## 1 Introduction  
Expansive soils, notably characterized by their high contents of montmorillonite and illite minerals, exhibit pronounced behaviors of swelling, shrinkage, and overconsolidation [1]. These properties can lead to significant engineering challenges, including uneven settlement, slope instability, and pavement having [2]. These detrimental effects have led to the metaphorical characterization of expansive soils as the \"cancer of highways\" [3]. The extensive distribution of these soils, coupled with the often concealed and long-term nature of the associated issues, results in global annual repair costs amounting to billions of dollars [4]. Consequently, the development of effective soil improvement techniques is essential for the rational and sustainable utilization of expansive soils. To address the challenges posed by expansive soils, researchers have extensively investigated a variety of physical and chemical stabilization techniques [5]. Physical stabilization methods typically involve the incorporation of non-expansive fillers, such as fine sand, crushed stone, or fibers, to reduce the montmorillonite and illite mineral contents, thereby suppressing the swelling and shrinkage behavior while simultaneously improving mechanical properties [6; 7]. For instance, [PERSON] et al. [8] demonstrated the effectiveness of basalt fiber in enhancing the stability of expansive soils. Their findings indicated that the addition of 4% basalt fiber significantly mitigated swelling and shrinkage while also enhancing the mechanical properties of the soil. Similarly, [PERSON] et al. [9] reported that the inclusion of rock powder, fine sand, and gravel improved both the stability and mechanical performance of expansive soils. However, it is important to note that the effectiveness of physical stabilization is somewhat limited and is primarily suitable for soils with low expansiveness. In contrast, chemical stabilization approaches focus on modifying the internal particle structure of expansive soils or inducing gel-like precipitations to enhance their swelling and shrinkage characteristics [10; 11]. The previous studies [12; 13] have shown that the addition of lime and cement positively influences the engineering properties and swelling-shrinkage behavior of expansive soils. For instance, [PERSON] et al. [14] effectively improved the swelling-shrinkage properties and enhanced the compressive and shear strength of expansive soils through various mixtures, including 6% lime with 12% zeolite, 1% cement with 10% fly ash, and sawdust ash. Furthermore, [PERSON] et al. [15] identified an optimal improvement in expansive soils with the incorporation of 0.6% cationic polyacrylamide. While these chemical methods can effectively reduce swelling and shrinkage, they often raise concerns regarding energy consumption and environmental pollution [16]. In recent years, microbially induced calcium carbonate precipitation (MICP) has emerged as a promising and eco-friendly technique for soil improvement [17]. MICP leverages the interaction between specific microorganisms and expansive soils to facilitate the formation of calcium carbonate precipitates, which subsequently alters the internal structure and mineral composition of the soil [18]. Numerous studies have demonstrated the effectiveness of MICP in enhancing various properties of expansive soils, including material properties, swelling rates, and mechanical characteristics [19; 20]. For instance, [PERSON] et al. [21] utilized MICP technology to treat the natural soil, resulting in significant improvements in key properties such as Afterberg limits and linear shrinkage, with optimal performance being achieved at a specific bacteria-to-cementation reagent ratio. Similarly, [PERSON] et al. [22] reported that MICP treatment led to substantial increases in both the compressive and shear strength of expansive soils. These findings underscore the potential of MICP as a viable solution for mitigating the challenges posed by expansive soil behavior [23].