## 4 Microscopic Structural Features and Mechanistic Analysis
### Microscopic Structural Mechanism Analysis  
Mechanism analysis of xanthan gum curing of gravelly soils: (1) Bonding and cementation mechanism: Xanthan gum, mainly composed of D-glucuronic acid, contains a large number of carboxyl groups (-COOH) and hydroxyl groups (-OH), which form a stable network space structure with the minerals in the gravel soil through electrostatic bonding  
Figure 10: Microstructure of xanthan gum and lignin-cured gravelly soils.  
synthesis or hydrogen bonding, resulting in strong tensile properties, which improve the bonding force of the geotechnical particles and the cohesion and strength of the gravel soil. In addition, the carboxyl group (-COOH) adsorbs cations such as Na\({}^{+}\), K\({}^{+}\), and Al\({}^{3+}\) in the gravel soil and forms ionic bonds with the anions in the minerals in the gravel soil, changing the electrically charged nature of their surfaces, which leads to complex aggregation of geotechnical particles and the formation of agglomerates, and strengthens the integrity of the soil body. Through SEM, it is found that xanthan gum has a strong adhesive force when it meets water, and it can bridge the particles in the soil body [40]. (2) Filling and film formation mechanisms: When in a hydrated state, xanthan gum forms a gel-like substance that fills the micropores between soil and rock particles, thereby enhancing the internal pore structure of the soil. This action serves to reduce stress concentration at the tips of microcracks and diminish the tensile stress field between soil particles. Furthermore, xanthan gum adsorbs onto the surfaces of soil and rock particles, thereby creating a polymeric film that isolates moisture from direct contact with the minerals and reduces the thickness of the hydration film [41]. A schematic diagram illustrating the mechanism of xanthan gum in stabilizing gravelly soil is provided in Figure 11.  
The mechanism of lignin-cured gravelly soil: (1) Ion exchange: Lignin dissolves in water, releasing more Ca\({}^{2+}\); the high valence Ca\({}^{2+}\) replaces the low valence Na\({}^{+}\) in the binding water film, and the cations adsorbed by the soil particles undergo an ion exchange reaction, weakening the thickness of the binding water layer adsorbed on the surface of the particles, leading to an increase in the inter-particle force, which improves the particles' cohesion [42]. (2) Adsorption and encapsulation effect: Lignin mixed into the gravel soil will undergo a protonation reaction to form positively charged lignin sulfate compounds, which will be adsorbed on the surface of the negatively charged soil, wrapping and covering the geotechnical particles to form agglomerates, forming a denser and more stable structure of the soil, and thus improving the compaction and stability of the gravel soil when bearing loads [43]. (3) Pore filling and cementation effects: The incorporation of lignin into gravelly soil results in the filling of larger voids between soil particles, thereby enhancing the soil's overall compaction. The cementing agents, formed by the calcium bridge of lignin, connect the soil particles. When the concentration of lignin in the soil is excessive, the lack of sufficient soil particles as a framework causes the surplus lignin to accumulate, resulting in localized softening of the soil and reduced compaction. This, in turn, results in a reduction in the soil's shear strength. A schematic representation of the lignin stabilization mechanism in gravelly soil is provided in Figure 12.  
Figure 11: Xanthan gum-cured gravel soil mechanism diagrams.