# Anisotropic Shear Strength Behavior of Soil-Geogrid Interfaces
## 3 Experimental Results
### Influence of Shear Direction  
Figures 7 and 8 show the influence of shear direction on the shear strength of fine sand-biaxial geogrid interfaces and fine sand-triaxial 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 fine sand-geogrid interfaces. Most of the curves show the lowest shear strength under the shear direction of 45\({}^{\circ}\), especially for fine sand-biaxial geogrid interfaces. However, there is not a consistent trend for the shear strength of fine sand-geogrid interfaces with varying shear direction for biaxial and triaxial geogrids. The interlocking effect of fine sand-biaxial geogrid interfaces is weak, which is mainly dominated by the friction effect, and the friction effect is related to the tensile strength of the geogrid, so the shear strength of soil-geogrid interfaces is greatly affected by the tensile strength of the geogrid. As the tensile strength of the geogrid increases, the difference in friction in different shear directions increases, which enhances the anisotropy. Therefore, the SS40 samples have the greatest influence on shear direction. Figure 7: The influence of shear direction on the shear strength of fine sand-biaxial geogrid interfaces. (**a**) SS20, (**b**) SS30, (**c**) SS40. Although the three biaxial geogrids have different tensile strengths (e.g., 26.29 kN/m for SS20, 33.97 kN/m for SS30, and 48.26 kN/m for SS40 under shear direction of 0\({}^{\circ}\)), as indicated in Table 1, the interface shear strengths between the soil and the three biaxial geogrids are not much different; for example, the shear strength values are approximately 50 kPa for the three soil-biaxial geogrid interfaces under the normal stress of 50 kPa. This confirms that the soil-geogrid interface shear strength mainly depends on the shear strength of the soil itself, rather than the tensile strength of the geogrid. A similar trend is also observed in triaxial geogrids, as shown in Figure 8. In addition, the shear strength values for soil-triaxial geogrid interfaces are also close to those for soil-biaxial geogrid interfaces under the same normal stress. This indicates that the shear strength of soil-geogrid interfaces is not significantly affected by the type of geogrid. In this study, the anisotropy of soil-geogrid interfaces is evaluated in terms of the percentage of shear strength variation \(A_{s}\), which is defined as the ratio of the difference between the maximum and minimum shear strength to the minimum shear strength under the same normal stress, and is expressed as follows:  
\[A_{s}=\frac{\tau_{\text{max}}-\tau_{\text{min}}}{\tau_{\text{min}}} \tag{1}\]  
Figure 8: The influence of shear direction on the shear strength of fine sand–triaxial geogrid interfaces. (**a**) TX150, (**b**) TX160, (**c**) TX170. where \(\tau_{\text{max}}\) and \(\tau_{\text{min}}\) are the maximum and minimum shear strength values of a soil-geogrid interface for different shear directions under the same normal stress. The higher the percentage of shear strength variation, the stronger the anisotropy of the interface. In Table 2, the percentages of shear strength variation range from 0.68% to 59.29% for soil-biaxial geogrid interfaces, and from 6.01% to 37.99% for soil-triaxial geogrid interfaces. The biaxial geogrid SS40 has the largest percentage of shear strength variation, \(A_{s}\) = 59.59%, under the normal stress of 50 kPa for soil-biaxial geogrid interfaces, and the triaxial geogrid TX160 has the largest value of \(A_{s}\) = 37.99% under the normal stress of 50 kPa. These large \(A_{s}\) values indicate strong anisotropic shear strength behavior of soil-geogrid interfaces. The anisotropy of soil-geogrid interfaces is ignored in the current geosynthetic testing standard, which could be important for the safe design of reinforced soil structures.