## 5 Microscopic Mechanism of Fiber Reinforcement  
From the above, we knew that both the UCS and STS of BFCSM are better than that of BFUSM. The changes in macromechanical properties are obvious, however, the reasons for the changes are not clear. Therefore, the microscopic images of SM0B, SM0.2B, SM2 POB, and SM2P0.2B after curing for 14 days are displayed in Figure 11 to explore the reasons for mechanical properties changes. From Figure 11a, with no fiber and cement incorporated, the formation of soil strength only depends on the bonding force between particles. The pores are large. The structure is a \"honeycomb structure\". The ability of interparticle bonding is poor, which leads to low strength. The schematic diagram as Figure 12a is presented. When BF is incorporated, the fiber is tightly wrapped by particles and plays the role of \"building bridges\" (Figure 11c,d). The integrity of the particles is improved and the pores decrease. Hence, the strength increases through physical action. The whole is presented as an \"attached skeleton\". The schematic diagram shows in Figure 12c. Figure 10: Relationship between experimental data and predicted data with a length of 12 mm for STS: (**a**) BFUSM and (**b**) BFCSM. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Variables**} & \multicolumn{2}{c}{\(p\)**-Value**} & \multicolumn{2}{c}{**Revised \(p\)-Value**} \\ \cline{2-5}  & \(q_{\text{tBFUSM}}\) & \(q_{\text{tBFCSM}}\) & \(q_{\text{tBFUSM-R}}\) & \(q_{\text{tBFCSM-R}}\) \\ \hline Intercept & \(5.3200\times 10^{-3}\) & \(8.4900\times 10^{-3}\) & \(4.9000\times 10^{-3}\) & \(1.4690\times 10^{-2}\) \\ \(B\) & \(4.7610\times 10^{-2}\) & \(5.6730\times 10^{-2}\) & \(2.1530\times 10^{-2}\) & \(4.6050\times 10^{-2}\) \\ \(D\) & \(9.3278\times 10^{-4}\) & \(1.1300\times 10^{-3}\) & \(8.2574\times 10^{-4}\) & \(2.0500\times 10^{-3}\) \\ \(B^{2}\) & \(1.5194\times 10^{-1}\) & \(1.2681\times 10^{-1}\) & - & - \\ \(D^{2}\) & \(2.9044\times 10^{-5}\) & \(3.3268\times 10^{-5}\) & \(2.5499\times 10^{-5}\) & \(6.0297\times 10^{-5}\) \\ \(BD\) & \(1.2900\times 10^{-3}\) & \(1.5800\times 10^{-3}\) & \(1.0700\times 10^{-3}\) & \(2.8600\times 10^{-3}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The \(p\)-values of coefficients corresponding to variables in MNLR models for predicting \(q_{\text{t}}\). Figure 11: SEM images of typical samples: (**a**) SM0B magnified 100\(\times\), (**b**) SM2P0B magnified 100\(\times\), (**c**) SM0.2B magnified 100\(\times\), (**d**) local marked magnified area of (**c**) (1000\(\times\)), (**e**) SM2P0.2B magnified 100\(\times\), and (**f**) local marked magnified area of (**e**) (1000\(\times\)). Figure 11b shows that the soil relies on the cement-soil skeleton formed by the hydration reaction to provide strength after the cement is incorporated. The particles have good integrity, small particles attach to the surface of large particles and fill pores, pore size, and pore numbers are reduced, and the strength is significantly improved by chemical action. The structure is a \"flocculation structure\". The schematic diagram presents in Figure 12b. From Figure 11e,\(f\), after the inclusion of BF and cement, the silty sand and hydration products are both attached to the surface of the fiber together, forming a better connection in the cement-soil skeleton, which enhances the interaction between the silty sand, cement, and fiber. The particle integrity is further improved, the pores are further decreased, and the strength is further significantly improved by the combined effect of physical action and chemical action. The whole is still presented as a \"flocculation structure\". The schematic diagram is shown in Figure 12d. In summary, due to a combination of the physical action of fiber reinforcement and the chemical action of cement hydration reaction, the bonding force between particles is made larger. Furthermore, due to the existence of fiber-cement-soil skeleton structure, improve the bonding capacity between particles, therefore, the strength is enhanced.