## 4 Analysis of Test Results  
### Stress-Strain Relationship Analysis of Fiber-Reinforced Soil  
The stress-strain curves of the fiber-reinforced soil show that for the fill mixed with fibers, the shape of the stress-strain curve did not change from that without fibers and still possessed strain-hardening characteristics [6; 8; 9], but the degree of hardening was enhanced. The effect of fiber incorporation on the mechanical properties of the fill showed that as the cohesion increased from 0 MPa to 0.11 MPa, the internal friction angle decreased from 38\({}^{\circ}\) to 37.7\({}^{\circ}\). Compared with the change in the internal friction angle, the increase in cohesion was even more pronounced with fiber incorporation, which is consistent with References [7; 8; 9; 22]. When the strain remained unchanged, the addition of fiber significantly increased the stress in the fill, indicating that fiber incorporation significantly enhanced the strength and stiffness of the fill, for an excellent reinforcement effect. The shear failure pattern of the fill did not change with the incorporation of fiber; the failure was still due to bulging deformation. The lateral deformation, which was large in the middle and small at the two ends of each specimen, showed plastic failure characteristics and no obvious cracks or failure in the surface [8; 9] (Figure 3). The analysis results showed that the fibers were randomly and disorderly dispersed in the soil, forming a 3D reticulated structure, which enhanced the frictional force and occlusal force of the fill. Fiber with good tensile reinforcement due to fiber bridging can bear part of the tensile stress, bear the external load, and transfer its force, inhibit the generation and development of microcracks, strengthen the soil integrity, and improve the ability of the soil to resist deformation and failure [8]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Soil Type**} & \multicolumn{2}{c}{**Confining**} & \multirow{2}{*}{**Fiber Content (\%)**} & \multirow{2}{*}{**Fiber Length (mm)**} & \multirow{2}{*}{**Test Number**} \\  & & & 0 & & M1 \\  & & 0.1 & & M3 \\  & 0.1 & 0.2 & & M4 \\  & & 0.3 & & M5 \\  & & & 0 & L1 \\  & & & 3 & L2 \\ Fill/Fiber-reinforced soil & 0.1 & 0.2 & 9 & L3 \\  & & & 12 & L4 \\  & & & 18 & L5 \\  & & & & WY1 \\  & 0.3 & 0.2 & 12 & WY2 \\  & 0.5 & & & WY4 \\  & 0.7 & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Triaxial compressive test conditions. The stress-strain curves of the fiber-reinforced soil with different fiber contents (Figure 4) show that when the strain remained unchanged, as the fiber content increased, the stress of the fiber-reinforced soil first increased and then decreased. When the fiber content was between 0% and 0.2%, the fibers filled the voids in the original soil skeleton. With the increase in fiber content, the compactness increased, the frictional force and occlusal force of the soil body were enhanced, and the constraint of the reticulated structure created by the fibers in the soil was strengthened, which enhanced the soil's load transfer and dispersion abilities. Therefore, a fiber content up to 0.2% enhanced the strength. When the fiber content was greater than 0.2%, the fibers in the soil were in an oversaturated state, and the fibers occupied the space of the original soil particles. As the fiber content increased, the compactness of the soil decreased, the rate of fiber entanglement and floating increased, the uniformity and effective reinforcement ratio decreased, and the reinforcement effect was weakened [23]. Thus, the strength decreased as the fiber content increased above 0.2%. The above analysis shows that there should be an optimal fiber content in fiber-reinforced soil. Here, the best reinforcement effect was achieved at a fiber content of 0.2%, which is in agreement with the findings of many experimental studies [5, 8, 9, 11, 12, 24, 25].