## 4 Discussion  
### Effect of Moisture Phase and Concentration of Hydrogel on the Interfacial  
_Shearing Performance_  
Generally, the shear behavior of cohesionless sand is dominantly governed by the inter-particle friction [43]. In this study, a wide range of experimental laboratory results showed that the effect of xanthan gum treatment on the internal and interfacial shearing performances was positive. Xanthan gum hydrogel exists as a viscous fluid in the static condition but exhibits a shear-thinning phenomenon where its viscosity decreases when a strain occurs. Thus, xanthan gum treatment provides an artificial force for mitigating the sand particle movement in the static state. However, its effect on the shearing behavior varies depending on the moisture state and \(M_{\text{XG}}/M_{\text{S}}\). Figure 12 presents the internal shear parameters (\(\phi\) and \(c\)) of xanthan gum-treated sand. When xanthan gum treatment in the hydrogel state was performed (i.e., 'initially wet' or're-wetting after dried'), the cohesion was seen to enhance with \(M_{\text{XG}}/M_{\text{S}}\) increase (Figure 12b); however, the friction angle was rather diminished owing to a decline in the surface dilatancy of sand particles, which was interrupted by the viscous xanthan gum hydrogel occupying the pores (Figure 12a) [18]. The phase of xanthan gum solution changes from the hydrogel to the film state at the critical water content condition. After this phase transformation, xanthan gum gains a tensile strength, which increases the friction angle as well as the cohesion in the dried state. Thus, the effect of \(M_{\text{XG}}/M_{\text{S}}\) on strengthening is much greater in the dried state than in other wet states. The result of internal shear parameters obtained in this study tends to coincide with those of xanthan gum-treated Sydney sand performed in triaxial compression tests [18]. However, the friction angle in the dried state in this study is higher owing to the larger mean particle size and angularity. Figure 12: Internal shear parameters of xanthan gum-treated sand (soil–soil shearing): (**a**) peak internal friction angle (\(\phi_{\text{peak}}\)), (**b**) peak cohesion (\(c_{\text{peak}}\)), (**c**) residual internal friction angle (\(\phi_{\text{residual}}\)), and (**d**) residual cohesion (\(c_{\text{residual}}\)). The effect of xanthan gum treatment on soil-to-soil shearing (direct shear test) also plays a key role in the interfacial shear behavior of xanthan gum-treated sand. The interface friction angle is found to be slightly diminished in the wet state (i.e., initial and resubmerged condition) (Figure 13a,e) and decrease of \(\phi\) in soil-to-soil shearing is responsible for the reduction of \(\delta\) at interfaces II, III, and IV containing asperity. At these interfaces, not only the failure of soil, but also the sliding shear behavior of interface I is seen. At the sliding occurring interface (I), the shear thinning of xanthan gum hydrogel provides a lubrication effect at low confinement pressures. Therefore, each decrease of \(\delta\) in interface II, III, and IV is triggered by the aforementioned two factors. Meanwhile, \(\delta\) at interface II, III, and IV is less sensitive with regard to the inclined angle in asperity slope throughout the peak and residual stages (Figures 13a and 14a). Because xanthan gum hydrogel in the initial state and re-swollen xanthan gum in the resubmerged state have a similar shear failure plane which approximates the boundary line regardless of the \(M_{\text{XG}}\)/\(M_{\text{S}}\) and asperity slope angle. In addition, \(\text{c}_{\text{a}}\) was not meaningfully affected by \(M_{\text{XG}}\)/\(M_{\text{S}}\) or asperity dimension (Figure 13b,f), except in the dried state. As explained above, \(\phi\) inside soil increases when xanthan gum experiences a gel phase transformation due to dehydration. In addition, an elastic tensile film bridge between the sand-xanthan gum mixture and structural surface is formed, thereby providing an additional adhesive force (Figure 13d).