## 3 Test Results and Analysis
### Effect of Biopolymer on Shear Strength of Granite Residual Soil  
#### 3.2.1 Shear Strength  
According to the results from the unconfined compressive tests, the sample strength peaked after 14 days of curing. Therefore, all samples were cured for 14 days before conducting the direct shear tests. The peak shear stresses obtained from the direct shear tests are presented in Figure 14. The shear strength of soil treated with biopolymer at different contents is shown in the line chart in Figure 14, and the shear strength increment  
Figure 12: Failure behavior of LBG-treated GRS in UCS test at different curing times: (**a**) 0; (**b**) 3; (**c**) 7; and (**d**) 14 days.  
Figure 13: Failure behavior of GG-treated GRS in UCS test at different curing times: (**a**) 0; (**b**) 3; (**c**) 7; and (**d**) 14 days.  
between the current and previous contents is shown in the bar chart in Figure 14. The variation in soil shear strength follows a trend similar to that of the UCS.  
Within the range of 0.5% to 1.5% XG content, a significant improvement in the shear strength of GRS is observed. Notably, the increase in shear strength is most pronounced at a 1.5% content. Specifically, under vertical stresses of 50 kPa, 100 kPa, and 150 kPa, the shear strength increases. At a vertical stress of 150 kPa, the shear strength reaches 265.3 kPa, marking an increase of 78.31 kPa compared to the untreated soil (186.99 kPa), which represents the highest increment. This indicates that XG has a strong cementing effect and can effectively enhance the soil's shear strength. As vertical stress increases, the cementing effect of XG becomes more pronounced. Especially at a vertical stress of 150 kPa, the overall structure of the soil stabilizes, allowing it to withstand greater shear forces. However, at a 2.0% content, the shear strength decreases. This decline is especially noticeable under vertical stresses of 100 kPa and 150 kPa. Excessive XG may lead to local aggregation within the soil, disrupting the optimal interaction between particles. This disruption can compromise the stability of the soil structure, resulting in a reduction in shear strength.  
In the application of LBG, the shear strength of GRS is significantly enhanced within the 0.5% to 1.5% content range, with the greatest improvement observed at 1.5%. The shear strength shows the most substantial increase under vertical stresses of 50 kPa, 100 kPa, and 150 kPa, particularly under 150 kPa, where it increased by 58.66 kPa, from 186.99 kPa to 245.65 kPa. LBG creates a binding network between soil particles, and as the content increases, this network strengthens, effectively binding the particles together and enhancing  
Figure 14: Shear strength of biopolymer-treated GRS: (**a**) XG; (**b**) LBG; and (**c**) GG.  
shear resistance. At 1.5% content, the LBG network is strong enough to resist shear forces, yielding the best performance. However, at 2.0% content, shear strength slightly decreases. Under vertical stresses of 100 kPa and 150 kPa, the shear strength at 2.0% is 8.59 kPa and 2.65 kPa lower than the shear strength at 1.5%, although still higher than untreated GRS. This suggests that excessive LBG content may negatively impact soil structure and reduce shear strength. Overall, LBG performance remains stable, peaking at 1.5%, with only a minimal decrease at 2.0%, indicating good stability.  
GG, in contrast, consistently improved the shear strength of GRS at all concentrations, with the most significant enhancement at 2.0%. As GG content increased, the shear strength of the soil continued to improve, aligning with previous research [38]. Notably, under 150 kPa vertical stress, shear strength at 2.0% GG showed a more significant increase compared to 50 kPa and 100 kPa, indicating that GG is more effective in enhancing shear strength under higher vertical stresses. Across all experimental conditions, 2.0% GG exhibited the best performance.