## 3 Test Results and Analysis
### Effect of Biopolymer on Shear Strength of Granite Residual Soil
#### 3.2.2 Direct Shear Strength Parameters  
The relationship between peak shear stress (\(\tau\)) and vertical stress (\(\sigma\)) for GRS treated with biopolymers was analyzed and is presented in Figure 15. The results show that biopolymers significantly influence the shear strength of GRS. As vertical stress increases, the shear strength of the soil exhibits a linear increase. Furthermore, the content of biopolymers affects the magnitude of the shear strength improvement, with different concentrations producing varying degrees of enhancement. Additionally, different types of biopolymers demonstrate distinct effects on the strength of GRS.  
Figure 15: Relationship between peak shear stress and vertical stress of biopolymer-treated GRS: (a) XG; (b) LBG; and (c) GG.  
Based on the intercept and slope of the fitted line in Figure 15, the cohesion and internal friction angle of the soil were calculated and are shown in Figure 16. The shear strength index of soil treated with biopolymer at different contents is shown in the line chart in Figure 16, and the shear strength index increment between the current and previous contents is shown in the bar chart in Figure 16. The results indicate that different concentrations of XG (0.5%, 1.0%, 1.5%) progressively increased the soil's cohesion. At a 1.5% concentration, the cohesion reached 155.29 kPa, significantly higher than the GRS cohesion of 114.44 kPa. However, at 2% XG content, the cohesion decreased to 121.39 kPa, suggesting that beyond a certain threshold, the thickening effect weakens. Excessive XG may affect the bonding force between particles, reducing cohesion. The thickening effect of LBG is also notable. Within the range of 0.5% to 1.5%, cohesion steadily increased, reaching 153.15 kPa at 1.5%, which is comparable to XG. At 2%, however, LBG caused a slight decrease in cohesion, dropping to 143.32 kPa. This reduction is due to excessive LBG, which leads to over-bonding between particles, affecting water absorption and soil stability. GG outperformed both XG and LBG in thickening effect. Cohesion continuously increased within the 0.5% to 1.5% range, reaching 177.68 kPa at 1.5%. At 2%, GG caused only a slight decrease, maintaining a high cohesion of 176.76 kPa, indicating that GG has a strong and stable effect in enhancing cohesion.  
The incorporation of XG significantly increased the soil's internal friction angle. Within the addition range of 0.5% to 1.5%, the internal friction angle steadily rose from 25.5\({}^{\circ}\) to 35.54\({}^{\circ}\), demonstrating XG's role in enhancing the soil's shear strength and friction properties. However, when the XG content increased to 2%, the internal friction angle dropped to 26.34\({}^{\circ}\). This decline may be attributed to the high XG content disrupting the soil structure, reducing particle contact, and lowering the friction angle. Similarly, the addition of LBG improved the internal friction angle, reaching 34.61\({}^{\circ}\) at 1.5%. Like XG, LBG showed a slight reduction in friction angle at 2%, decreasing to 32.72\({}^{\circ}\). This could be due to excessive cementation, which reduces particle friction and lowers the internal friction angle. The incorporation of GG had the most significant impact on increasing the internal friction angle. At 0.5%, the internal friction angle rose to 31.38\({}^{\circ}\), and at 2%, it further increased to 39.01\({}^{\circ}\). This shows a consistent and stable improvement, indicating that GG is particularly effective in enhancing the soil's friction properties. In contrast, XG and LBG displayed reduced effects at the 2% concentration.  
Figure 16: Cohesion and internal friction angle of biopolymer-treated GRS: (**a**) cohesion; and (**b**) internal friction angle.