## 3 Results and Discussion  
### Undrained Behavior of Expansive Soil-Rubber Fiber (ESR)  
Figure 3 shows the deviator stress, effective stress path, and pore water pressure for the ESR with different rubber fiber contents at a confining pressure of 200 kPa. The test results show that the deviator stress of the ESR increased with the rubber fiber contents. The deviator stress of ESR-1 is the smallest and that of ESR-3 is the largest. Compared with ESR-1, the stress-strain response of ESR varied with the addition of rubber fibers, as shown in Figure 3a. ESR-1 exhibits an initial strain hardening behavior in the nonlinear deviator stress-strain curve. It attains a peak deviator stress of 116.2 kPa under 14% axial strain. As the shear proceeds, the ESR tends to display strain softening behavior. This is manifested by a decrease in deviator stress. The peak deviator stress decreases from 116.2 kPa to 112.1 kPa. This agrees with the results of [PERSON] et al. [29]. That study investigated the mechanical response of root-reinforced loess by triaxial tests. ESR-2, ESR-3, ESR-4, and ESR-1 showed similar deviator stress-strain trends. Moreover, significant deviator stress peaks were observed, namely, 128.6 kPa, 134.5 kPa, and 124.4 kPa, respectively. Conversely, no significant peak deviator stress was observed for ESR-5. Figure 3d shows the pore water pressure curves for the ESR. These exhibit a trend similar to that of the deviator stress. The maximum pore water pressure (80.5 kPa) was obtained for  
Figure 3: Results of undrained triaxial tests of ESR with different rubber fiber contents at 200 kPa confining pressure: (**a**,**b**) deviator stress–strain, (**c**) effective stress path strain behavior, (**d**) pore water pressure. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Rubber Fiber Content** & **Frequency** & **Echo Interval** \\  & \(f\) (\%) & **(kHz)** & **(ms)** \\ \hline ESR-8 & 0 & & \\ ESR-9 & 5 & & \\ ESR-10 & 10 & & \\ ESR-11 & 15 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Nuclear magnetic resonance test scheme. ESR-4, and the minimum (73.2 kPa) was obtained for ESR-1. During the undrained shear test, the pore water pressure was related to the tendency of contraction and expansion. The effective stress path of ESR is shown in Figure 3c. The slope of the ESR stress path to the left indicates that it generates a positive pore water pressure during shearing, which reflects the contraction behavior of the ESR (Figure 3c). The deviator stress-strain curve for the ESR in the 0-4% strain range was selected, as shown in Figure 3b. Figure 3b shows that under equal deviating stress, the strain of the rubber fiber-reinforced expansive soil was higher than that of the unreinforced expansive soil. This indicated that the rubber fibers reduced the initial stiffness of the expansive soil. The undrained triaxial test results of the ESR at confining pressures of 100, 200, and 300 kPa are shown in Figures 4 and 5. The results reveal that the incorporation of rubber fibers increased ESR deviator stress, which was more significant at higher confining pressures (200 and 300 kPa). However, the effect on the deviator stress at 100 kPa was negligible. This is consistent with the results reported by [PERSON] et al. [30], who observed that the effectiveness of fibers for soil shear strength enhancement increases with an increase in confining pressure. Prior to shearing, the interaction between the soil particles and fibers improved owing to the increased confining pressure, and the fibers stretched gradually. As the presence of well-dispersed rounded particles destroys the sliding surface to increase the shear strength, the rougher surface of the rubber fibers makes the adhesion between the fibers and the soil play a significant role. The adhesion between the rubber fibers and the soil particles is greater than the friction force within the soil. At the beginning of the shear phase, the fibers improved the distribution of the applied shear stress over a wider area.