## 3 Results and Discussions
### Shear Strength
#### 3.3.2 Dry Sample (UU Test)  
The deviatoric stress-axial strain curves obtained from UU triaxial tests for the dried untreated and biopolymer-treated specimens are shown in Figure 7.  
Soil strength improvement is seen due to the biopolymer additive. Adding higher percentages of XG shows a larger enhancement in soil strength. Regardless of XG content, all specimens showed higher strength under higher confining pressures. Specimens treated with 1% XG caused a 46% increase in maximum deviatoric stresses. This shows that adding only 1% biopolymer can significantly enhance soil strength.  
Once the XG-treated soil dried, the gel converted to a firm plastic material leading to the strong bonding between soil particles. This brings a higher resistance against the shearing of soil.  
Previous studies from direct shear tests showed that changes in shear parameters depend on biopolymer and soil type. [PERSON] et al. [28] investigated the effect of gellan gum on the soil cohesion and friction angle. They used a mixture of sand and clay and reported that adding gellan gum increased the cohesion of pure sand. At the same time, its friction angle remained relatively constant with increasing gellan gum content. In terms of pure clay, adding gellan gum to the soil caused an increase in both cohesion and friction angle. [PERSON] et al. [29] reported that even though adding agar and starch biopolymers to sand may effectively increase the cohesion of the soil, it leads to a reduction of soil friction angle when compared with untreated soil. By conducting the direct shear test, [PERSON] et al. [30] showed that xanthan gum and guar gum significantly increase soil cohesion but slightly reduce soil friction angle. [PERSON] et al. [8] investigated the shear behavior of silty sand treated with xanthan gum, guar gum, and beta-glucan biopolymers by performing a direct shear test. Similarly, they reported adding biopolymers increases soil cohesion but causes soil friction angle reduction.  
In the current study, the results of UU triaxial tests for the given XG concentrations under various confining pressures were used to obtain the shear parameters.  
\begin{table}
\begin{tabular}{c c c} \hline
**Soil Reference** & **Effective Cohesion, \(\boldsymbol{\zeta}^{\prime}\) (kPa)** & **Effective Internal Friction Angle, \(\boldsymbol{\phi}^{\prime}\) (\(\boldsymbol{\zeta}\))** \\ \hline Pure soil & 24 & 28.9 \\
1\% XG & 17 & 21.8 \\
2\% XG & 1 & 18.0 \\ \hline \end{tabular}
\end{table}
Table 2: Effective friction angle and cohesion of pure soil and XG-treated soil.  
Figure 7: Results of UU triaxial test at 50 kPa confining pressure; stress–strain curves.  
The failure envelope curves were plotted (Figure 8), and the corresponding shear parameters were determined (Table 3).  
The addition of XG resulted in a drastic improvement in soil cohesion, as adding only 1% XG doubled the soil cohesion. A similar trend can be seen for the 2% XG-treated soil with the above threefold soil cohesion improvement. The main reason is that adding XG acts like glue and supplies strong binding between soil particles, causing a significant increase in soil cohesion. While XG covers the soil particle surfaces, causing a reduction in grain angularity and thereby leading to decreases in internal friction angle. The obtained results have consistency with the reported outcomes from previous studies [8; 30].