## 4 Discussion
### Microscopic Mechanism of Biopolymers  
As shown in Figure 21, when biopolymers combine with water, they form a gel-like substance that enhances the bonding between soil particles. This colloidal effect contributes to the formation of a more stable soil structure and improves the soil's aggregate composition. The biopolymers can form a network structure upon interacting with water, which further enhances the soil's water retention capacity. XG, LBG, and GG all contain hydrophilic carboxyl or hydroxyl groups, as well as hydrophobic sugar chain components. These structures can undergo physical cross-linking with the surface of soil particles, improving the soil's water and gas properties. All three biopolymers are rich in hydroxyl (-OH) functional groups, which form hydrogen bonds with water molecules, facilitating interactions with soil moisture. As shown in Figures 2-4, GG has the highest hydroxyl content, followed by LBG, with XG having the lowest. The strength of the soil treated with biopolymers correlates with their hydroxyl content. GG-treated soil exhibits the highest strength, followed by LBG, and XG shows the weakest strength. A higher hydroxyl content in a biopolymer leads to greater strength in treating the soil. The hydrophobic portion of the biopolymer interacts with hydrophobic minerals in the soil, enhancing the soil's hydrophobicity and reducing moisture loss. Moreover, the combination of hydrophilic and hydrophobic sections in the biopolymer forms water-soil-biopolymer complexes in the soil. On the surface of soil particles, the biopolymer adsorbs moisture through its hydrophilic groups, while its hydrophobic segments interact with the soil particles. XG, LBG, and GG all contain functional groups such as C-OH and C-O, which interact with the soil. In GRS containing iron minerals, iron oxide reacts with these functional groups through acid-base interactions, leading to the formation of covalent bonds, such as C-O-Fe bonds [40]. Biopolymers interact with iron oxide particles through both covalent and non-covalent electrostatic interactions, which enhances the macro-stability and coagulation effect of the soil [41]. Additionally, biopolymers typically contain charged groups, which impart a certain charge density in aqueous solutions. These charged groups can attract soil particles via electrostatic forces, adsorbing onto the surface of the particles. This interaction forms charge pairs or complexes, stabilizing the connections between soil particles and improving the overall soil structure. Through physical adsorption, biopolymers bind with soil particles, creating a stable cross-linked structure. This cross-linking effect increases the cohesion between soil particles, leading to a more compact soil structure. The physical cross-linking also helps fill the tiny pores in the soil, further optimizing the soil's overall structure. Biopolymers interact with the mineral components in granite residual soils at the microscopic scale, forming more stable compounds.  
Figure 21: Interaction between biopolymers and GRS.  
As shown in Figure 22 are the changes in moisture caused by water loss during the curing process are another key factor influencing the effectiveness of biopolymer treatment. In GRS, pore water is evenly distributed in the initial state. As the soil moisture gradually decreases, the adhesive force between soil particles is mainly provided by iron oxide. For soil treated with biopolymers, a thin film of biopolymer initially forms on the surface of the soil particles. As moisture decreases, the biopolymer enhances its bonding effect. The adhesive force between soil particles gradually transitions, with the combination of iron oxide, biopolymer gel, and iron oxide-biopolymer complexes providing the primary bonding force.