# Anisotropic Shear Strength Behavior of Soil-Geogrid Interfaces
## 4 Conclusions  
This paper presented an experimental study on the anisotropic shear strength behavior of soil-geogrid interfaces. A series of interface shear tests were carried out for different biaxial geogrids and triaxial geogrids under the shear directions of 0\({}^{\circ}\), 45\({}^{\circ}\) and 90\({}^{\circ}\). The results, concerning the influences of shear direction and particle size on the shear strength behavior of soil-geogrid interfaces, are presented and discussed. The following conclusions are reached from the conditions of the study:  
1. The interface shear strength under the same normal stress varies with shear directions for all biaxial and triaxial geogrids, which indicates the anisotropic shear strength behavior of soil-geogrid interfaces. The anisotropy decreases with the increase in normal stress because greater normal stress could generate a more stable interlocking structure between the soil particles and the geogrid aperture.
2. The percentages of shear strength variation could be relatively large (e.g., 59.59% for the soil-biaxial geogrid interface and 37.99% for the soil-triaxial geogrid interface). These large values indicate strong anisotropic shear strength behavior of soil-geogrid interfaces, which is ignored in the current soil-geosynthetic interface testing standard, but could be important for the safe design of reinforced soil structures.
3. The soil-biaxial geogrid interface shows stronger anisotropic shear strength behavior than the soil-triaxial geogrid interface. This is because the triangular aperture of the triaxial geogrid is more stable than the rectangular aperture of the biaxial geogrid, which makes the soil-triaxial geogrid interaction less sensitive to shear direction. The stable aperture shape of the triaxial geogrid could improve the interlocking effect between the soil particles and geogrid apertures, and thus show less anisotropic shear strength behavior for the soil-triaxial geogrid interface.
4. As the particle size increases, the friction and interlocking between the soil and geogrid plays a more significant role. Particle size also has a great influence on the anisotropy behavior of soil-geogrid interfaces. With the increase in particle size, the anisotropy behavior of the soil-triaxial geogrid interface becomes stronger, but the anisotropy behavior of the soil-biaxial geogrid interface becomes weaker.  
Conceptualization, J.Z.; methodology, M.J.; formal analysis, Y.J.; investigation, C.W.; resources, C.M.; data curation, Y.J.; writing--original draft preparation, J.Z.; writing--review and editing, M.J. and Y.Z.; supervision, Y.Z.; project administration, Z.Z.; funding acquisition, J.Z. All authors have read and agreed to the published version of the manuscript.  
This work is funded by the National Key R&D Program of China (Grant Number: 2018 YFB1600100).  
Not applicable.  
This work is funded by the National Key R&D Program of China (Grant Number: 2018 YFB1600100). The authors are grateful for the financial support.  
The authors declare no conflict of interest.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Particle**} & \multicolumn{3}{c}{**Normal Stress (kPa)**} \\ \cline{2-4}  & **50** & **100** & **150** \\ \hline Fine sand & 6.01 & 18.03 & 9.85 \\ Coarse sand & 17.54 & 14.25 & 9.12 \\ Gravel & 30.06 & 19.44 & 27.24 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Percentages of shear strength variation (%) for fine sand–triaxial geogrid, coarse sand–triaxial geogrid and gravel–triaxial geogrid interfaces.