## 3 Results  
### Shearing Behavior of Xanthan Gum-Treated Sand (Soil-to-Soil Shearing) in Hydrogel State  
The representative direct shear stress (\(\tau\))-horizontal displacement (\(\delta\)) behavior of xanthan gum-treated sand according to different normal stress, moisture state, and \(M_{XG}/M_{S}\) values are presented in Figure 5. The \(\tau\) values were calculated by dividing the measured shear force by the effective area of specimens, which is found to decrease along the shearing process, as shown in Figure 5a,c,e. The vertical strain (\(\varepsilon_{p}\)) values were calculated as the ratios of vertical displacement in samples to the initial height of samples (25 mm). Positive values indicate a volumetric dilation, whereas negative values indicate a volumetric contraction. The maximum dilation angle (\(\psi_{max}\)) under shearing was derived from the largest ratio (i.e., slope) of the vertical and horizontal displacement as shown in Figure 5b,d,f. Figure 5a,b shows the shear behavior of xanthan gum-treated (1%) sand using vertical confinement pressures of 50, 100, 200, and 400 kPa. Overall, the direct shear stress and vertical strain were seen to vary depending on the normal stress. As the normal stress increased, the direct shear stress also increased (Figure 5a), while the soil became less dilative (Figure 5b) owing to the stronger interlocking among sand grains [21]. Figure 5c,d presents the shear behavior at three different moisture states (i.e., initial, dried, and resubmerged) of xanthan gum-treated sand. The xanthan gum hydrogel occurring in pore spaces in the sand showed different trends of its strengthening effect depending on the hydraulic state. A phase change in xanthan gum solution from a hydrogel (initial state) to the xanthan gum film occurred with dehydration (dried) [16], which generated tensile strengths. Thus, an obvious peak of the shear strength exists prior to failure followed by a residual strength with more dilation compared to other samples having presence of water. Moreover, the condensed xanthan gum film becomes disturbed and reaches a swollen state when it is exposed to water (resubmerged) and exhibits a more contractive behavior than  
Figure 4: Schematic diagram of experimental setup: (**a**) direct shear test; (**b**) interface shear test. that of uniform hydrogel at the initial state [40]. Figure 5e,f reveals that the direct shear stress of xanthan gum-treated sand increases with the \(M_{XG}/M_{S}\) in dried state. Compared to pure sand, xanthan gum-treated sand shows a remarkable strengthening effect and dilation at dried conditions, and this effect is greater at higher \(M_{XG}/M_{S}\) conditions, which increases the thickness of xanthan gum films in dried conditions. The Mohr-Coulomb failure envelopes for the peak and residual direct shear strengths of xanthan gum-treated sand with varying moisture states are presented in Figure 6. The shear parameters determined from the linear regression of the failure envelope are listed in Table 4. Generally, both the peak and residual shear strengths linearly increase with an increasing degree of normal stress. The strengthening effect of xanthan gum is rarely observed in its initial state (Figure 6a); however, the cohesion (c) slightly increases the pressure to 23.2 kPa with a decrease of the internal friction angle (\(\phi\) = 39.0\({}^{\circ}\) to 34.6\({}^{\circ}\)) according to \(M_{XG}/M_{S}\). The residual strength of xanthan gum-treated sand is found to be lower than that of pure sand (Figure 6b). Figure 5: Direct shear test results of xanthan gum-treated soil: (**a,c,e**) direct shear stress (\(\tau\))-horizontal shear displacement (\(\delta\)) curves; (**b,d,f**) vertical strain (\(\varepsilon_{\flat}\))-horizontal shear displacement (\(\delta\)) curves. Meanwhile, xanthan gum treatment increases the strength of soil significantly in terms of the internal friction angle and cohesion with \(M_{\text{XG}}/M_{S}\) increase in the dried state.