# Triaxial Test of Coarse-Grained Soils Reinforced with One Layer of Geogrid
## 3 Test Results
### Geogrid Influence  
Post-test images provide visual evidence of the different behaviors exhibited by specimens without and with geogrid reinforcement (see Figure 6). Notably, the presence of geogrid in the specimens introduces a significant change in how the shearing surfaces are formed.  
In the case of specimens without geogrid reinforcement (Figure 6 above), the shearing surfaces extend uniformly across the entire height of the specimen. This results in a relatively uniform distribution of shear forces throughout the specimen, as is typical in triaxial compression testing.  
However, when geogrid reinforcement is introduced (Figure 6 below), the shearing surfaces are localized either in the upper or lower part of the specimen. This localization indicates that geogrid reinforcement plays a pivotal role in redistributing and concentrating shear forces. This concentrated behavior might explain the presence of two deviatoric stress peaks in tests with geogrid reinforcement, as observed earlier (Figure 4).  
Figure 4: Relationship between deviatoric stress and vertical strain of soil No. 5.  
Figure 5: Geogrid appearance after testing: (**a**) single peak during shear; (**b**) dual peaks during shear.  
It is important to mention that although the soils used for the tests are classified as non-cohesive, they contained fine particles, the amount of which did not exceed 5%, and also the suction effect due to the water in the sample allowed the samples to be kept stable after the test. Stable gravelly soils were not maintained in almost all cases; therefore, their photos are with a membrane.  
The maximum deviatoric stresses and their dependency on cell pressure are presented in Table 5. The influence of cell pressure on the increase in deviatoric stress in geogrid-reinforced specimens is notable. At a cell pressure of 20 kPa, the reinforcement effect showed a range from 1.05 to 1.37. When the cell pressure increased to 50 kPa, the reinforcement effect still showed a substantial range, spanning from 1.09 to 1.31. As the cell pressure further increased to 70 kPa, the range of the reinforcement effect remained considerable, varying from 1.02 to 1.13. These results demonstrated that cell pressure was the dominant factor influencing reinforcement, overshadowing the influence of soil type, whether sand or gravel, which did not significantly affect the observed outcomes.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample No. \({}^{*}\)**} & \multirow{2}{*}{**Test Conditions**} & \multicolumn{2}{c}{**Maximum Deviatoric Stress, kPa**} & \multicolumn{3}{c}{**Reinforcement Effect R\({}_{\sigma}\)**} \\ \cline{3-8}  & & \(\sigma_{3}\) = 20 kPa & \(\sigma_{3}\) = 50 kPa & \(\sigma_{3}\) = 70 kPa & \(\sigma_{3}\) = 20 kPa & \(\sigma_{3}\) = 50 kPa & \(\sigma_{3}\) = 70 kPa \\ \hline \multirow{2}{*}{\(1\)} & Without geogrid & 144.68 & 281.32 & 428.96 & 1.26 & 1.30 & 1.08 \\  & With geogrid & 182.92 & 364.40 & 465.12 & & & \\ \hline \multirow{2}{*}{\(2\)} & Without geogrid & 195.97 & 380.48 & 475.56 & & & \\  & With geogrid & 242.31 & 418.88 & 538.55 & & & \\ \hline \multirow{2}{*}{\(3\)} & Without geogrid & 141.78 & 284.98 & 394.80 & & & \\  & With geogrid & 178.30 & 310.54 & 422.40 & & & \\ \hline \multirow{2}{*}{\(4\)} & Without geogrid & 106.94 & 208.04 & 339.94 & & & \\  & With geogrid & 145.98 & 244.19 & 364.48 & & & \\ \hline \hline \end{tabular}
\end{table}
Table 5: Summary of the results of the triaxial compression test for maximum deviatoric stress and reinforcement effect.  
Figure 6: The shape of failure: (**above**)—without geogrid; (**below**)—with geogrid.