## 4 Discussion on the Mechanism of Damage
### Mechanism of Microscale Damage  
The capacity of sandy red clay to exhibit expansion and contraction is fundamentally attributed to its distinctive mineralogical composition, characterized by a substantial abundance of clay minerals, serving as a pivotal determinant in the occurrence of expansion and contraction phenomena [35]. Water serves as a crucial external stimulus that induces expansion and contraction in the soil. Put simply, if the water content of the soil specimen remains constant, no expansion or contraction will occur. During the wetting phase, water molecules infiltrate the interior of the soil sample through existing pores and fissures under the influence of osmotic pressure. The hydrophilicity of the clay mineral particle surfaces, coupled with the polar structural features of water molecules, facilitates the adsorption  
Figure 8: (**a**) Stress–strain curves of sandy red clay; (**b**) relationship between shearing strength and \(N\); (**c**) relationship between shear strength index and \(N\).  
of water molecules around the clay minerals under the influence of electrostatic forces, leading to the expansion of the particles. Simultaneously, the bilayer repulsive force that arises between the mineral particles augments the inter-particle spacing. Consequently, this not only results in an increase in the original pore size but may also generate new pores, ultimately leading to enhanced porosity and soil volume expansion (Figure 9(a-2)) [36]. During the drying phase, water molecules within the pores migrate to the atmosphere via the soil surface, leading to evaporation. As evaporation persists, the water film surrounding the mineral particles diminishes, forming a curved liquid surface between the particles and generating capillary water pressure. Under the synergistic effect of capillary water pressure and surface tension, the mineral particles are drawn closer together, resulting in a reduction in pore aperture, porosity, and soil volume contraction (Figure 9(a-3)) [37].  
Although the specimen undergoes expansion during the wetting phase and contractions during the drying phase, the reversibility of these dimensional changes during wet-dry cycles is incomplete, attributable to irreversible alterations within the microstructure. The directionality of mineral particle expansion and contraction exhibits disorderliness [38], with variability in the magnitude of these changes among diverse mineral constituents. Clay minerals, predominantly composed of illite, exhibit heightened sensitivity to variations in water content, with particularly pronounced expansion and contraction effects observed during wet-dry cycling [39]. These effects induce alterations in particle structure and arrangement, as well as modifications in the stress regimes of particles and pore structures. Consequently, the microstructure is unable to revert to its initial state. Simultaneously,  
Figure 9: Degradation mechanism: (**a-1, a-2, a-3**) microscale; (**b-1, b-2, b-3**) mesoscale.  
water migration during dry-wet cycling results in the rounding, dissolution, destabilization, or detachment of certain minerals, thereby augmenting the size and number of pores within the specimen (Figure 9(a-2)). Furthermore, the interconnection and coalescence of these pores ultimately culminate in the formation of microscopic cracks (Figure 9(a-3)). Hence, the cumulative phase of microstructural deterioration in the soil corresponds to the initiating stage of mesoscopic cracking.