## 3 Test Results and Analysis
### Conventional Triaxial Compression Strength
#### 3.2.2 Strength Parameter Analysis  
The peak deviatoric stresses for gravel soil samples stabilized with different amounts of xanthan gum and lignin are shown in Figure 5. It can be seen from the figure that the peak deviatoric stress generally increases with the addition of xanthan gum and lignin, with lignin reaching a maximum value at 4%. As the xanthan gum content increases to 6%, the peak deviatoric stress shows a linear increase of approximately 70.13%. The presence of xanthan gum increases the bonding strength between soil and rock particles, thereby improving soil strength. Conversely, the peak deviatoric stress shows a tendency to first increase and then decrease with increasing lignin content, reaching a maximum of 66.70% at 4%. This behavior is similar to that observed in unconfined compressive tests, where excessive lignin can locally soften the soil, reducing the overall strength. The strength of the xanthan gum-stabilized soil is higher than that of the lignin-stabilized soil at 2% and 6% xanthan gum content, whereas at 4% lignin content, the strength of the lignin-stabilized soil exceeds that of the xanthan gum-stabilized soil. In addition, the difference in strength between the two types of stabilized soil decreases as the confining pressure increases.  
As the amounts of xanthan and lignin increase, the strength characteristics of the samples show significant changes. Therefore, the effectiveness of xanthan gum and lignin in improving gravel soil is evaluated by calculating the rate of increase in peak deviatoric stress \(D\) using Equation (1). The results are shown in Figure 6.  
\[D=\frac{p_{i}}{p_{0}}-1 \tag{1}\]  
Figure 4: Stress–strain curves in conventional triaxial tests on consolidated gravel soil.  
where \(p_{0}\) represents the peak deviatoric stress of the untreated gravel soil; \(p_{i}\) is the maximum deviatoric stress of the stabilized gravel soil.  
Figure 6 shows that a higher xanthan content results in a higher rate of increase in peak deviatoric stress, indicating a better stabilizing effect. At 4% lignin content, the rate of increase in peak deviatoric stress reaches a threshold, indicating that the stabilizing effect is optimal at this concentration. The increase in the growth rate of bias stress with increasing perimeter pressure indicates that the effect of xanthan gum and lignin on the strength of gravelly soils is also enhanced under conditions of increasing perimeter pressure. The weakening of the growth rate of bias stress suggests that at higher perimeter pressures, the interstitial space of the rock and soil particles decreases, resulting in a reduction in  
Figure 5: Peak bias stress as a function of xanthan gum and lignin admixture.  
Figure 6: Peak bias stress growth rate as a function of xanthan gum and lignin dosing.  
the amount of xanthan gum and lignin filling, leading to a weakening of the curing effect. Therefore, higher xanthan gum content results in better stabilization of gravel soils, while 4% lignin content provides the best performance. In addition, at higher confining pressures, the cementing effect of xanthan gum and lignin becomes weaker relative to the strength provided by the confining pressure, demonstrating a reduced influence of additive content on the shear strength of gravel soil.