# The Shear Strength of Granite Weather Soil Under Different Hydraulic Paths
## 4 Discussion  
### Effect of Particle Size Gradation and Mineral Composition on Shear Strength  
According to the interaction of soil particles in different mineral compositions and water holding states, it is assumed that there are three different forms of contact between particles in the soil as shown in Figure 7: Figure 7 displays that when the saturation is high, the pores are filled with water, and a small amount of meniscus mainly provides the shear strength. When the saturation is low, as shown in Figure 7, there are three modes of inter-particle contact: (1.) The soil particles are in direct contact with the adsorbed water film, due to the relative largely density, the adsorbed water cannot flow, which can be regarded as a solid or semi-solid [28]. (2.) There is a meniscus contact by the capillary between the soil particles. (3.) The cement contact in which solidified cementation materials bind soil particles tightly together. (1)When there is no cementation substance and the saturation is low, the contact types between the soil particles are mainly direct contact and meniscus contact. The content of the capillary  
Figure 6: Relationship between suction and strength of weathered granite soil: (**a**) fully weathered soil; (**b**) residual soil. Figure 7: Soil particle contact model: (**a**) Saturated state; (**b**) Unsaturated state. meniscus increases first and then decreases as the saturation increases. The optimal saturation occurs when there is the largest soil-water contact area in the soil, with the most meniscus contact, thereby increasing the shear strength of the soil. On the other hand, the pore size and distribution in the soil body will affect the characteristics of the meniscus, while the particle size of the soil particles and the mineral composition affect the pore size and distribution of the soil [23]. According to the Young-Laplace equation, small pores have a more substantial capillary effect than large pores [29]. Therefore, the particle size gradation and the mineral composition in the soil control the contact form and relative content of the soil particles, as well as their intensity of action, which in turn affects the law of shear strength. 2. When there are cementation materials in the pore fluid, the cement contact forms between soil particles in addition to direct contact and meniscus contact. Part of the iron oxide in the soil will be distributed on the whole or part of the surface of the clay particles in the form of \"envelope\", and it will be cemented together in the form of \"bridge\" to increase the strength of the soil, and the other part will exist as free iron oxide. The key to the \"envelope\" formation lies in the saturation of the soil and the content of free iron oxide. With low saturation, the solid particles of iron oxide in the soil cannot be dissolved in the pore fluid to form a soil; with high saturation, the water film between the soil particles is so thick that it cannot create cementation but exists in a free state. The free iron oxide will gradually develop a colloidal \"envelope\" as the saturation decreases from the wet state to the dry state. It is known from Table 1 that the more severely weathered residual soil contains higher residual Fe\({}_{2}\)O\({}_{3}\), Al\({}_{2}\)O\({}_{3}\), and organic matter than the fully weathered soil (other oxides cannot be dissolved to form free oxides), which makes the residual soil contain more cementation. On the other hand, the larger specific surface area and cation exchange capacity of the residual soil can make the soil particles adsorb more ions and increase the soluble salt in the pore fluid, which can produce the same cementation effect as free iron oxide [30]. The cementation makes the shear strength of the residual soil higher than that of the fully weathered soil and makes the shear strength of the dehydrated specimen higher than that of the prepared sample.