## 3 Results
### Xanthan Gum-Treated Sand Shearing at Interface with Singular Asperity
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Hydrogel State**} & \multirow{2}{*}{\(M_{\text{XG}}/M_{S}\) [\%]} & \multicolumn{2}{c}{**Peak Strength**} & \multicolumn{2}{c}{**Residual Strength**} \\ \cline{3-6}  & & **Adhesion** & **Interface Friction** & **Adhesion** & **Interface Friction** \\  & & **[kPa]** & **Angle [\({}^{\circ}\)]** & **[kPa]** & **Angle [\({}^{\circ}\)]** \\ \hline \multirow{4}{*}{Initial} & 0.0 & 3.1 & 28.6 & 4.1 & 27.1 \\  & 0.5 & \textless{}1 & 26.610 & \textless{}1 & 26.1 \\  & 1.0 & 2.2 & 26.3 & 2.2 & 26.3 \\  & 2.0 & \textless{}1 & 26.7 & 1.2 & 26.6 \\ \hline \multirow{4}{*}{Dried (\({}^{1}\))} & 0.0 & 3.2 & 28.6 & \textless{}1 & 28.3 \\  & 0.5 & 7.1 & 30.1 & \textless{}1 & 28.7 \\  & 1.0 & 15.1 & 30.7 & 3.1 & 29.6 \\  & 2.0 & 21.5 & 35.1 & \textless{}1 & 32.8 \\ \hline \multirow{4}{*}{Resubmerged (\({}^{2}\))} & 0.0 & 3.1 & 28.6 & 4.1 & 27.1 \\  & 0.5 & \textless{}1 & 28.3 & \textless{}1 & 26.6 \\ \cline{1-1}  & 1.0 & 2.8 & 26.9 & 2.3 & 27.0 \\ \cline{1-1}  & 2.0 & 2.0 & 27.6 & \textless{}1 & 27.5 \\ \hline \hline \end{tabular} \({}^{(1)}\) Initial to dried (tested after oven-drying for 21 days at 40 \({}^{\circ}\)C), \({}^{(2)}\) Tested after resubmerging dried specimens for 24 h at room temperature.  
\end{table} 
Table 5: Mohr-Coulomb failure parameters determined from linear regression (_Interface I_). According to the moisture conditions of xanthan gum, the results of shear failure according to the slope angle of asperity were different under all \(M_{\text{XG}}/M_{\text{S}}\) conditions. When the xanthan gum remained in a hydrogel state, the effect of the asperity dimension did not appear remarkably; in contrast, a distinct variation along the slope angle is seen in the dried state. Figure 9: Interfacial shear test results of xanthan gum-treated soil shearing including asperity (interfaces II, III, IV): (**a,c,e**) interface shear stress (\(\tau\))-horizontal shear displacement (\(\delta\)) curves; (**b,d,f**) vertical strain (\(\epsilon_{v}\))-horizontal shear displacement (\(\delta\)) curves. Figure 10: Peak strengths of xanthan gum-treated sand obtained from direct and interfacial shear tests. Figure 11: Residual strengths of xanthan gum-treated sand obtained from direct and interfacial shear tests. Xanthan gum-treated sand at both wet initial state and resubmerged state shows low cohesion because of the low viscosity of xanthan gum hydrogels (i.e., low \(M_{xg}/M_{w}\)). Thus, the interface interaction and failure between xanthan gum-treated sand and the structural member are regarded to be mostly affected by the relatively weaker xanthan gum-sand medium inside the asperity. Meanwhile, xanthan gum-treated sand at the dried state has distinctively large cohesion due to the high elastic tensile strength of dehydrated (i.e., high \(M_{xg}/M_{w}\)) xanthan gum biofilms forming a continuous matrix among granular particles [16; 18]. Highly condensed xanthan gum biofilms can exist as remaining particle coats or viscous fragments in intergranular pore spaces even after the shear failure [18; 21], where this can be the possible reason why dried cases show residual adhesion with \(M_{XG}/M_{5}\) increase as listed in Table 6.  
Under the resubmerged condition, the strength induced in the previously dried state was not fully dissipated upon wetting at high \(M_{XG}/M_{5}\) conditions (1% and 2%). Meanwhile, the strength dropped to its initial state at a low \(M_{XG}/M_{5}\) (0.5%) because sufficient dissolution and diffusion of xanthan hydrogel occurred. On the contrary, the shear failure envelopes of the residual state were similar to those of soil-to-soil shearing at the initial and resubmerged states as seen from Figure 11. Moreover, there is little difference caused by the asperity condition in the dried state.