## 4 Results and Discussion
### Shear Strength  
According to the confining pressure and the peak stress of the numerical triaxial compression test, the variation in the shear strength index of the samples can be determined by drawing the Mohr strength envelope. The three points in the legend represent the shear strength obtained by fitting the numerical test results under a confining pressure of 150, 200, and 250 kPa. The five points in the legend also indicate the shear strength obtained by fitting the numerical test results under confining pressures of 150, 200, 250, 300, and 400 kPa. Figure 10: The deviatoric stress-axial strain curves of the LGSCR samples at different thicknesses of sand layers and confining pressures of (**a**) 150 kPa, (**b**) 250 kPa, and (**c**) 400 kPa. Figures 11 and 12 present the peak stress-confining pressure curves and the shear strength index of the samples at different numbers of sand layers, respectively. Figure 11 shows that the \(\sigma_{\text{p}}\) of the samples enlarges with an increase in the confining pressure. At a constant confining pressure, the \(\sigma_{\text{p}}\) rises with an increase in the \(n\) value; the influence of \(n\) on the \(\sigma_{\text{p}}\) also continues to increase as the confining pressure enlarges. A higher confining pressure causes a greater difference in the \(\sigma_{\text{p}}\) of the samples with different numbers of sand layers, leading to a significant difference in the shear strength indices obtained by the two different fitting methods in Figure 12. According to the numerical triaxial compression test results, the fitted values of cohesion (\(c\)) obtained in a confining pressure range of 150-400 kPa are all lower than those obtained in a confining pressure range of 150-250 kPa. However, the variation in the regularity of the internal friction angle (\(\varphi\)) is precisely the opposite. Hence, a reasonable test pressure should be determined according to the actual stress state of the LGSCR structure in practical engineering. It can be seen in Figure 12 that increasing the number of sand layers reduces the cohesion of the samples significantly but enlarges the internal friction angle. After laying the  
Figure 11: The peak stress-confining pressure curves of samples at different numbers of sand layers. Figure 12: The variation in the shear strength index of the samples with the number of sand layers. geogrid in the sand layer of the SCL sample, the \(\sigma_{\rm p}\) of the samples increases significantly. The higher the confining pressure, the greater the increasing degree of the \(\sigma_{\rm p}\). Comparing the shear strength indices of the LGSCR samples with those of the SCL samples demonstrates that the geogrid embedment improves the cohesion of the samples; the internal friction angle also rises when the number of sand layers is equal to or greater than two, that is, \(n\geq 2\). Figures 13 and 14 delineate the peak stress-confining pressure curves and the shear strength index of the samples at different thicknesses of sand layers, respectively. Figure 13 shows that the \(\sigma_{\rm p}\) of the samples enlarges with an increase in the confining pressure, and the influence of the thickness of sand layers on the \(\sigma_{\rm p}\) improves continuously as the confining pressure rises. The higher the confining pressure, the more significant the difference in the \(\sigma_{\rm p}\) of the samples at different thicknesses of sand layers. Therefore, there is also a marked difference in the shear strength calculated by the two different fitting methods. According to the numerical triaxial test results, the fitted values of cohesion obtained in a confining pressure range of 150-400 kPa are all lower than those obtained in a confining pressure range of 150-250 kPa. However, the internal friction angle shows an opposite variation trend. According to Figure 14, raising the thickness of sand layers reduces the fitted cohesion of the samples but enlarges the internal friction angle.