## 4 SWCC Model Considering Fibre Reinforcement  
Among the commonly used SWCC models are VG (van Genuchten) [39], Gardner-Russo [40], Mckee and Bumb [41], and Fredlund and Xing [42]. The SWCCs fitted by these four models are shown in Figure 11. The VG model fits well on the SWCC of clean sand. It fully represents the saturated, transition, and dry sections of the curve. In the region close to saturation, the continuity of the function is maintained, and the fitting effect is satisfactory. The fitting curve of the Gardner-Russo model for sand is not smooth, and the fitting effect is inadequate in the saturated section. The relative accuracy of the Fredlund and Xing model in fitting parameters exceeds 0.97. However, the form of the model is complicated; consequently, its application is affected detrimentally to a certain extent. The coefficients of determination of the McKee and Bumb model for fitting the curves of five different samples all exceed 0.96. The McKee and Bumb model has a simpler form than the Fredlund and Xing model; however, its water-retention fitting curves also appear discontinuous and nonconvergent in the saturated section. The relevant parameters of the VG model (including air-entry value (AEV), VG model parameter (\(\alpha\), \(n\)), and residual saturation (\(S_{rr}\)) are listed in Table 5. The addition of fibre can reduce the \(\alpha\) value of sand. Such reduction is more distinct, especially for samples with low contents of 6 mm-long and 12 mm-long fibres. The fibres have a slight effect on the value of \(n\). The \(S_{rr}\) value of the fibre-reinforced sand is significantly higher than that of clean sand; it increases with the fibre content and length. This indicates that the addition of fibres improves the water-retention capacity of soil. This is mainly because numerous randomly distributed fibres interweave, form a spatial network structure in the soil, interlock with soil particles, and form a fibrous soil-wrapped ball. The presence of fibres provides an additional contact area for water molecules. With a wide surface area of soil particles, more water is required to satisfy the energy requirements for the interaction between particles and pore water, corresponding to high suction. Consequently, the water-retention characteristics improve [52]. Figure 10: Saturation change at the same matric suction. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multirow{2}{*}{**Clean Sand**} & **PP-6** & **PP-6** & **PP-12** & **PP-12** \\  & & **mm-0.25\%** & **mm-0.50\%** & **mm-0.25\%** & **mm-0.50\%** \\ \hline \(\alpha\) & 0.52 & 0.37 & 0.34 & 0.33 & 0.47 \\ AEV (kPa) & 1.92 & 2.70 & 2.94 & 3.03 & 2.13 \\ \(n\) & 1.95 & 2.20 & 2.16 & 2.32 & 2.03 \\ \(S_{rr}\) (\%) & 0.014 & 0.021 & 0.031 & 0.028 & 0.032 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Parameter values used in the VG model. The main factors affecting the total soil suction are capillary action [53, 54], short-range adsorption (interaction between particles and pore water) [55], and infiltration [56]. The foregoing analysis indicates that fibres affect the water-retention characteristics of soil. Moreover, the additional contact area between the fibre and soil affects pore structure and compactness. The main factors affecting capillary action are the changes in soil-pore structure and pore-size distribution, which are inevitably affected by the addition of fibres. Therefore, the fibres affect the capillary action of soil and soil water-retention characteristics. Because the fine soil particles used in this study only account for approximately 10% of the dry weight of the soil, the short-range adsorption is low. Moreover, hydration cannot be quantified; hence, it is ignored. Infiltration is the result of the dissolution of solutes in pore water. However, the fibres do not change the chemical properties of the soil; therefore, the effect of fibres on infiltration is negligible.