Enhancing the Compressibility of Seasonally Frozen Subgrade Clay Subjected to Freeze-Thaw Cycles Using Lignin Fibers

Abstract

Repeated freeze-thaw cycles in seasonally frozen regions significantly degrade the mechanical properties of clay, posing serious challenges to geotechnical infrastructure stability. This study investigates the compressibility behavior of lignin fiber-reinforced clay under freeze-thaw conditions through one-dimensional consolidation tests and microstructural analysis. Clay specimens containing 0.0%, 0.5%, 1.0%, 1.5%, and 2.0% lignin fibers by mass were subjected to 0, 1, 4, and 10 freeze-thaw cycles to simulate typical seasonal variations. The results indicate that reinforcement with lignin fibers markedly enhances the soil’s resistance to freeze-thaw-induced degradation. Specifically, in unreinforced clay, 10 freeze-thaw cycles reduced the pre-consolidation pressure from 139 kPa to 97 kPa. With 2.0% lignin fiber, the pressure increased to 186 kPa under unfrozen conditions and remained at 120 kPa after 10 cycles. SEM and MIP analyses revealed that lignin fibers form interconnected networks that inhibit the formation and expansion of strip pores and constrained pore coarsening caused by freeze-thaw action, effectively stabilizing the soil structure. A model incorporating both fiber content and freeze-thaw cycle effects was proposed to predict compression behavior, and the model accurately captured the experimental compression curves across all test conditions. This study provides a theoretical and experimental basis for the application of natural fiber-reinforced clay in cold-region geotechnical engineering, offering a sustainable and effective alternative to traditional stabilization methods.

1. Introduction

Clay soils are extensively employed in civil engineering projects due to their availability and inherent properties [1,2,3]. Their mechanical and hydraulic behavior plays a critical role in determining the stability and safety of infrastructures such as foundations, roadways, and slopes [4,5,6,7,8]. Particularly in regions experiencing seasonal freezing and thawing, clays are vulnerable to repeated thermal cycles that significantly alter their structural integrity [9,10,11]. During freezing conditions, the transformation of pore water into ice causes volumetric expansion, disrupting internal particle arrangements and inducing micro-cracks. Upon thawing, the ice melts, leading to soil contraction and further internal structural degradation [12,13,14]. The continuous repetition of these freeze-thaw cycles progressively weakens the soil, compromising its compressive property, shear capacity, and water retention ability, ultimately jeopardizing infrastructure reliability [15,16].

To address the degradation of clays induced by freeze-thaw cycles, various soil stabilization and improvement methods have been explored. Chemical stabilization methods involving additives like cement, lime, and fly ash facilitate the formation of cementitious or pozzolanic compounds within the soil matrix, enhancing bonding strength and resistance to freeze-thaw damage [17,18,19]. Despite their effectiveness, these chemical methods raise environmental concerns due to high carbon footprints and potential secondary pollution, limiting their suitability in sustainable construction practices.

Physical soil improvement techniques, including soil replacement, vacuum consolidation, dynamic compaction, and soil reinforcement with synthetic materials, provide alternative solutions [20,21,22,23,24]. Soil replacement involves substituting problematic clay with granular materials to reduce susceptibility to freeze-thaw-induced deformations. Vacuum consolidation enhances settlement control through accelerated pore water drainage, while dynamic compaction densifies soil via mechanical impact. Reinforcement using geosynthetics or fibers creates composite systems that effectively distribute loads and limit deformations. Nonetheless, these approaches often incur high construction costs, prolonged construction schedules, environmental impacts, or community disturbance, highlighting the necessity for alternative methods that integrate sustainability, cost-efficiency, and environmental compatibility.

Recently, natural fiber reinforcement has garnered increasing attention due to its alignment with sustainable development goals, offering advantages such as renewability, cost effectiveness, ease of construction, and minimal environmental impact [25,26,27]. Unlike synthetic reinforcements that rely on petrochemical derivatives and may generate microplastic pollution, natural fibers are biodegradable and originate from renewable biomass sources. Among natural fibers, lignin fibers—extracted as a byproduct from the paper industry—stand out due to their durability, chemical stability, and biodegradation resistance, making them suitable for harsh climatic conditions [28,29,30]. Studies have shown that the inclusion of lignin fibers in clayey soils improves unconfined compressive strength, shear strength, and ductility by promoting particle interlocking and reducing strain localization [31,32]. Additionally, lignin fibers have demonstrated favorable durability under cyclic environmental conditions. For example, Fan et al. [33] observed that lignin-reinforced clay exhibited significantly lower degradation in stiffness and strength after multiple wetting-drying and freeze-thaw cycles compared to untreated soil. From an environmental perspective, lignin fiber is a renewable, biodegradable, and carbon-neutral byproduct of the pulp and paper industry. Unlike synthetic geofibers that may generate persistent microplastic pollution, lignin fibers naturally decompose without leaving harmful residues [34]. These attributes make lignin fiber a more sustainable and environmentally responsible alternative for geotechnical applications. Given these environmental and mechanical advantages, lignin fiber reinforcement is particularly suitable for use in rural or ecologically sensitive areas where access to conventional stabilizers is limited and sustainable construction practices are prioritized.

Despite preliminary recognition of lignin fiber’s beneficial effects on mechanical performance, systematic studies investigating its role in reducing compressibility and modifying microstructure under repeated freeze-thaw conditions remain scarce. Addressing this research gap is essential not only for improving geotechnical performance, but also for promoting eco-efficient construction practices. To this end, the current research systematically evaluates how varying lignin fiber contents affect the compressibility of compacted clay through one-dimensional consolidation experiments subjected to controlled freeze-thaw cycles. In addition, scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) analyses are conducted to elucidate the underlying microstructural evolution. Finally, a coupled compressibility model is proposed, which accounts for both fiber content and freeze-thaw effects, offering a sustainable and theoretically robust approach for applying lignin fiber-reinforced clays in cold-region geotechnical engineering.

2. Compression Tests and Results Analysis

2.1. Materials and Experimental Procedures

2.1.1. Materials

The clay used in this study was collected from a highway construction site in Heilongjiang Province, China. After removing impurities and humic substances, the clay was crushed manually and sieved through a 2 mm mesh to achieve uniform particle size, as shown in Figure 1a. The basic physical properties of the clay are summarized in

Table 1. Basic physical properties of the clay.

Properties	Values
Specific gravity, Gs	2.68
Liquid limit, wL/%	54.0
Plastic limit, wP/%	24.4
Maximum dry density, dmax/(g/cm3)	1.75
Optimum water content, wopt/%	22.7
Sand, %	11
Silt, %	48
Clay, %	41
Classification	CL

. Mineralogical analysis using X-ray diffraction (XRD) revealed that the primary mineralogical components were quartz, illite, albite, and calcite. The major chemical constituents of the clay and their corresponding mass percentages were as follows: SiO₂—60.48%, Al₂O₃—18.53%, Fe₂O₃—6.63%, K₂O—3.05%, and CaO—4.00%. Lignin fibers, supplied by Qingjun Cellulose Factory in Shijiazhuang, Hebei Province, China, were used for reinforcement, as shown in Figure 1b. The fibers were produced from waste wood through a series of processes including soaking, shredding, pulverization, acid precipitation, filtration, and drying. Chemically, they are primarily composed of amorphous polymeric compounds consisting of carbon (C), oxygen (O), hydrogen (H), and nitrogen (N), which contribute to their chemical stability and resistance to biodegradation. The fibers appear as fine brown strands, with lengths ranging from 6 to 8 mm. Prior to mixing with soil, the fibers were passed through a 1 mm sieve to ensure the uniformity of the mixture.

2.1.2. Specimen Preparation

Previous studies indicated that lignin fiber contents below 2.0% effectively improve clay mechanical properties without causing significant fiber aggregation [33]. To ensure both mixture homogeneity and reliable test performance, fiber contents in this research ranged from 0.0% to 2.0% at intervals of 0.5%. The lignin fibers were first passed through a 1 mm sieve to eliminate oversized agglomerates. Specimen preparation involved a layered dry mixing method, where dried clay and fibers were gradually blended in multiple stages to promote uniform fiber dispersion. Water was added incrementally while mixing to achieve the optimum moisture content. The mixtures were then sealed in plastic bags for 24 h to allow uniform moisture redistribution before compaction. This procedure was adopted to ensure consistent initial conditions across all test specimens and minimize variability due to fiber clustering or moisture heterogeneity.

2.1.3. Application of Freeze-Thaw Cycles

To simulate the seasonal freeze-thaw conditions in Heilongjiang Province, compacted specimens were wrapped tightly in plastic film and placed in a temperature-controlled chamber. Each freeze-thaw cycle comprised freezing at −20 °C for 12 h followed by thawing at 20 °C for 12 h. To eliminate moisture content gradients, specimens were kept at room temperature (25 °C) for 24 h after completing each set of cycles. Given that previous studies have indicated that freeze-thaw-induced damage stabilizes after approximately 10 cycles [35,36,37], tests were conducted with 0, 1, 4, and 10 freeze-thaw cycles to investigate the impact of cycle frequency on fiber-reinforced clay compressibility.

2.1.4. One-Dimensional Compression Tests

Following freeze-thaw treatment, specimens were vacuum-saturated for 2 h to remove residual air bubbles and then placed in a one-dimensional consolidation apparatus. A gradual loading sequence of 12.5, 25, 50, 100, 200, 400, 800, 1600, 3200, and 4000 kPa was applied, with each stress level maintained for 24 h to ensure consolidation equilibrium. Compression curves (e-lgσ_v) were plotted, and compressibility indices (C_c), recompression indices (C_r), and pre-consolidation pressures (σ_vp) were calculated to assess the effects of lignin fiber content and freeze-thaw cycles on clay compressibility.

2.2. Results and Discussion

2.2.1. Effect of Lignin Fiber Content

Figure 2 presents compression curves for clays reinforced with varying lignin fiber contents (C_F). Increasing fiber content clearly reduced compressibility, as evidenced by the flatter e-lgσ_v curves, indicating enhanced compressive resistance. Pre-consolidation pressures, determined using the Casagrande method [38], increased significantly from 139 kPa to 186 kPa as fiber content rose from 0.0% to 2.0%. This increase in σ_vp reflects the delayed onset of plastic deformation, attributed to the development of fiber–soil interlocking structures that restrict particle movement and provide additional mechanical reinforcement. Moreover, both the compression index (C_c) and recompression index (C_r) decreased with higher fiber content, confirming that the lignin fibers enhanced stiffness during both the elastic and plastic deformation stages. These results demonstrate that even small additions of lignin fiber can effectively improve the structural integrity of compacted clay, offering practical benefits for applications where deformation control is critical.

2.2.2. Effect of Freeze-Thaw Cycles

To evaluate the damage from freeze-thaw cycles and fiber reinforcement efficacy, specimens with fiber contents of 0.0% and 2.0% were subjected to 0, 1, 4, and 10 freeze-thaw cycles. Figure 3 illustrates the corresponding e-lgσ_v curves. The first freeze-thaw cycle caused the most significant structural damage, indicated by substantial downward shifts in compression curves. Subsequent cycles resulted in smaller incremental changes, suggesting structural stabilization. Comparing Figure 3a–c, higher fiber content (2.0%) substantially reduced the freeze-thaw induced compressibility deterioration.

Figure 4 shows the variation in pre-consolidation pressure (σ_vp) of clay specimens subjected to different numbers of freeze-thaw cycles with varying lignin fiber contents. A pronounced decline in σ_vp is observed after the first freeze-thaw cycle for all specimens, regardless of fiber content. Specifically, more than 50% of the total reduction in σ_vp after 10 cycles occurs during the first cycle, indicating that the initial freeze-thaw transition imposes the most severe damage to the soil microstructure. As the number of cycles increases from 1 to 10, the rate of σ_vp reduction gradually stabilizes, suggesting that the majority of structural damage is concentrated in the early cycles, while subsequent cycles contribute to more moderate degradation. However, this trend is notably mitigated by the addition of lignin fibers. When the fiber content is increased from 0.0% to 2.0%, the magnitude of σ_vp reduction after each freeze-thaw cycle significantly decreases. In specimens with 2.0% fiber content, σ_vp remains relatively high even after 10 cycles, highlighting the superior freeze-thaw durability imparted by lignin fiber reinforcement. The ability of lignin fibers to maintain higher σ_vp values under cyclic thermal loading is critical for ensuring long-term deformation resistance and structural stability in cold-region geotechnical applications.

3. Microstructural Tests and Results Analysis

3.1. Microstructural Test Procedures

To elucidate the microstructural evolution of lignin fiber-reinforced clay subjected to varying freeze-thaw cycles, SEM and MIP tests were performed on specimens after 0, 1, 4, and 10 freeze-thaw cycles. The SEM testing procedure involved the following steps:

• Small cubic samples approximately 10 mm in size were cut from compacted specimens after freeze-thaw treatment. These samples were rapidly frozen using liquid nitrogen, followed by freeze-drying to preserve the original microstructure by removing pore water.
• After freeze-drying, samples were carefully trimmed to approximately 5 mm cubes using diamond tools, ensuring flat and even surfaces.
• The specimens were then gold-coated to enhance surface conductivity and reduce charging effects under electron beam irradiation. SEM imaging was conducted using a JEM-2100 scanning electron microscope (Tokyo, Japan) at a magnification of 200×.
• The MIP procedure was as follows:
• Small clay samples weighing 1–2 g were taken from similarly treated specimens, rapidly frozen in liquid nitrogen, and subsequently freeze-dried to retain the original pore structure.
• After drying, samples were gently ground to particles smaller than 2 mm, avoiding pore structure damage. The prepared samples were placed into a PoreMaster 33GT mercury porosimeter (Quantachrome Instruments, Boynton Beach, FL, USA), where pressure was gradually increased from vacuum to a maximum of 400 MPa.
• The pore size distribution was determined by measuring mercury intrusion volume as a function of applied pressure, converted into pore diameter via the Washburn equation.

3.2. Analysis and Discussion

3.2.1. SEM Analysis

Figure 5 presents the SEM images of lignin fiber-reinforced clays at various fiber contents (0.0%, 1.0%, 2.0%) before and after 10 freeze-thaw cycles. Initially, untreated clay exhibited intrinsic small strip pores and cavernous pores, negatively affecting its mechanical behavior, as shown in Figure 5a. Compared with Figure 5a,c,e, it can be seen that, after lignin fibers were introduced, fibers were observed interspersed among soil particles, forming bridging and interconnected three-dimensional networks. Increasing the fiber content significantly enhanced network density, promoting stress distribution and reducing the stress concentration in the soil. After 10 freeze-thaw cycles, untreated clay showed significant strip pore widening and propagation, developing connected strip pores. However, specimens with 1.0% fiber exhibited noticeably smaller and more strip pores, highlighting the fiber’s ability to retard strip pore growth and particle rearrangement, as shown in Figure 5d. At 2.0% fiber content, a more intact fiber–soil matrix was evident, with strip pores predominantly localized around fibers and at fiber–soil interfaces, as shown in Figure 5f. The majority of these strip pores failed to propagate extensively, demonstrating the fibers’ substantial protective role against freeze-thaw damage.

3.2.2. MIP Analysis

Figure 6 shows pore size distribution curves for lignin fiber-reinforced clays with different fiber contents before and after 10 freeze-thaw cycles. Pores were categorized as micropores (<0.1 μm), mesopores (0.1–5 μm), macropores (5–40 μm), or ultra-macropores (>40 μm) [39]. Initially, untreated clay demonstrated a unimodal distribution dominated by micropores. With fiber addition (1.0%), a new peak emerged in the mesopore range (0.5–2 μm), indicating that lignin fibers occupied existing micropores, shifting pore distribution toward larger mesopores. At 2.0% fiber content, this dual peak trend became more pronounced, with further decreases in micropore volumes and corresponding increases in mesopores, alongside reductions in macropore proportions.

After 10 freeze-thaw cycles, untreated clay showed a marked shift toward macropores and ultra-macropores, signifying severe structural damage due to pore enlargement and interconnection. Specimens with 1.0% fiber content also displayed pore size enlargement, but the increase in macropore volume was notably less pronounced than in untreated samples. Specimens with 2.0% fiber exhibited the strongest resistance to pore enlargement, with substantially reduced transitions from mesopores to macropores. These observations underline the effectiveness of lignin fiber networks in stabilizing pore structures and mitigating freeze-thaw-induced deterioration.

Both the SEM and MIP results provide consistent evidence for microstructural improvements induced by lignin fiber reinforcement. The SEM images showed that, in fiber-reinforced samples, especially at 2.0% content, fibers interspersed among soil particles formed spatially connected networks that restrained the development and propagation of strip pores. The MIP results further indicated that fiber addition shifted the pore size distribution from macropores to mesopores and effectively limited the formation of interconnected larger pores, even after 10 freeze-thaw cycles. These observations suggest that lignin fibers primarily function as internal structural bridges that restrict the rearrangement of soil particles and inhibit the coalescence of pores during freeze-thaw cycling. At lower fiber contents (e.g., 1.0%), partial suppression of macropore formation was observed, but with increasing fiber content to 2.0%, the fiber network became more continuous and dense, effectively limiting the transition from mesopores to macropores. Freeze-thaw cycles tend to enlarge pores and connect them into strip-like structures, but the presence of lignin fibers disrupts this process by physically restraining particle movement and preserving pore isolation. This pore-stabilizing effect underlies the improved compressibility resistance observed at higher fiber contents and under severe freeze-thaw conditions and highlights the importance of sufficient fiber dosage in maintaining microstructural integrity.

4. Compression Model

4.1. Modified Van Genuchten Model for Clay Compression

Conventional models used to simulate the compressibility of clay, including logarithmic, power-law, and modified SWCC-based equations, have been successful in fitting one-dimensional compression curves under controlled laboratory conditions [40,41]. However, their applicability in scenarios involving environmental degradation (e.g., freeze-thaw cycles) and material modification (e.g., fiber reinforcement) remains limited. Most existing models do not account for the evolving internal structure of the soil, especially the microstructural changes induced by cyclic loading and reinforcement interactions.

To address these limitations, this study proposes a modified van Genuchten-based model [42], in which the key parameters a and n are explicitly formulated as functions of lignin fiber content and the number of freeze-thaw cycles. This approach not only preserves the fitting flexibility of SWCC-based formulations, but also enhances the model’s generalization and predictive capabilities under varying environmental and material conditions. The model enables dynamic simulation of compressibility evolution in natural fiber-reinforced soils subjected to environmental degradation, thereby providing a practical tool for geotechnical design in cold regions. The modified van Genuchten equation, describing the relationship between void ratio (e) and vertical stress (σ_v), is expressed as follows:

$$e={\displaystyle \frac{e_0}{{\left[1+{\left({\sigma }_{\mathrm{v}}/a\right)}^n\right]}^m}}$$

(1)

where e₀ is the initial void ratio (0.531 in this study), and a, n, and m are fitting parameters. Parameter a primarily controls the curve’s inflection point and correlates closely with σ_vp. Parameter n predominantly influences the slope in the high-stress range, associated with C_c, and parameter m affects the slope in the low-stress range, related to C_r. Research has shown a semi-empirical relationship between parameters n and m: m = 1 − (1/n) [9]. By substituting this relation into Equation (1), the model simplifies to

$$e={\displaystyle \frac{e_0}{{\left[1+{\left({\sigma }_{\mathrm{v}}/a\right)}^n\right]}^{1-\frac{1}{n}}}}$$

(2)

4.2. Coupling Effect of Fiber Content and Freeze-Thaw Cycles on the Parameters in the Model

Equation (2) was used to perform nonlinear fitting of the experimental compression curves (e-lgσ_v) with varying lignin fiber contents C_F, as shown in Figure 2. The obtained optimal parameter a values with different fiber contents and numbers of freeze-thaw cycles are illustrated in Figure 7. With a fixed freeze-thaw cycle number, parameter a exhibits an approximately exponential increase with rising C_F. Conversely, with a constant fiber content, parameter a demonstrates a rapid initial decline followed by gradual stabilization as the freeze-thaw cycle number increases. This indicates that parameter a effectively reflects material improvement and resistance to freeze-thaw deterioration. Parameter a can be approximated as

$$a(\kappa ,\mu ,\eta )=a_0\cdot \left(1+\kappa \cdot e^{-\mu N_{\mathrm{FT}}}\right)\cdot \left(1+\eta \cdot C_{\mathrm{F}}\right)/\left(1+\kappa \right)$$

(3)

where a₀ is the value of parameter a without freeze-thaw cycles; κ, μ, and η are fitting parameters.

Figure 8 illustrates the variations in parameter n with the number of freeze-thaw cycles (N_FT) at different fiber contents (C_F). Parameter n gradually increases as N_FT rises from 0 to 10 cycles, with a more pronounced increase at lower fiber contents (e.g., 0% or 1%). At higher fiber contents (e.g., 2%), parameter n exhibits minimal change, indicating that increased fiber content effectively mitigates freeze-thaw deterioration effects. Consequently, parameter n can be fitted using the following exponential decay formula:

$$n(\lambda ,\gamma )=n_0\cdot \left(1+\lambda \right)/\left(1+\lambda \cdot e^{-\gamma N_{\mathrm{FT}}}\right)$$

(4)

where n₀ is the initial parameter value without freeze-thaw cycles; λ and γ are fitting parameters.

Figure 9 displays the relationship among parameters λ, γ, and fiber content C_F, indicating clear downward trends with increasing C_F. This result aligns well with the experimental findings, confirming that fiber content significantly reduces the freeze-thaw sensitivity of parameter n.

4.3. Integrated Compression Model Development

Combining Equations (3) and (4) into Equation (2), the compression model considering both fiber content and freeze-thaw cycle effects is

$$e={\displaystyle \frac{e_0}{{\left[1+{({\sigma }_{\mathrm{v}}/a(\kappa ,\mu ,\eta ))}^{n(\lambda ,\gamma )}\right]}^{1-\frac{1}{n(\lambda ,\gamma )}}}}$$

(5)

Figure 10 shows the fitting results of the proposed compression model across various lignin fiber contents (C_F) and numbers of freeze-thaw cycles (N_FT). Comparison of the modeled and experimental compression curves reveals a high degree of consistency, with the model successfully capturing both the initial void ratio and the nonlinear compression behavior across the full stress range. The model reflects the distinct flattening of the compression curves with increasing fiber content, as well as the progressive reduction in compressibility with increasing freeze-thaw damage, demonstrating its robustness in simultaneously incorporating the dual effects of reinforcement and environmental degradation. Notably, at low fiber contents and high freeze-thaw cycle numbers—conditions typically associated with severe structural deterioration—the model still maintains excellent predictive capability, highlighting its applicability in extreme environmental scenarios.

5. Discussion

A comparative analysis between the present findings and previous studies reveals both commonalities and unique aspects. Regarding the mitigation of freeze-thaw damage by fibers, Liu et al. [43] reported that cotton stalk fibers effectively reduced crack development in clay soils subjected to freeze-thaw cycles. The underlying mechanism, attributed to a physical bridging effect, is consistent with the “interlocking” function of lignin fibers observed in this study, thereby reinforcing the general efficacy of natural fibers in crack suppression. In a study conducted by Fan et al. [33], it was noted that high lignin fiber contents (>2.0%) tend to induce fiber agglomeration during mixing. This observation highlights the importance of the refined preparation protocol adopted in the present study—specifically, pre-sieving the fibers to 1 mm and implementing a layered mixing approach—which ensured uniform fiber distribution and effectively prevented clumping.

With respect to compression modeling, modified soil–water characteristic curve (SWCC) models have previously been employed to describe compression behavior [9]. However, those models typically rely on parameters derived from initial soil states or singular stress paths. The core innovation of the present model lies in the explicit expression of key fitting parameters as functions of both fiber content and the number of freeze-thaw cycles (Equation (5)). This parametric dependency enables the model to dynamically account for both environmental effects (freeze-thaw deterioration) and material modification (fiber reinforcement), significantly extending its predictive capacity and applicability. As such, the proposed model serves as a valuable complement to existing frameworks for compressibility evaluation.

Based on the above findings, several engineering recommendations are proposed:

• Optimization of lignin fiber content and economic considerations: The experimental results indicate that fiber contents ranging from 0.5% to 2.0% are effective, with the range of 1.0 to 1.5% identified as offering optimal performance-to-cost ratios. Below 1.0%, the reinforcement effect deteriorates rapidly with increasing numbers of freeze-thaw cycles, while above 1.5%, further improvements are marginal despite increased cost. The selection of fiber content in engineering practice should therefore be based on the anticipated severity of freeze-thaw conditions and settlement control requirements.
• Key construction procedures: To ensure optimal fiber performance, uniform dispersion must be prioritized, as shown in Figure 11. It is recommended that procedures similar to those adopted in this study be implemented: pre-sieving of fibers, dry mixing in layers with soil followed by the gradual addition of water, and sufficient resting time to ensure moisture equilibrium. Bulk mixing of fibers or direct incorporation into wet soil should be avoided to prevent fiber clumping.
• Compression model applicability: The proposed compression model can be directly embedded into numerical simulations or simplified design analyses for foundation settlement in seasonally frozen regions. By inputting the expected equivalent number of freeze-thaw cycles over the service life and the selected fiber content, the compressibility characteristics (e-lgσ_v relationship) of the modified soil can be predicted, thereby providing more reliable design parameters and overcoming the limitations of conventional methods that cannot account for long-term environmental effects.
• Long-term performance and durability: While this study focused on the effects of up to 10 freeze-thaw cycles, during which mechanical degradation tended to stabilize, actual engineering applications may involve dozens or even hundreds of cycles, as well as additional environmental stressors such as wetting–drying alternation and microbial activity. Although the lignin fiber-reinforced clay specimens exhibited stable mechanical and microstructural performance under the tested conditions, long-term durability under more extreme environmental exposures remains to be evaluated. While the aromatic structure of lignin provides some inherent resistance to biodegradation, its sustained performance in complex soil environments warrants further experimental verification to support its use in infrastructure projects with extended service lifespans.
• Engineering applicability and sustainable deployment scenarios: The use of lignin fiber-reinforced clay is not only technically effective, but is also well-aligned with current sustainability goals in geotechnical engineering. Its application is particularly advantageous in scenarios requiring minimal environmental impact, such as temporary embankments, ecological slope protection, low-volume roads, and other infrastructure within natural reserves or water conservation zones. In such contexts, lignin fiber can serve as a viable substitute for traditional chemical stabilizers, reducing both carbon emissions and the risk of secondary pollution while maintaining adequate soil performance. This highlights its potential as an eco-friendly and practical solution for sustainable infrastructure development, especially under conditions where environmental constraints limit the use of conventional treatment methods.

6. Conclusions

This study systematically evaluated the effects of lignin fiber content and freeze-thaw cycles on the compressibility of compacted clay and proposed a coupled constitutive model. The following conclusions were drawn:

(1) Incorporation of lignin fibers (up to 2.0%) significantly increased the pre-consolidation pressure and reduced the compression and recompression indices of clay. The improvement is attributed to the formation of three-dimensional fiber networks that enhance stress distribution and delay plastic deformation.

(2) Repeated freeze-thaw cycles severely degraded the structure of untreated clay, particularly after the first cycle. However, fiber-reinforced specimens exhibited substantially reduced deterioration. The most pronounced protective effect was observed at a fiber content of 2.0%, indicating the effectiveness of lignin fibers in mitigating freeze-thaw-induced compressibility loss.

(3) The SEM and MIP results demonstrated that lignin fibers inhibited the formation and expansion of strip pores and constrained pore coarsening caused by freeze-thaw action. The fibers served as internal bridges, reducing structural rearrangement and maintaining pore integrity.

(4) The compression model, derived from a modified van Genuchten equation, accurately predicted the void ratio–normal stress relationships for clay subjected to different environmental conditions by incorporating fitting functions for fiber content and freeze-thaw cycles. This model provides a practical theoretical tool for engineering applications and settlement control in fiber-reinforced clay in seasonal frost regions. It is currently applicable to fiber contents between 0.0% and 2.0% and up to 10 freeze-thaw cycles; further validation is needed before applying the model beyond these tested ranges.