \) is lower since it varies linearly with \(\gamma \) and \({K}_{H}\) will remain nearly constant since it is a function of \(\mathrm{\varnothing }\), which will not change much or slightly decrease. On the other hand, the vertical weight and the shear strength of the soil will decrease upon using GTR. Therefore, the whole stability of the wall must be examined before such option is employed.

In low lying areas in Kuwait such as Sabah Al Ahmad new residential city, the ground level will be raised by about 2 m from the present level. GTR may be used as an additive to the sand fill in compacted layers to reach the desired elevation prior to road construction or before placement of pipelines. The resulting lighter surcharge will minimize elastic settlement. The resulting lighter surcharge will minimize elastic settlement, ensuring that the settlement of the fill is minimal due to its exposure to only light loads.

Comparing the effects of fine and coarse rubber is informative. Fine rubber additives increase compressibility and decrease shear strength more than coarse rubber. The rate of increase of the permeability is larger with the addition of coarse GTR compared with fine GTR. This is due to the fine rubber filling the voids and soil particle contacts, creating a softer, less permeable, and more compressible soil structure.

Reviewing Fig. 4 and Table 3, reveal that the coefficient of permeability for samples at 100% relative compaction increases by 3.11 times with 20% coarse GTR and 1.73 times with fine GTR. While the increase is significant, the values reached are small, and they do not match those for free-draining granular soil. Therefore, for applications such as drainage layers in landfill construction (Reddy and Marella 2001) or in road and highway construction (Cetin et al. 2006), either a higher percentage of GTR or larger scrap tire shreds is necessary. Alternatively using sub layers of 100% GTR may be necessary.

The above applications require large quantities of GTR aggregates or, if applicable, larger size shredded tires. The material will be transformed into a useful earth construction material. This approach could significantly mitigate issues related to disposal site shortages in Kuwait and the Gulf region, while limiting the environmental impact.

6 Conclusions and Recommendations

As the world increasingly focuses on sustainable practices, the repurposing of waste materials in construction and environmental management has become a crucial area of research. This study, investigating into the use of GTR in desert sands, contributes significantly to this work. By exploring the transformative effects of GTR additives on sand, it not only addresses the issue of tire waste management but also opens new avenues for sustainable construction practices. The findings presented here highlight the potential of integrating recycled materials into conventional construction methods, thereby promoting environmental conservation and resource efficiency. The conclusions drawn from this research are not only relevant for desert regions but also set a precedent for global sustainable practices in material science and engineering.

Based on the results of this testing program, the following conclusions are drawn:

  1. (a)

    Mixing surface sand with GTR produces a lighter material, with compacted densities 10% and 20% lower for rubber contents of 10% and 20%, respectively. The density reduction is slightly greater with very fine rubber compared to coarser rubber.

  2. (b)

    The coefficient of permeability for samples compacted to 100% relative compaction increased by 1.92 and 3.11 times with 10% and 20% coarse rubber additives, respectively. With fine rubber, the increases were 1.57 and 1.73 times.

  3. (c)

    Shear parameters c and ∅ showed that the angle of internal friction ∅ remained unchanged with coarse rubber additive. However, with fine rubber, ∅ decreased by 3°–4° at 100% relative compaction and by 2°–4° at 95% relative compaction. The cohesion c was negligible and remained nearly constant. Shear displacement at maximum shear stress generally increased substantially with GTR additive.

  4. (d)

    Compressibility, as indicated by the compression index \({C}_{c}\) and swell index \({C}_{s}\), increased significantly with rubber additive. With 10% and 20% fine rubber, \({C}_{c}\) increased by 3 and 6 times, respectively. Coarse rubber resulted in a \({C}_{c}\) increase of 2–3.8 times compared to clean soil. Similarly, \({C}_{s}\) increased by 1.3–3.4 times with fine rubber and 1.1–2.4 times with coarse rubber.

  5. (e)

    For samples compacted to 95% relative compaction, the conclusions were similar, but the strength was lower, and compressibility was higher than for samples at 100% relative compaction. This was evident from the reduction in the angle ∅ by 1.1°–2.6° and a 10% to 63% increase in \({C}_{c}\) with GTR content ranging from 0 to 20%.

  6. (f)

    Sand mixed with GTR additive is recommended for various practical applications, including embankment construction on soft ground, backfill around retaining walls, as a drainage layer, and as an additive in road construction bituminous mixes. However, it disadvantages including increased compressibility and decreased strength must be considered along with any environmental impacts from its use in construction.