西北内陆干旱区广泛分布粗粒盐渍土,盐分类型多样,工程性质复杂,有别于一般的细粒盐渍土,对工程建设危害严重。引入2种不同表面特性的纤维,结合室内纤维吸水试验、扫描电镜试验(SEM)、X射线能谱分析(EDS)和无侧限抗压强度试验等方法,对不同含盐量盐渍土改性前后的强度变化规律开展研究。试验结果表明,含盐量增大使盐渍土应力-应变关系逐渐由应变软化型向应变硬化型转变,无侧限抗压强度呈现先增大后减小的趋势,含盐量1.5%时最大,较非盐渍土提高约40.3%;不同纤维对盐渍土抗压强度的影响效果和机制与纤维的微观构造、盐分溶解度和结晶度、水盐吸附作用等密切相关。木质素纤维具有良好的吸水吸盐性,提高了盐分的溶解比例,盐分多聚集在纤维表面,对弱-中-强盐渍土的抗压强度有改善效果,最优掺量在2%左右,强度增幅12%~118%,但对超盐渍土,强度反而降低;聚丙烯纤维对盐分的吸附效应不明显,纤维在土中交织成网络,更易发挥其抗拉性能,显著增强盐渍土的强度,随纤维掺量的增加,强度增幅呈现先增后减的趋势,弱-中-强盐渍土最优纤维掺量为0.35%,强度最大增幅可达150%~213%。

[Objective] This study investigates the mechanical properties and microstructural evolution of coarse-grained sulfate saline soils in the arid inland regions of Northwest China, modified with lignin fiber (LF) and polypropylene fiber (PP). It aims to clarify the effects of fiber dispersion methods, salt dissolution behavior, and fiber-salt interactions on soil strength with different sodium sulfate (Na₂SO₄) contents, thereby providing engineering strategies to mitigate soil-related hazards in construction. [Methods] Lignin fiber (hydrophilic) and polypropylene fiber (hydrophobic) were incorporated into saline soils using dry and wet mixing methods, respectively, to ensure uniform dispersion. A series of laboratory tests were performed, including fiber water absorption measurements, scanning electron microscopy (SEM) for microstructural observation, energy dispersive X-ray spectroscopy (EDS) for elemental analysis, and unconfined compressive strength (UCS) tests on soil samples with Na₂SO₄ contents ranging from 0% to 6%. The effects of salt content, fiber type (LF or PP), dosage, and their interactions on compressive strength were evaluated through multifactor analysis of variance (ANOVA) using SPSS software. [Results] (1) Fiber characteristics: The lignin fiber, hydrophilic and flat with a ribbon-like structure, had a porous, rough surface, demonstrating a water absorption ratio of 6.70 in pure water (an 8% reduction in saline solutions). Salt crystals adhered to its surface in the form of scales or layers, leaving minimal salt in soil pores. In contrast, the polypropylene fiber, hydrophobic and smooth with a cylindrical shape, exhibited a lower water absorption ratio (4.25 in pure water, 63% of that of LF), with further reduction (7%-23%) in saline conditions. Salt crystallized within the soil pores rather than on the fiber surface. (2) Salt dissolution dynamics: At salt contents ≤3%, complete salt dissolution occurred regardless of fiber type. At 6% salt content, approximately 83% dissolved, while the remaining 17% formed needle-like crystalline clusters that reinforced the soil skeleton. Lignin fiber increased the optimal water content in high-salt soils, promoting nearly complete salt dissolution. Polypropylene fiber had no significant effect on salt dissolution. (3) Strength behavior: Salt content primarily influenced UCS trends. Strength peaked at 1.5% salt (a 40.3% increase compared to untreated soil), then declined sharply, with a 37% reduction at 6% salt. Stress-strain curves shifted from strain-softening to strain-hardening behavior as salt content increased. Polypropylene fiber consistently enhanced UCS and residual strength. Optimal dosages were 0.35% for low-to-moderate salt soils (<3%, with strength increases of 150%-213%) and 0.45% for hypersaline soil (6%). Its smooth structure allowed salts to remain dissolved, while the fiber network facilitated load redistribution. Lignin fiber improved strength in low-to-moderate salt soils (12%-118% increase at 2%-4% dosage, optimal at 2%) but weakened high-salt soils (4%-17% decrease at 6%) due to salt aggregation on its rough surface. (4) Statistical significance (ANOVA): For LF-modified soils, salt content had the most significant effect on strength, followed by the interaction between salt content and fiber dosage. In PP-modified soils, salt content and fiber dosage were significant independent factors, with minimal interaction effects. [Conclusion] (1) Fiber performance hinges on microstructure and hydrophilicity. Polypropylene fiber, with its hydrophobic nature and load-distributing network, strengthens soil across all salt levels but requires higher dosages in hypersaline conditions. Lignin fiber enhances the strength of low-to-moderate salt soils by promoting salt dissolution but destabilizes high-salt soils due to surface-induced salt crystallization. (2) Salt dissolution and crystallization play a critical role in the mechanical properties. At high salt contents, undissolved crystals become part of the soil structure, while the fiber type influences both salt solubility and distribution. (3) Engineering strategies must align fiber selection with salt content. Polypropylene fiber is recommended for hypersaline soil, while lignin fiber proves more effective under moderately saline conditions. This study provides practical guidelines for managing sulfate saline soils in arid regions, emphasizing microstructure-driven design approaches.