## 4 Analysis of Test Results
### Stress-Strain Relationship Analysis of Fiber-Reinforced Soil
This study found that the optimal fiber content varied depending on the soil properties, fiber type, and particle size but was mostly in the range of 0.1-0.3% [12, 26, 27, 28]. The stress-strain curves of the fiber-reinforced soil with different fiber lengths (Figure 5) showed that when the fiber length was 0-12 mm, the strength of the fiber-reinforced soil increased with fiber length, and when the fiber length was greater than 12 mm, the strength of the fiber-reinforced soil decreased with the increase in fiber length. This suggests that there is an optimal fiber length for the fiber-reinforced soil. The best reinforcement effect was achieved when the length was 12 mm, which is  
Figure 4: Stress-strain relationships of the fiber-reinforced soil with different fiber contents. Figure 3: Failure characteristics of subgrade fill and fiber-reinforced soil samples: (a) Fill; (b) Fiber-reinforced soil. consistent with the conclusions of other studies [12; 24]. At the same time, the optimal fiber length in many studies was 10-15 mm [22; 25; 29; 30; 31; 32]. The above analysis shows that the optimal fiber length is in the range of 12-18 mm. This length can avoid the problems of fiber twisting, agglomeration, and floating due to the long lengths of the monofilament bundles and the problem of its inability to form a 3D reticulated structure due to the fiber lengths being too short. In the force-induced deformation process of reinforced soil specimens with 12-mm-long fibers, with a continuously increasing load, the fiber approached the critical state of sliding, pulling out, and breaking, and the frictional and occlusal forces in the soil approached the tensile strength of the fiber itself. At this time, the fiber reinforcement effect was optimal. Therefore, the choice of fiber length is particularly important. When the length is optimal, the fiber is close to the critical state, the tensile strength of the fiber and its tensile stress in the soil are similar, and the fiber reinforcement effect is the best [33]. (3) Effect of confining pressure  
The stress-strain curves of the fiber-reinforced soil under different confining pressures (Figure 6) showed that at the beginning of loading, as the strain increased, the stress of the fiber-reinforced soil under different confining pressures stayed basically the same, which meant that the contribution of the confining pressure to the fiber reinforcement effect was small when the strain was small. As the load continued to increase, with the increase in the confining pressure, the stress of the fiber-reinforced soil increased rapidly, and the confining pressure had a significant effect on the strength of the fiber-reinforced soil. That is, the higher the confining pressure was, the better the reinforcement effect. This is because when the confining pressure was high, the ability to confine the lateral deformation of the soil was strong, and the voids in the soil closed quickly. The fibers could effectively distribute the shear stress in the soil, change the forms of the external load forces, and optimize the force structure. The stronger the constraint on the soil was, the better the fiber reinforcement effect. Figure 5: Stress-strain relationships of soils reinforced with fibers of different lengths. Figure 6: Stress-strain relationships of the fiber-reinforced soil under different confining pressures.