# Anisotropic Shear Strength Behavior of Soil-Geogrid Interfaces
## 3 Experimental Results
### Influence of Particle Size  
The results for coarse sand and gravel are presented to evaluate the influence of particle size on the anisotropic shear strength behavior of soil-geogrid interfaces. For the coarse sand, taking the biaxial geogrid SS40 and the triaxial geogrid TX150 as examples, the interface shear test results of coarse sand in different shear directions are shown in Figures 9 and 10, respectively. The shear stress-shear strain curves of the coarse sand-biaxial geogrid (SS40) interface and the coarse sand-triaxial geogrid (TX150) interface both show that the shear stress remains stable, or has slight strain softening, after reaching the peak values, and that the strain softening behavior becomes more obvious with the increase in normal stress. However, the strain softening behavior of the shear stress-shear strain curves of the coarse sand-triaxial geogrid (TX150) interface is not as obvious as that of the coarse sand-biaxial geogrid (SS40). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Geogrid**} & \multicolumn{4}{c}{**Normal Stress (kPa)**} \\ \cline{2-5}  & **50** & **100** & **150** & **200** \\ \hline SS20 & 41.55 & 18.14 & 13.59 & 0.68 \\ SS30 & 11.72 & 29.01 & 11.65 & 11.52 \\ SS40 & 59.29 & 33.19 & 22.97 & 36.15 \\ TX150 & 6.01 & 18.03 & 9.85 & 14.00 \\ TX160 & 37.99 & 16.20 & 22.60 & 10.20 \\ TX170 & 29.39 & 11.62 & 7.83 & 11.42 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Percentages of shear strength variation (%) for fine sand–biaxial geogrid interfaces and fine sand–triaxial geogrid interfaces. The interface shear test results of gravel in different shear directions are shown in Figures 11 and 12, respectively. The shear stress-shear strain curves of the gravel-biaxial geogrid (SS40) interface and gravel-triaxial geogrid (TX150) interface are nearly consistent with those for the coarse sand, with both showing different behavior between the soil-biaxial geogrid (SS40) interface and soil-triaxial geogrid (TX150) interface, which may be affected by the shape of the geogrid aperture. By comparing the shear stress-shear strain curves of fine sand, coarse sand and gravel, it can be observed that particle size has a significant influence on the interface stress-strain behavior. The shear stress-shear strain curves of the fine sand-geogrid interface show stress hardening, while the coarse sand-geogrid interface and gravel-geogrid interface show a certain degree of strain softening. This may be because the shear strength of the soil-geogrid interface largely depends on the interlocking effect between the geogrid aperture and soil particles, and this interlocking effect is closely related to particle size. [PERSON] [34] showed that with the improvement in the matching degree between the average particle size and the aperture size of the geotextile, the shear strength of the soil-geogrid interface obtained in the direct shear test also increases. In this study, the matching degree of the fine sand and the aperture size of the geogrid are obviously smaller than those of the coarse sand and gravel, resulting in a weaker interlocking effect. Due to the small particle size of fine sand, this interlocking effect is weak, and it gradually develops with the progress of shearing, it does not reach  
Figure 9: Shear stress–strain curves for coarse sand–biaxial geogrid (SS40) interfaces. (**a**) Shear direction of 0\({}^{\circ}\), (**b**) shear direction of 45\({}^{\circ}\), (**c**) shear direction of 90\({}^{\circ}\). the peak value, and it manifests as strain hardening. However, for coarse sand and gravel with a larger particle size, this interlocking effect is strong, and reaches the peak value quickly with the progress of shearing. However, after reaching the peak value, the original interlocking mechanism is destroyed due to the continuous shear action, resulting in the reduction in shear strength, showing a certain degree of strain softening.