## 4 Discussion
### Analysis of Interface Evolution  
The deterioration of red-bed soft rocks under the influence of water is characterized by a number of processes, including softening, occlusion, scouring, adsorption and lubrication. Additionally, the cementing capacity of the cementing material between the particles at the interfaces is weakened, and stretching, compression, torsion and translation occur between the rock particle skeletons (Schoner et al. [27]). The aforementioned physical effects result in the expansion of the cemented material and subsequent dissolution in water. This is evidenced by the emergence of pore spaces within the soft rock that dilate and connect to form fractures. Over time, the fractures expand and connect with each other, allowing the particles to detach from their parent bodies (Parcerisa et al. [28]). This section presents an analysis of point A1 in the SEM test. As illustrated in Figure 9a, the microstructure of the specimens subjected to different water saturation times reveals that the particle boundaries underwent significant deterioration and dissolution after one week of saturation. The particles that were originally tightly cemented by the cementing material spread out due to the deterioration and dissolution of the cementing material, while the loosely cemented part was gradually detached from the parent rock and washed away by the aqueous solution with the increase in the water saturation time, as shown in Figure 9b. After one month of water saturation, the deterioration of the cemented \begin{table}
\begin{tabular}{c c c} \hline \hline
**Immersion Time** & **Fractal Coefficient** & **3D Fractal Dimension** \\ \hline
0 Day & 2.30 & 1.34 \\
7 Days & 1.33 & 2.25 \\
30 Days & 1.58 & 1.89 \\
90 Days & 1.83 & 1.64 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Results of 3D fracture analysis for each zone of red-bed soft rock with different water saturation times. Figure 8: Fracture evolution diagram. material described above continued, resulting in the shedding of more debris particles. Consequently, new pores emerged and the fractures expanded, as illustrated in Figure 9c,d. The phenomenon was observed in all samples during the test. It can be observed that the microscopic interfacial softening mechanism of red-bed soft rocks is the dissolution and destruction of the internal cemented interface. As the water saturation time increases, the cementation process breaks down, resulting in the detachment of particles from the parent rock and their suspension in the water. The high clay mineral content of the red-bed soft rocks facilitates the ingress of water into the particle interstitials, thereby accelerating the softening process depicted in Figure 10a-d.  
Figure 10: Schematic diagram of particle interface disruption and mechanical effects at the particle interface ((**a–d**) particle interface disruption; (**e**) hydrodynamic pressure; (**f**) hydrostatic pressure). Figure 9: Softening and dissolution of granular cementing material. The stretching, compression, torsion and translation between the rock particle skeletons are mainly manifested as changes in the mechanical effects between particles. The mechanical effects between particles are mostly due to the water pressure, because they act with water, followed by intermolecular forces. The water pressure is divided into hydrodynamic pressure and hydrostatic pressure, both of which play a major role. Hydrodynamic pressure is generated when water flows through a rock mass that is not saturated. This generates osmotic pressure (tangential thrust) on large particles in the vicinity of the channel in the same direction as the flow direction. Additionally, tiny particles with diameters smaller than the flow channel move with the flow of water and even flow out of the interior of the skeleton, exacerbating the damage and deterioration of the soil particles, as demonstrated by Figure 10e.