## 3 Results
### Xanthan Gum-Treated Sand Shearing at Interface without Asperities (Interface I)  
Results of the interfacial shearing (Interface I) of xanthan gum-treated sand without asperities are presented in Figure 7. The interface shear stress (\(\tau_{i}\))-horizontal displacement (\(\delta\)) curve shows a vertical confinement-dependent behavior similar to the results of the direct shear test with a quite less dilation being the only difference. On the other hand, the shear behavior of initial condition of untreated and xanthan gum-treated sand on interface I exhibits a sliding behavior in which \(\tau_{i}\) maintains a constant value along displacement (Figure 7a,b). Moreover, depending on the hydrated state of xanthan gum, a peak and strain-softening behavior is observed when xanthan gum-treated sand is dried, while \(\tau_{i}\) is restored to be similar to that of the initial state after exposure to water (i.e., resubmerged state in Figure 7c,d). Xanthan gum treatment in initial state caused a slight decrease of \(\tau_{i}\), but the difference was not remarkably observed depending on the \(M_{\text{XG}}/M_{\text{S}}\) (Figure 7e,f). Xanthan gum in the hydrogel state retains enough moisture due to its hydrophilic property. A slight lubrication effect is generated between the particles and the solid surface due to the pseudoplasticity of xanthan gum hydrogel, resulting in the reduction of the frictional resistance [42]. Meanwhile, dehydration of xanthan gum hydrogel showed increases in \(\tau_{i}\) and \(\psi_{max}\) with brittle behavior in which \(\tau_{i}\) rapidly decreased after reaching a clear peak failure, and this behavior occurred distinctively with a sequential increase of \(M_{\text{XG}}/M_{\text{S}}\) (Figure 7g,h). The Mohr-Coulomb failure envelopes for peak and residual interfacial shear strengths of xanthan gum-treated sand (Interface I) according to varying moisture states are presented in Figure 8. The shear parameters determined from the linear regression of failure envelope are listed in Table 5. As mentioned above, xanthan gum treatment in the initial state slightly reduces the interface friction angle (\(\delta\)) from 28.6\({}^{\circ}\) to 26.7\({}^{\circ}\) regardless of \(M_{\text{XG}}/M_{\text{S}}\) (Figure 8a). Adhesion (c\({}_{\text{a}}\)) values were found to be less than 3 kPa, which indicates little contribution to the peak strengths (Figure 8b) and even for residual strengths at interface I. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Hydrogel State**} & \multirow{2}{*}{
\begin{tabular}{c} **Xanthan Gum** \\ \(M_{\text{XG}}\)/\(M_{\text{S}}\) **[\%]** \\ \end{tabular} } & \multicolumn{2}{c}{**Peak Strength**} & \multicolumn{2}{c}{**Residual Strength**} \\ \cline{3-6}  & & **Cohesion** [kPa]** & **Friction Angle [\({}^{\circ}\)]** & **Cohesion** [kPa]** & **Friction Angle [\({}^{\circ}\)]** \\ \hline \multirow{4}{*}{Initial} & 0.0 & \textless{}1 & 39.0 & \textless{}1 & 33.6 \\  & 0.5 & 2.5 & 38.1 & \textless{}1 & 31.1 \\  & 1.0 & 8.5 & 36.9 & \textless{}1 & 29.6 \\  & 2.0 & 23.2 & 34.6 & \textless{}1 & 31.7 \\ \hline \multirow{4}{*}{Dried (Initial \(\rightarrow\) Dried)} & 0.0 & \textless{}1 & 39.0 & \textless{}1 & 33.6 \\  & 0.5 & 117.1 & 45.4 & 28.2 & 36.8 \\ \cline{1-1}  & 1.0 & 307.7 & 51.6 & 34.9 & 39.5 \\ \cline{1-1}  & 2.0 & 563.7 & 53.7 & 45.1 & 46.3 \\ \hline \multirow{4}{*}{Resubmerged (Initial \(\rightarrow\) Dried \(\rightarrow\) Resubmerged)} & 0.0 & \textless{}1 & 39.0 & \textless{}1 & 33.6 \\ \cline{1-1}  & 0.5 & 11.4 & 38.6 & 3.6 & 30.7 \\ \cline{1-1}  & 1.0 & 23.3 & 37.9 & 12.8 & 30.3 \\ \cline{1-1}  & 2.0 & 41.1 & 35.3 & 21.7 & 29.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Mohr–Coulomb failure parameters determined from linear regression (soil–soil shearing).