## 3 Reinforcement Mechanism of Soil Nail Support  
The soil nail support mechanism is dependent on soil nail inclination, stiffness, soil nail length, spacing, nail-soil interface interaction, soil parameters, and other factors. Accordingly, the aim of this research is to study their roles in said mechanism. Therefore, this study focuses on analyzing the reinforcement mechanism of soil nail support. The shear strength of a soil body is low, and its tensile strength is almost negligible. In order to provide the soil with a certain tensile capacity, a specific number and density of reinforcing elements must be placed within the soil. The synergistic effect between the soil and reinforced body not only accounts for the soil body's tensile and shear strength to improve the overall stiffness of the soil body, but also allows the soil to utilize its structural strength potential to the maximum, altering the deformation and failure characteristics of slopes. The reinforcement theory of soil nail support is based on the interaction at the nail-soil interface, where the tensile capacity of the soil nail is transferred to the soil body, so that the soil body's strength is reinforced. Engineering practices and research show that soil nail support can significantly improve the strength and stability of a soil body. In Figure 3 ([PERSON] et al. [34]), it is evident that the damage strength of the soil nail support is significantly improved, with the two lines being essentially parallel. This indicates that the soil nail support does not significantly change the angle of internal friction. However, the damage envelope of the soil body does not pass through the origin, indicating quasi-viscous cohesion. It can be seen that the lateral deformation of the soil body is restrained owing to the tensile bonding between the nail and soil, thus improving the strength and stability of the soil body. Figure 2: Four forms of rupture surfaces assumed in engineering. Combined with Figure 3, it follows from the M-C theory that for unreinforced soil,  
\[\sigma_{1}=\sigma_{3}\tan^{2}(45^{\circ}+\varphi/2) \tag{1}\]  
and for reinforced soil,  
\[\sigma_{1}=\sigma_{3}\tan^{2}(45^{\circ}+\varphi/2)+2c\tan(45^{\circ}+\varphi/2) \tag{2}\]  
where \(\sigma_{1}\) and \(\sigma_{3}\) are the maximum and minimum principal stresses, respectively, \(c\) is the cohesive force, and \(\varphi\) is the angle of internal friction. When Equation (2) is deformed, we obtain  
\[\sigma_{1}=(\sigma_{3}+2c/\tan(45^{\circ}+\varphi/2))\tan^{2}(45^{\circ}+ \varphi/2) \tag{3}\]  
such that \(\Delta\sigma_{3}=2c/\tan(45^{\circ}+\varphi/2)\). Accordingly, \(c=\Delta\sigma_{3}\tan(45^{\circ}+\varphi/2)/2\); \(c=0\) in the unreinforced soil itself. Using \(c=0\) in Equation (3), \(c=\Delta\sigma_{3}\tan(45^{\circ}+\varphi/2)/2\) can be considered \(\Delta c\). At this point, \(c=\Delta\sigma_{3}\tan(45^{\circ}+\varphi/2)/2\) can be transformed into  
\[\Delta c=\Delta\sigma_{3}\tan(45^{\circ}+\varphi/2)/2 \tag{4}\]  
Equations (2)-(4) indicate that the strength of the soil body increases significantly when the reinforced material is added to the soil body. The increment is shared by \(\Delta c\) or \(\Delta\sigma_{3}\), as shown in the reinforced soil unit in Figure 4, where \(T\) is the rupture strength of the reinforced soil, \(s_{v}\) is the vertical spacing of the reinforced soil, \(\varphi\) is the friction angle within the soil, and \(L\) is the width of the wedge. This is known from the triangular equilibrium condition for forces:  
\[\tan(45^{\circ}+\varphi/2)=\sigma_{1}L/[\sigma_{3}\tan(45^{\circ}+\varphi/2)+ TL\tan(45^{\circ}+\varphi/2)/s_{v}] \tag{5}\]  
Figure 4: Soil reinforcement unit. Figure 3: Experimental effect of soil reinforcement.