# Experimental Study on Cyclic Shear Performance of the Four-Way Geogrid Reinforcement-Soil Interface
## 3 Results and Discussion  
### Shear Stress  
#### 3.1.1 Effect of the Normal Stress  
The four-way geogrid-soil interface was tested under a \(f\) of 1 Hz and \(A_{w}\) of 1.5 mm for 1000 cycles of shear. Figure 5 shows the resulting different cycles of the geogrid-soil interface shear stress and shear displacement curves. Referring to the literature [24], the average of the absolute values of the maximum and minimum shear stresses in a hysteresis loop is defined as the average peak shear stress of the hysteresis loop; the average peak shear stress of the first hysteresis loop is defined as the initial interfacial shear stress; the maximum average peak shear stress that occurs in all the hysteresis loops is defined as the peak interfacial strength, and the average peak shear stress that achieves the stabilized residual stage is defined as the interfacial residual strength. It is evident that the shape of the hysteresis loop at the interface between the geogrid and soil undergoes significant alterations with an increasing number of cycles. During the initial stages of the cyclic shear test (\(n\leq 50\)), the hysteresis curve appears long and elliptical, but as the number of cycles increases (\(100\leq n<500\)), the middle segment of the hysteresis loop protrudes outward. The specimen displays a lengthy pike-shaped structure with a broad mid-section flanked by two pointed ends. Subsequently, as \(n\) ranges from 500 to less than 1000, the bulging middle exhibits increasing fullness, and the hysteresis loop adopts a parallelogram shape. Figure 4: Geogrid fixing diagram. \begin{table}
\begin{tabular}{c c c} \hline
**Normal Stress \(\sigma\)/kPa** & **Shear Displacement \(A_{w}\)/mm** & **Shear Frequency \(f\)/Hz** \\ \hline
60 & 1.5 & 1 \\
90 & 1.5 & 1 \\
120 & 1.5 & 1 \\
150 & 1.5 & 1 \\
120 & 1.5 & 0.5 \\
120 & 2.5 & 0.5 \\ \hline \end{tabular}
\end{table} 
Table 3: Cyclic shear test conditions. Figure 6 displays the graphs for the average peak shear stress variation at the interface of the soil reinforced with a four-way geogrid subjected to various normal stresses and numbers of cyclic shear. The results demonstrate that both the initial shear stress and peak interfacial strength increase as normal stress is raised. Specifically, when \(\sigma\) increases from 60 kPa to 150 kPa, there is a 31.02 kPa increase in initial shear stress and a 41.60 kPa increase in peak interfacial strength. This aligns with the Moore-Coulomb strength criterion, indicating that material shear strength increases with increasing normal stress. When the stress is equal to 60 kPa or 90 kPa, the peak shear stress rises with the growing number of cycles, ultimately stabilizing. Furthermore, the larger the stress, the greater the number of cycles required for stabilization. Moreover, the geogrid reinforcement-soil intersection depicts shear-strengthening properties. When the stress level is 120 kPa and 150 kPa, the peak shear stress increases with the number of cycles, subsequently decreasing and eventually stabilizing. Additionally, the number of cycles needed to attain the interfacial residual strength increases with the growth in stress level. During this phase, the interfacial residual strength does not undergo significant alterations, and the geogrid-soil interface displays shear-softening characteristics. This result is very similar to the experimental results of [PERSON] et al. (2019). The possible cause of this phenomenon is that the value of \(\sigma\) is too high, which exacerbates the deformation and wear of the grid during the shearing process. As a result, the interlocking effect between the soil and the grid weakens. The greater the vertical stress, the smaller the corresponding cycle times when reaching the peak shear stress can be observed simultaneously ([PERSON] et al., 2019). With increased vertical stress, soil particles densify at a faster rate, leading to earlier interlock between soil particles and grille ([PERSON] et al., 2019). Consequently, the appearance of peak shear stress occurs earlier within the entire cycle.