## 3 Results and Discussions
### Mechanical Property Test Results
#### 3.1.2 Direct Shear Test Results  
The shear strength of the reinforced sand clearly exceeds that of the unreinforced soil under different levels of vertical stress, as shown in Figure 6. The shear strength increases to different degrees with increasing fibre length and content. Previous studies have shown that a critical fibre content exists with respect to the contribution of fibres to the shear strength or other strength indices of soil. When the fibre content exceeded 0.75%, the fibre contribution to the shear strength of soil was no longer positive or even became negative [50]. In the current study, only two contents, 0.25% and 0.5%, were used, and both were within the critical fibre-content range. Consequently, the influence of the critical fibre content is not observed in this study. Results show that the shear strength of soil with 0.5% fibre content was better than that with 0.25% fibre content. Therefore, the addition of fibres has improved the shear strength of sand. In addition, moisture content is also an important factor affecting the shear strength. The shear strength of clean and reinforced sands first increases and then decreases with increasing moisture content. By comparing the shear strengths of clean and reinforced sands at different moisture contents, it is found that, when the moisture content of the soil is 10%, the shear strength increases more rapidly with increasing vertical stress compared with those of soils with other moisture contents. The soil with 10% moisture content and reinforced with 6 mm-long fibres exhibits the highest shear strength. This indicates that 10% is the optimal moisture content for this sample type, and the shear strength decreases when the moisture content exceeds 10%. An increase in moisture content was found to lead to the capillary tension of pore water [26], increasing apparent cohesion, which could prevent the movement of soil particles. However, other studies reported contradictory conclusions. According to the study of [PERSON] et al. [51], the interfacial peak strength and interfacial residual strength of fibre-reinforced soil decrease with increasing moisture content. Figure 6: _Cont._To further analyse the reinforcement mechanism of fibres, the effects of fibre length, fibre content, and moisture content on the shear strength parameters are analysed. As shown in Figure 7, the addition of fibres did not show the obvious effect on the internal friction angle of sand. This is similar to the results reported in the study of [PERSON] et al. [26] in which the addition of fibres to sand does not significantly change the final friction angle. Notably, the addition of a small amount of fibre to the sample with 2.5% moisture content can increase the internal friction angle of sand; however, a further increase in fibre content decreases the internal friction angle. In Figure 7a,b, the increase in fibre length is observed to reduce the effect of water content on the friction angle of the soil. This causes the internal friction angle to fluctuate within a small range with different moisture contents. In general, the 12 mm fibres have a negligible effect on the internal friction angle of unsaturated silty sand compared to the 6 mm fibres. On another hand, distinct differences in the contribution of moisture content to the internal friction angle at different intervals are observed. For moisture contents ranging from 10% to 12.5%, a significant decrease in the internal friction angle occurs. In contrast, for moisture contents ranging from 12.5% to 15%, the internal friction angle does not change significantly. Hence, the influence of moisture content on the internal friction angle of soil does not show a simple linear relationship. Adding a small amount of water to sand can fill the pores and increase the friction angle. After reaching a critical value, a continuous increase in moisture content can decrease the soil suction and internal friction angle.