# Anisotropic Shear Strength Behavior of Soil-Geogrid Interfaces
## 3 Experimental Results
### Influence of Particle Size
Figures 13 and 14 show the influence of shear direction on the shear strength of coarse sand-geogrid interfaces and gravel-geogrid interfaces, respectively. The interface shear strength under the same normal stress varies with shear directions for all the biaxial and triaxial geogrids, which indicates anisotropic shear strength behavior of coarse sand-geogrid interfaces and gravel-geogrid interfaces. As the normal stress increases, the anisotropic shear strength behavior becomes more obvious. This is the opposite of the findings regarding fine sand. Because the particle size of fine sand is small, with increasing normal stress, the friction between the geogrid and soil interface plays a dominant role, while the friction along different shear directions has little difference, resulting in less obvious anisotropy. However, for coarse sand and gravel with a larger particle size, with the increase in normal stress, the interlocking between the geogrid and soil interface plays a dominant role, while the interlocking along different shear directions is greatly affected by the contact condition of the particles, resulting in more obvious anisotropy. Figure 10: Shear stress–strain curves for coarse sand–triaxial geogrid (TX150) interfaces. (**a**) Shear direction of 0\({}^{\circ}\), (**b**) shear direction of 45\({}^{\circ}\), (**c**) shear direction of 90\({}^{\circ}\). Figure 12: Shear stress–strain curves for gravel–triaxial geogrid (TX150) interfaces. (**a**) Shear direction of 0\({}^{\circ}\), (**b**) shear direction of 45\({}^{\circ}\), (**c**) shear direction of 90\({}^{\circ}\). Figure 13: The influence of shear direction on the shear strength of coarse sand–biaxial geogrid (SS40) and coarse sand–triaxial geogrid (TX150) interfaces. (**a**) SS40, (**b**) TX150. the anisotropy behavior of the interface shear strength becomes more obvious with the increase in particle size. As shown in Table 4, the percentages of shear strength variation range from 6.01% to 9.85% for fine sand-triaxial geogrid interfaces, and from 19.44% to 30.06% for gravel-triaxial geogrid interfaces. For the shear strength of soil-triaxial geogrid (TX150) interfaces and soil-biaxial geogrid (SS40) interfaces, this anisotropy behavior has the opposite law, which may be due to the influence of the geogrid aperture shape. Existing studies have shown that geogrids with triangular apertures are more effective than rectangular apertures due to the uniform stress distribution [31]. In addition, the triangular aperture has a more stable interlocking mechanism [29]. For biaxial geogrids, the stability of the square aperture structure is low. With the increase in particle size, the interlocking effect also increases, which restricts the deformation of the aperture structure, enhances the stability of the aperture structure, and reduces the anisotropy. However, the triangular aperture of the triaxial geogrid is more stable than that of the biaxial geogrid, and the restraint effect on soil particles is stronger. With the increase in particle size and the increasing interlocking effect, the damage effect on ribs and nodes is strengthened, which reduces the stability of the mesh structure and enhances the anisotropy. In conclusion, it can still be concluded that the particle size has a great influence on the anisotropy behavior of soil-geogrid interfaces. Figure 16: The influence of particle size on the shear strength of soil-triaxial geogrid (TX150) interfaces. (**a**) Normal stress of 50 kPa, (**b**) normal stress of 100 kPa, (**c**) normal stress of 150 kPa.