## 1 Introduction  
Granite Residual Soil (GRS) is a product of the long-term weathering of granite, and it is widely distributed in the southern regions of China, particularly along the southeastern coastal areas. GRS is primarily influenced by subtropical and tropical climatic conditions, as well as the properties of the parent rock, resulting in complex pore and fracture structures. During weathering, a variety of oxides, such as iron oxide and aluminum oxide, are generated, with their content varying according to the degree of weathering the soil has undergone [1]. This gives the soil a brown-red, brown-yellow, and off-white surface character. These surface traits provide an intuitive basis for assessing the degree of weathering in GRS. GRS exhibits a loose structure, high permeability, and is easily softened and disintegrated when in contact with water, and it shows remarkable anisotropy [2]. These properties make GRS highly susceptible to external factors, such as rainfall, which can trigger landslides [3]. Therefore, GRS is used in engineering construction, and it needs to be strengthened. Soil solidification is a technique that enhances the bearing capacity and shear strength of soil by incorporating hardening agents or curing materials. In engineering applications, chemical curing agents such as lime and cement are commonly used to improve the properties of GRS. These agents work by initiating physical and chemical reactions, significantly increasing the soil's bearing capacity and disintegration resistance [4]. However, disintegration resistance like lime and cement release substantial amounts of greenhouse gases during both production and application, resulting in direct or indirect negative environmental impacts. Microbial-Induced Calcite Precipitation (MICP) technology can also reinforce GRS [5], improving not only the shear strength but also reducing its permeability, making the soil more stable against external loads and water erosion, and improve its anti-disintegration ability [6; 7]. However, the microbial reaction process of MICP technology is more sensitive to environmental conditions and the cost is relatively high. Biopolymers are high-molecular compounds derived from living organisms or their metabolic products. These polymers are typically produced through biosynthesis processes and include proteins, polysaccharides, nucleic acids, and other substances. As a renewable material [8], biopolymers can be recycled through natural biodegradation or artificial treatment, aligning with the concept of sustainable development [9; 10]. When mixed with soil, biopolymers form composite materials that show great potential for use in geotechnical and construction engineering, positioning them as a promising and environmentally friendly option for ground improvement [11]. Biopolymers commonly used for soil solidification include xanthan gum (XG), sodium alginate, carrageenan, locust bean gum (LBG), agar, gellan gum [12], thermogels [13], dextran [14], and guar gum (GG) [15]. Among these, XG and GG are particularly notable for their widespread use in soil solidification [16; 17]. Research has demonstrated that these biopolymers exhibit excellent soil solidification properties and can be effectively applied to improve and reinforce soils. The inclusion of biopolymers in soil has several advantages: they enhance soil structure, reduce erosion, and increase the soil's load-bearing capacity [18]. Biopolymers combine with water in soil to form hydrogels, which have good hydrophilicity and degradability [19; 20]. The hydrogel film has a good water retention ability [21], thereby increasing the water retention performance of the soil and helping to alleviate the problem of soil drought. Furthermore, biopolymers contribute nutrients to the soil, fostering the growth and activity of soil microorganisms and plants [22], thus improving soil fertility and health. As a result, the appropriate application of biopolymers can positively impact soil texture, moisture content, nutrient levels, and other key properties.