## 4 Test Results Analysis
### Analysis of Test Results of Lignin Fiber-Time Composite-Improved Soil
#### 4.2.2 Direct Shear Test  
Based on the results of the unconfined compressive strength test for the composite-improved soils, it was determined that the optimum content of lignin fibers mixed into 2% lime-improved soil is 8%. Therefore, a direct shear test was conducted with four groups of soil samples for a comparative analysis: untreated soil, soil improved with 2% lime, soil improved with 4% lime, and soil improved with 8% lignin fibers and 2% lime. After sample preparation, tests were performed after 1, 3, 7, and 14 days of curing. Figure 11: The relationship between the lignin fiber contents and the compressive strength of the composite-improved soil. As shown in Figure 13, the results reveal that the addition of lime significantly increases the shear stress of the improved soil, which then experiences a rapid reduction after reaching its peak value. This shift leads to a change in the failure mode from plastic to brittle failure. This phenomenon can be attributed to the reactions between the lime and clay minerals, leading to the creation of cementation that fills the pores between soil particles and enhances their bonding force. Moreover, after the addition of 8% lignin fibers, an improvement in the ultimate strain corresponding to the peak strength was observed. As illustrated in Figure 14, the shear stress shows a linear increase with vertical stress, with the fitting line for the improved soils significantly surpassing that of the untreated soil. This indicates a substantial improvement in the shear strength of the soil. Simultaneously, the inclination angle of the fitting line increases, indicating a gradual increase in the internal friction angle of the improved soil. Figure 12: The relationship between the compressive strength and the curing days of the improved soils with different treatments. Figure 13: The relationship between the shear stress and the horizontal displacement of the improved soils with different treatments (200 kPa). The relationship between the cohesion and internal friction angle of the improved soils with different treatments at different curing ages is depicted in Figures 15 and 16. The cohesion of the improved soil with different treatments increases with the curing age. After 1 day of curing, the cohesion of the composite-improved soil reaches its peak. After 7 and 14 days of aging, the cohesion of the composite-amended soil reached 91.59% and 96.95% of the cohesion of the 4% lime-improved soil, respectively, and increased by 7.76% - 12.17% for the 2% lime-improved soil. This indicates that with smaller curing ages, the reaction between the lime and soil particles is insufficient. At this stage, the bridging and filling effects of the fibers in the soil are more pronounced, consequently increasing cohesion in the composite-improved soil. As the curing age increases, the cohesion of the lime-improved soil with a higher dosage gradually exceeds that of the composite-improved soil. Figure 14: The fitting curves of the shear stress and vertical stress of the improved soils with different treatments. Figure 15: The relationship between the cohesions and the curing days of the improved soils with different treatments. Figure 16 shows the changes in the internal friction angle over time for the different soil treatments. The internal friction angle of the composite-improved soil increased from 44.63\({}^{\circ}\) to 50.56\({}^{\circ}\). For the improved soil with 4% lime, the internal friction angle increased from 47.03\({}^{\circ}\) to 55.56\({}^{\circ}\). In contrast, the internal friction angle of the improved soil with 2% lime remained around 42\({}^{\circ}\), showing minimal variation but still higher than that of untreated soil. In the case of lime treatment, cementitious materials gradually fill the pores and bind the soil particles together, consequently increasing the soil strength. During curing, a continuous increase in strength can be observed because of the increasing quantity of cementitious compounds generated.