Introduction

As a widely distributed engineering material, the mechanical properties and long-term durability of soils directly affect the safety and stability of structures including subgrades, slopes, and foundations. For decades, researchers have focused on improving the soil mechanical properties to withstand various external changes through different stabilization techniques1,2,3,4. The commonly used soil stabilizers, including cement, lime, and asphalt, typically contain chemical additives that can alter soil chemical properties and disrupt soil environment5,6,7,8. Through mixing and curing, a series of hydration and pozzolanic reactions occur between soil and stabilizers, leading to the formation of cementitious materials that enhance the physico-mechanical properties of soil. However, the production of these chemical stabilizers is energy-intensive and contributes to the rapid natural resources consumption. Furthermore, these production processes contribute substantially to global climate change via significant carbon dioxide emissions9. Therefore, there is an urgent need to explore cost-effective and environmentally friendly alternatives for soil stabilization10,11,12.

Owing to their reduced size, nanomaterials exhibit distinct properties compared to their macroscopic counterparts, positioning them as a current research focus across materials science, physics, and chemistry. With the rapid advancements in material science, researchers have introduced nanomaterials as a promising alternative to traditional soil stabilizers. Nanomaterials provide a novel approach to modifying soil structure and geotechnical properties via pore-filling mechanisms13,14,15. For instance, Chen et al16. investigated the soil stabilization effects of nano-materials and found that adding 2.5% nano-silica significantly increased the soil strength by reducing porosity. Yesilmen et al17. further reported that nanosilica incorporation effectively improved the strength of cemented soils, although its effect on ductility was limited. Increasing the nano-silica content further improved the resilient modulus of the stabilized soil18. It significantly enhanced soil cohesion, although the internal friction angle decreased slightly. The maximum shear strength was achieved at a dosage of 4%, beyond which further increases in dosage did not significantly improve shear strength19. Although nano-silica show its advantages by significantly enhancing the soil strength, it also exhibits challenges such as brittleness and limited efficiency at higher dosages.

In contrast, fiber reinforcement offers an alternative method that increases soil ductility with its high tensile properties, presenting higher strength increment20,21,22,23. This technique involves the incorporation of randomly distributed fibers into soil to improve its mechanical properties24,25,26. Under applied loading, frictional interactions and bonding between fibers and soil particles collectively enhance the overall soil strength27,28,29,30. Various natural fibers and synthetic fibers have been applied to explore for soil stabilization, with polypropylene fibers being the most commonly used. Studies have shown that polypropylene fibers improve soil strength, reduce cracking, enhance permeability, and mitigate swelling in soils. For example, Murray et al31. observed in consolidated undrained (CU) triaxial tests that adding 1% polypropylene fibers to sandy silt soils increased undrained shear strength and resulted in more ductile post-peak behavior. Navagire et al29. investigated the effectiveness of polypropylene fibers in stabilizing Indian black cotton soil and found that the 0.8% dosage of polypropylene fibers increased the unconfined compressive strength (UCS) by 60%. Ghazavi32 studied the effects of polypropylene and steel fibers on the properties of kaolinite clay. The results indicated that the unconfined compressive strength of clay samples increased by 160% and 7%, respectively before freeze-thaw cycles. However, after undergoing freeze-thaw cycles, the strength improvement rates dropped to 60% and 6%, respectively, compared to untreated samples. This highlights that polypropylene fibers exhibit superior durability and stabilization performance.

However, the individual application of polypropylene fibers or nano-silica has a relatively limited effect on improving the initial stiffness and strength of soil15,33,34. Several studies have been carried out to explore the combined use of multiple additives to enhance the mechanical properties and durability of soils35,36,37. For instance, Pashabavandpouri et al. found that incorporating polypropylene fibers into lime-stabilized soil increased the soil strength by 11.7 times while reducing the lime dosage by 60%36,37. Chen et al.38 observed that adding polypropylene fibers to xanthan gum-stabilized soil not only improved the unconfined compressive strength (UCS) but also mitigated the brittleness induced by xanthan gum. Therefore, the simultaneous application of nano-silica and polypropylene fibers for soil stabilization exhibits significant mechanistic complementarity. Nano-silica modifies the bonding and microstructure of the soil matrix at the nanoscale, whereas polypropylene fibers provide physical reinforcement and toughness enhancement at the mesoscale30,39. Their combination is expected to synergistically produce composite stabilized soils with high strength, toughness, and excellent durability. However, there is currently a lack of systematic studies on the synergistic mechanism, optimal mix ratio, and effects on long-term environmental durability (e.g., wet-dry and freeze-thaw cycles) of these two materials in soil stabilization.

To investigate the mechanical properties, durability, and micro-mechanisms of soil stabilized with a composite of nano-silica and polypropylene fibers, this study carried out a series of laboratory tests to examine the effects of varying contents of these two additives on soil compressive strength under different wet-dry and freeze-thaw cycles. Nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) were then used to further analyze the reinforcement mechanisms in soils treated with these additives.

Material

The soil used in this study was sourced from a construction site in Tibet. Initially, the soil was cleaned to remove impurities such as gravel and plant roots, then dried in an oven at 105 °C for 24 h. After drying, the soil was crushed and sieved through a standard 2 mm mesh. The particle size distribution of the soil is shown in Fig. 1. The liquid limit (LL) and plastic limit (PL) of the soil, determined via the combined liquid-plastic limit test, were 32.9% and 19.3%, respectively. According to the GB/T 50,123 − 2019, the soil was classified as sandy medium liquid limit clay (CL). Additionally, compaction test results indicated that the optimum moisture content (OMC) and maximum dry density (MDD) of the soil were 21% and 1.66 g/cm³, respectively.

Fig. 1
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Soil particle size distribution.

The nano-silica used in this study had a particle size ranging from 1 to 100 nm and BET surface area of 200 m²/g, which allows for excellent dispersion and uniform distribution in composite materials. Additionally, nano-silica is non-toxic, chemically stable, and environmentally friendly, making it suitable for a wide range of applications. Polypropylene fibers were chosen for their high durability, cost-effectiveness, and broad applicability in soil stabilization. The basic physical and mechanical properties of the polypropylene fibers used are summarized in Table 1.

Table 1 The properties of polypropylene fiber.
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Method

Sample Preparation

Previous studies have demonstrated that the soil stabilization effectiveness of nano-particles and fiber additives tends to decrease when their dosage exceeds 2%39,40, Thus, a dosage range of 0% to 2% was adopted in this study. In this study, four types of samples were prepared for the unconfined compressive test. (1) Untreated Soil: Soil without any additives. (2) Polypropylene fiber reinforced soil: Soil reinforced with polypropylene fibers at 0.5%, 1%, and 2% by dry soil weight. (3) Nano-Silica reinforced soil: Soil stabilized with nano-silica at 0.5%, 1%, and 2% by dry soil weight. (4) Soil reinforced with polypropylene fibers (0.5%, 1%, and 2%) and nano-silica (0.5%, 1%, and 2%) as composite additives. Dry soil was first mixed with the specified amounts of additives (polypropylene fibers, nano-silica, or both) until homogeneous. The proper amount (water content 21%) of deionized water was then added to the dry mixture while stirring continuously to ensure uniformity and avoid clumping. Finally, mixture was poured into the cylindrical mould with a diameter of 39.1 mm and a height of 80 mm and compacted layer by layer to reach the designed dry density of 1.58 g/cm3. The samples were wrapped in the plastic bag and preserved in constant temperature (20℃) and humidity (95%) chamber for 7 days for the following test. During preliminary tests, it was found that the curing conditions employed in this study could effectively ensure both the soil strength and curing efficiency. Schematic diagram of test procedure was presented in Fig. 2.

Dry-wet cycles and freeze-thaw cycles

For the dry-wet cycles, the samples were first dried in an oven at a constant temperature until their weight remained stable, ensuring complete moisture removal. The dried samples were then immersed in water for 24 h to initiate the first dry-wet cycle. This process was repeated for a total of 10 cycles. For the freeze-thaw cycles, the samples were stored in a temperature-controlled chamber at −20 °C for 24 h, followed by a 24-hour period at 20 °C. This freeze-thaw process was repeated for 10 cycles. The mechanical properties of the samples were tested after the 1 st, 2nd, 5th, and 10th dry-wet or freeze-thaw cycles. The unconfined compressive strength (UCS) were measured according to the China National Standard GB/T 50,123 − 2019 with loading speed of 1 mm/min. Each test was conducted in triplicate and average result was calculated to minimize the errors.

Nuclear magnetic resonance (NMR)

NMR has been widely recognized as a powerful and non-destructive analytical tool in geotechnical engineering and soil science to systematically characterize the microscopic properties of soil41. By providing quantitative and high-resolution insights into microscale features, NMR contributes significantly to the understanding of soil pore size behavior. In this study, the sample was first subjected to saturation to ensure complete water filling of internal pores. During the scanning, the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence was used to acquire the time-evolution curve of nuclear magnetic signals of the sample. By conducting inversion processing on the CPMG curve, the distribution characteristics of the transverse relaxation time T2 within the sample can be determined. Analysis of the T₂ distribution curve further enabled characterization of the sample’s pore size characteristics.

Scanning electron microscopy (SEM)

For the SEM testing, the sample was firstly subjected to freeze-drying to remove moisture without structural collapse. Prior to imaging, samples were sputter-coated with a 10 nm thick gold layer to enhance electrical conductivity. The morphology of the sample was then examined using SEM at multiple magnifications.

Fig. 2
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Schematic diagram of test procedure.

Results and discussion

Single effect of nano-silica or polypropylene fibers on soil UCS

The unconfined compressive strength (UCS) of soil reinforced with 0%, 0.5%, 1%, and 2% nano-silica content is shown in Fig. 3a. The results demonstrate that the addition of nano-silica significantly enhances the mechanical properties of the soil, with the improvement becoming more pronounced at higher nano-silica contents. For unreinforced soil, the UCS was approximately 130.5 kPa. Incorporating 0.5% nano-silica increased the UCS to 153.3 kPa, reflecting a 12.5% improvement. This initial increase highlights the beneficial effect of nano-silica in enhancing soil strength. When the nano-silica content was raised to 1%, the UCS rose to 195.9 kPa, representing a substantial 27.8% improvement compared to the 0.5% content. Further increasing the nano-silica content to 2% led to a UCS of 245.4 kPa, a 25.12% improvement compared to the 1% content. While the rate of strength increase began to decelerate, the overall UCS exhibited a consistent upward trend. These results confirm that nano-silica effectively strengthens soil, although the incremental benefit diminishes at higher concentrations42,43. A similar phenomenon was also observed in the study by Kalhor et al39. While nano-silica enhances the unconfined compressive strength (UCS) of soil by filling pores and forming a denser soil structure44,45. Its lightweight particles lead to reduced soil density when added excessively, which is detrimental to UCS improvement. This further indicates that the soil strength enhancement capacity of pure nano-silica particles is limited.

The unconfined compressive strength (UCS) of polypropylene fiber-reinforced soil at different fiber contents is presented in Fig. 3b. Similar to nano-silica, the addition of fibers significantly enhances the UCS of the soil. As the fiber content increases from 0% to 0.5%, the UCS rises substantially from 130.5 kPa to 203.7 kPa, reflecting a 56.1% improvement. Within this lower fiber content range, fibers effectively contribute to soil strength enhancement. When the fiber content is increased to 1%, the UCS shows a dramatic increase from 203.7 kPa to 521.1 kPa, representing an extraordinary 155.9% growth. At a fiber content of 2%, the UCS further improves to 749.4 kPa, an additional 43.8% increase compared to the 1% content. These results demonstrate that the UCS consistently increases with higher fiber content, with the magnitude and efficiency of this improvement surpassing those observed with nano-silica. This highlights the superior reinforcement potential of fiber inclusion for improving soil strength46. Randomly distributed polypropylene fibers form a continuous and uniform three-dimensional reinforcement network in the soil, which provides omnidirectional constraints to the soil29,46,47. When the soil deforms, the polypropylene fibers generate relative displacement with the surrounding soil particles, and the interfacial friction is activated. The interfacial friction transfers stress from the soil to the polypropylene fibers, inhibits soil deformation, and significantly enhances soil stability48.

Fig. 3
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Stress strain curve of (a) nano-silica treated soil (b) polypropylene fibers treated soil.

Combined effect of nano-silica and polypropylene fibers on soil UCS

Figure 4 presents the peak compressive strength of soil samples reinforced with varying contents of nano-silica (0%, 0.5%, 1%, and 2%) and polypropylene fibers (0%, 0.5%, 1%, and 2%). The results demonstrate that the combined use of nano-silica and polypropylene fibers produces significantly higher compressive strength compared to their individual effects. For instance, soil treated with 0.5% nano-silica and 0.5% polypropylene fiber achieved a compressive strength of 238.5 kPa. This value is markedly higher than that of soil treated with 0.5% nano-silica alone (153.3 kPa) or 0.5% polypropylene fiber alone (203.7 kPa). When the additive content increased to 2%, the compressive strength rose further to 923.1 kPa, highlighting the synergistic effect of combining nano-silica with polypropylene fibers.

Fig. 4
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UCS of nano-silica (0%, 0.5%, 1%, and 2%) associated with (0%, 0.5%, 1%, and 2%) polypropylene fibers treated soil.

To quantify the efficiency of the combined reinforcement, Table 2 presents the improvement ratios of nano-silica and polypropylene fiber relative to their individual effects. It was defined as fiber improvement ratio and nano-silica improvement ratio. The improvement ratios were calculated as follows:

Fiber improvement ratio = (Strength of combined treatment − Strength of nano silica treated soil)/Strength of nano silica treated soil×100%.

Nano silica improvement ratio = (Strength of combined treatment − Strength of fiber treated soil)/Strength of fiber treated soil×100%.

For example, in the case of 0.5% nano-silica and 0.5% polypropylene fiber treatment, the fiber improvement ratio was calculated as (238.5-153.3.5.3)/153.3 × 100%=55.6%, while the nano-silica improvement ratio was (238.5-203.7.5.7)/203.7 × 100%= 17.1%. When the nano-silica content was kept constant, the improvement ratio of soil strength increased with a higher fiber content. A similar trend was observed for the nano-silica improvement ratio, where a higher nano-silica content resulted in a greater enhancement of soil strength. However, the fiber improvement ratio consistently outperforms the nano-silica improvement ratio, suggesting that polypropylene fibers provide a greater contribution to soil strength enhancement.

Table 2 Fiber improvement ratio and nano-silica improvement ratio in nano-silica associated with polypropylene fibers treated soil.
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Long term resistance to dry-wet or freeze-thaw cycles

Dry-wet and freeze-thaw cycles often induce significant changes in the soil mechanical properties. To mitigate these effects, the inclusion of fibers and nano-additives enhances soil resistance by forming a cohesive matrix that reduces degradation under multiple cycles. Based on the preceding experimental results, soil treated with 2% nano-silica and 2% polypropylene fiber was selected for evaluating strength deterioration under multiple dry-wet and freeze-thaw cycles. The corresponding results are presented in Fig. 5.

The strength retention ratio after multiple dry-wet or freeze-thaw cycles, defined as the ratio of compressive strength after the i-th cycle (Ui/U₀, where i = 1, 2, 5, 10) to the initial strength (U₀), was used to quantify the degradation. For untreated soil, compressive strength decreased with an increasing number of cycles, with the most significant loss occurring after the first cycle49,50. The dry-wet or freeze-thaw strength retention ratio were 49.2% and 40.2%, respectively after the first cycle. Strength degradation stabilized after the second cycle, with final rates of 37.1% and 32.5% after the 10th cycle. In contrast, soil treated with 2% nano-silica and 2% polypropylene fiber exhibited significantly higher strength retention ratio across all cycles, demonstrating enhanced resistance to both dry-wet and freeze-thaw processes. For instance, after the first dry-wet or freeze-thaw cycles, strength retention ratio for the treated soil were 63.9% and 57.0%, considerably higher than the untreated soil (49.2% and 40.2%, respectively). This number was 54.6% and 50.4% respectively for the treated soil and 37.1% and 32.5% even after 10 cycles.

During dry-wet or freeze-thaw cycles, the soil undergoes repeated expansion and contraction51,52. Water infiltration during wetting or freezing increases pore water pressure, causing particle expansion, while drying or thawing induces contraction as water evaporates or melts. These processes disrupt the soil structure, increasing porosity, weakening particle bonding, and creating microcracks53. With repeated cycles, the microcracks expand, progressively reducing soil strength. The addition of nano-silica and polypropylene fiber mitigates this degradation. Nano-silica and fibers form a three-dimensional network within the soil, binding particles and providing structural reinforcement23,54. This network reduces internal displacement caused by volume changes, minimizing the formation and growth of microcracks. Consequently, the combined use of nano-silica and fibers effectively enhances soil resilience to cyclic degradation, maintaining structural integrity under dry-wet and freeze-thaw conditions. For example, while the incorporation of polypropylene fibers alone can improve the shear strength of soil, it compromises the water stability of the modified soil, as fibers increase water infiltration pathways55,56,57. In contrast, when nano-silica is used in combination with polypropylene fibers, the micro-filling effect of nano-silica enhances soil compactness and optimizes pore structure, reducing water penetration and evaporation, thereby improving soil water stability37,55. This synergistic effect enables the co-reinforced soil to exhibit superior crack resistance and strength retention capacity during wet-dry and freeze-thaw cycles58.

Fig. 5
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2% content nano-silica associated with 2% content polypropylene fibers treated soil deterioration after 10 dry-wet or freeze-thaw cycles.

Pore structure analysis via NMR

The transverse relaxation time (T₂) distribution curve from nuclear magnetic resonance (NMR) is linearly positively correlated with soil pore size59. Based on the linear relationship between total NMR signal intensity and pore size distribution, the T₂ distribution curve of soil can be quantitatively converted into a pore size distribution curve, which more accurately reflects the intrinsic pore size distribution of the soil. Figure 6 present the T2 curve and pore size distribution curve of clean soil, 2% nano-silica treated soil, 2% polypropylene fibers, 2% content nano-silica associated with 2% content polypropylene fibers treated soil. All soil samples exhibited two distinct peaks in their T2 curves, representing larger pores and smaller pores, respectively. The larger pores, composed of macropores, were primarily distributed in the range of 0–0.4 μm. The smaller pores, consisting of micropores, were mainly distributed between 0.4 and 20 μm. Soil treated with 2% polypropylene fibers showed a slight reduction in both peaks, indicating a limited effect of fibers on altering pore structure. Polypropylene fibers were randomly distributed in soil pore space, thereby reducing soil porosity. Furthermore, the three-dimensional skeleton formed by interwoven fibers can markedly constrain relative displacement between particles and inhibit crack propagation60. In contrast, the two peaks in the T₂ curve of soil treated with 2% nanosilica decreased significantly. Nano-silica possesses a larger specific surface area, enabling stronger adsorption onto soil particle surfaces to form stable aggregates. It penetrated existing pore channels within the soil, directly occupying pore spaces and reducing the total number of pores. Notably, the minimum T2 value remained unchanged while the maximum T2 value decreased, indicating that no smaller pores were generated and that the filling effect of nano-silica reduced the size of large pores. When nano-silica and polypropylene fibers were added simultaneously, the two peaks in the T2 curve decreased more significantly than those in soil modified with nano-silica or fiber alone. Fibers provided a stable filling space for nanoparticles, while nanoparticles strengthened the interfacial bonding between fibers and soil particles. This synergy induces significant microstructural optimized, including refined pore size distribution and enhanced aggregate stability61. Through such synergistic interaction, the two materials collectively contribute to the optimization of the soil pore structure.

Fig. 6
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T2 curve and pore size distribution curve of clean soil, 2% nano-silica treated soil, 2% polypropylene fibers, 2% content nano-silica associated with 2% content polypropylene fibers treated soil (a:T2 curve; b: pore size distribution curve).

Then, pores in soil can be classified into macropores (> 4 μm), mesopores (0.4–4 μm), small pores (0.04–0.4 μm), and micropores (< 0.04 μm). Figure 7 presents the pore size proportion diagram of clean soil, 2% nano-silica treated soil, 2% polypropylene fibers and 2% content nano-silica associated with 2% content polypropylene fibers treated soil. As indicated in the Fig. 7, the proportion of all pore types decreases in both the soil mixed with polypropylene fibers and that with nano-silica.

After the incorporation of polypropylene fibers, the micropores and mesopores in the soil remained almost unchanged, while the small pores and macropores decreased slightly. Consequently, the total soil porosity was reduced, leading to an optimized soil structure. This phenomenon can be attributed to the random dispersion of polypropylene fibers within the soil, which directly filled or covered the original pores, thereby resulting in a reduction in soil porosity. For soil treated with nano-silica, the decrease in micropores and small pores was more significant than that in mesopores and macropores. Nano-silica particles typically had a particle size distribution ranging from 1 to 100 nm. Their particle size distribution exhibited high compatibility with the micropores and small pores in the soil. For mesopores and macropores with pore sizes ranging from 0.4 to 40 μm, their dimensions far exceeded the effective action radius of nano particles, rendering the filling effect of individual nano-silica particles negligible62.

Fig. 7
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Pore size proportion of clean soil, 2% nano-silica treated soil, 2% polypropylene fibers and 2% content nano-silica associated with 2% content polypropylene fibers treated soil.

Micro interaction analysis via SEM image

The role of SEM provide a microscopic explanation for the improvement of polypropylene fiber and nano-silica reinforcement soil macro-mechanical properties (Fig. 8). SEM images of fiber-treated soil reveal the mechanisms underlying the improved performance of fiber-reinforced soil. Polypropylene fibers form an intertwined network with soil particles, creating a complex and stable structure63. This network effectively inhibits the expansion of pores and microcracks under external loads, thereby reducing the progression of soil deformation. The fibers within the soil matrix are subject to interfacial friction and adhesion forces, which resist relative slippage between the fibers and soil particles47. As fiber content increases, these interfacial forces grow stronger, further enhancing the soil’s compressive strength. However, excessive fiber content may lead to clumping, which reduces effective contact between fibers and soil particles, ultimately diminishing reinforcement efficiency. Nano-silica particles, with diameters typically ranging from 1 to 100 nanometers, are significantly smaller than the pore spaces between soil particles. This allows nano-silica to infiltrate and fill these microscopic voids effectively. The reduction in porosity enhances soil compaction and contributes to higher UCS values. Additionally, nano-silica particles adhere to the surfaces of soil particles, forming strong interparticle bonds. The combined effects of reduced porosity and enhanced bonding significantly improve the mechanical properties of the soil64. Nano-silica particles adhere to the surfaces of soil grains and fibers, increasing surface roughness and enhancing interfacial interactions. Additionally, the nano-silica fills soil pores, improving soil compaction. This compaction exerts pressure on both the polypropylene fibers and soil particles, binding them more tightly together. The interaction between nano-silica and fibers forms a complex, intertwined structure within the soil matrix. The fibers and nano-silica are compressed by soil particles, creating a stronger bonding layer that strengthens the connection between fiber surfaces and nano-silica soil aggregates65. Nano-silica contributes by filling microscopic pores and forming strong bonds, while polypropylene fibers provide tensile reinforcement. The synergy between nano-silica and polypropylene fibers makes this combination an effective and sustainable solution for soil stabilization66.

Fig. 8
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SEM of (a) untreated soil, (b) nano-silica associated with polypropylene fibers treated soil.

Conclusion

This study presents a comprehensive evaluation of the combined effects of nano-silica and polypropylene fibers on soil stabilization, offering valuable insights into their synergistic reinforcement mechanisms and practical applications in geotechnical engineering. The following conclusions can be obtained:

  1. (1)

    Effectiveness of nano-silica in soil stabilization: Nano-silica significantly enhances soil mechanical properties by reducing porosity and forming strong interparticle bonds. The addition of 0.5% nano-silica resulted in an 17.5% improvement in unconfined compressive strength (UCS) compared to untreated soil, demonstrating its efficacy as a soil stabilizer.

  2. (2)

    Role of polypropylene fibers: Polypropylene fibers improved soil ductility and strength by creating an interwoven network that inhibits crack propagation and deformation. A 0.5% fiber content led to a 56.1% increase in UCS, showcasing the superior reinforcement potential of fibers.

  3. (3)

    Synergistic reinforcement effects: The combination of nano-silica and polypropylene fibers yielded a synergistic effect, significantly enhancing soil strength and stability beyond the individual contributions of each additive. Peak UCS values were achieved with 2% content of both nano-silica and fibers, emphasizing the advantages of composite stabilization techniques.

  4. (4)

    Resilience to dry-wet and freeze-thaw degradation: Soil treated with the combined additives exhibited superior resistance to dry-wet and freeze-thaw cycles. The strength retention ratio was 54.6% and 50.4% respectively even after 10 dry-wet or freeze thaw cycle, much higher than untreated soil (37.1% and 32.5%), which underscores the durability and long-term performance of the composite stabilization method.

  5. (5)

    Microstructural insights: NMR results revealed that the addition of both polypropylene fibers and nano-silica reduced soil porosity and refined the soil pore structure, with nano-silica demonstrating superior efficacy in these aspects relative to polypropylene fibers.

  6. (6)

    Microstructural insights: Scanning electron microscopy (SEM) analysis revealed that nano-silica fills soil pores and binds to particle surfaces, while fibers form a three-dimensional network, collectively enhancing soil compaction and structural integrity. This microstructural reinforcement explains the observed improvements in mechanical properties and resilience.

  7. (7)

    The synergistic interaction between nano-silica and polypropylene fibers has a great impact on geotechnical engineering design and applications. By changing the mix proportions of the two additives, the mechanical behavior of soils can be systematically optimized, enabling more accurate and flexible design parameters. These improvements in soil strength and long-term degradation extend the applicability of stabilized soils to challenging environments, including soft clays, water-rich strata, seasonal frozen regions, and slopes subjected to cyclic environmental loading. This adaptability provides a practical pathway for improving the resilience of geotechnical infrastructures under diverse and extreme service conditions.