###### Abstract  
Expansive soil exhibits significant swellings and shrinkages, which may result in severe damage or the collapse of structures built upon it. Calcium-based admixtures, such as lime, are commonly used to improve this problematic soil. However, traditional chemical additions can increase significant environmental stress. This paper proposes a sustainable solution, namely, the use of lignin fiber (LF) from the paper industry to partially replace lime as an amendment for expansive soils. Both the macroscopic and microscopic characteristics of the lignin fiber-treated expansive soil are extensively studied. The results show that the mechanical properties of expansive soil are improved by using lignin fiber alone. Under the condition of an optimal dosage of 8%, the compressive strength of lignin fiber-modified soil can reach 193 kPa, the shear strength is increased by 40% compared with the untreated soil, and the water conductivity is also improved with the increase in dosage. In addition, compared with 2% lime-modified soil, the compressive strength of 8% lignin fiber- and 2% lime composite-treated expansive soil increased by 50%, the cohesion increased by 12%, and the water conductivity decreased significantly. The microstructure analysis shows that at an 8% lignin fiber content, lignin fibers interweave into a network in the soil, which effectively enhances the strength and stability of the improved soil. Simultaneously, the fibers can form bridges across the adjacent micropores, leading to the merging of pores and transforming fine, dispersed micropores into larger, connected macropores. Line promotes the flocculation of soil particles, forming larger aggregates and thus resulting in larger pores. The addition of fibers exerts an inhibitory effect on the flocculation reaction in the composite-improved soil. In conclusion, lignin fibers are an effective addition used to partially replace calcium admixture for the treatment of expansive soil, which provides a sustainable and environmentally friendly treatment scheme for reducing industrial waste. chemical improvement. The physical improvement method usually includes the addition of fibers [5; 6; 7], geotextiles [8], etc., to the soil or changes to the particle gradation by adding coarse particles such as steel slag [9], sand [10; 11], rubber particles [12], gravel [13], and construction waste [14], thereby enhancing the physical and strength characteristics of the soil. In addition to these materials, bentonite has been successfully used to improve soil compactness, reduce permeability, and enhance mechanical stability [15]. At present, the chemical modification method mainly involves adding lime [16; 17], cement [18], or chemical slurry [19] to the soil and aims to achieve soil improvement by adjusting the mineral composition or structure inside the soil. However, the production and use of alkaline chemical materials such as lime and cement will produce a large amount of carbon dioxide [20]. Nowadays, more and more attention has been paid to green and economically improved materials in the field of soil treatment. Lignin fiber, as an economical byproduct of the paper industry with a wide range of sources, has a high level of toughness and is insoluble in water. It is non-toxic, tasteless, and environmentally friendly [21]. In particular, lignin fibers have been treated at high temperatures and chemically modified to provide excellent resistance to degradation. This resistance allows lignin fibers to maintain their structural stability in alkaline or other harsh environments, which is not the case with many other natural fibers. Currently, lignin fiber has been relatively widely used in improving the performance of asphalt pavement [22; 23; 24], but its application in the geotechnical field is still rare. [PERSON] et al. [25] treated soil with lignin fibers and found that the shear strength of the improved soil was significantly improved by longer fibers. [PERSON] et al.