# The Shear Strength of Granite Weather Soil Under Different Hydraulic Paths
## 4 Discussion
### Effect of Section on Shear Strength and Model Establishment  
At present, the mechanism of matric suction on strength is mainly aimed at capillary pressure caused by surface tension. According to the microcosmic study of soil particle contact model above, the matric suction not only includes the suction generated by the capillary phenomenon, but also includes capillary and adsorptive suction. Other scholars also hold unanimous views [31; 32]. The adsorptive part is a complex interaction between soil and water, including short-range van der Waals forces, long-range electrostatic forces, and other hydration, namely physico-chemical interactions [16; 33]. Cement contact caused by cementation substances such as free oxides, soluble salts, and organic matter is a typical part of adsorptive suction. At high saturation, the adsorptive part also occupies a large proportion; at low saturation, the capillary effect is no longer effective, and the dominant is the clay mineral and the soil-water interaction that adsorbs water [25; 31]. In the existing unsaturated strength equation [14; 26], the effect of capillary and suction on strength is confused, and the mechanism of capillary action is relatively clear, and the mathematical treatment is relatively simple. Therefore, in the quantitative research of unsaturated soil mechanics, matric suction is often equated with the capillary part and the adsorptive part is ignored. The constitutive models established for unsaturated soils are mostly based on the capillary mechanism, so these models are not suitable for high plastic clays, soils containing cementation, or low water content. According to the previous micro research and the existing problems in the current model, this paper divides the shear strength of soil into capillary and adsorptive parts as shown in Equation (3). [PERSON] and [PERSON] [34] used the capillary model to clarify the relationship between the capillary strength generated by the capillary water and the matric suction. A capillary model of a typical silty soil was obtained, and the capillary strength was first increased and then decreased with the matric  
Figure 8: Scanning electron microscope (SEM) of granite weathered soils: (**a**) Preparation sample of fully weathered soil; (**b**) Dehydrated sample of fully weathered soil; (**c**) Preparation sample of residual soil; and (**d**) Dehydrated sample of residual soil. suction. Referring to the ideal capillary model of silt, the strength is shown in Equation (4) when only the capillary effect is considered, and the adsorptive effect is ignored. According to Equation (4), the relationship between soil strength and matric suction is shown in Figure 9. The shear strength of soil increases first and then decreases with the increase of matric suction  
\[\tau_{f}=\tau_{f}^{cap}+\tau_{f}^{ads} \tag{3}\]  
\[\tau_{f}^{cap}=c\
u+\left[\overline{\sigma}_{n}+s\left(\frac{S_{r}-S_{r,r}}{1- S_{r,r}}\right)^{\frac{0.55}{4}}\right]\tan\varphi\
u \tag{4}\]  
where \(\tau_{f}\) is the total shear strength, \(\tau_{f}^{cap}\) is the shear strength of the capillary effect, \(\tau_{f}^{ads}\) is the shear strength of the adsorptive effect, \(\overline{\sigma}_{n}\) is net stress, \(S_{r,r}\) is residual saturation, \(S_{r}\) is total saturation, and \(\lambda\) is the parameter fitted according to the experimental data. For special soils, such as granite weathered soil containing cementation, the contact form between the soil particles under the dehumidification path includes not only meniscus contact, but also cement contact. Equation (4) is for soils dominated by capillary models such as low plastic silty soil or sandy soil, so it is necessary to introduce the influence of adsorptive suction. [PERSON] et al. [35] believed that the adsorptive part is mainly a variable structure suction that changes with saturation. The variable structure suction includes the cementation between the soil particles, the electric double layer suction, the bite force, and the physical-chemical force and other forces.