## 4 Test Results Analysis
### Analysis of Test Results of Lignin Fiber-Time Composite-Improved Soil
#### 4.2.4 Microstructure Investigations  
Figure 18 presents the cumulative pore distribution curves of the improved soil with various treatments. It is evident that within the pore size range of 0.1 to 2 \(\upmu\)m, as the pore size decreases, the total mercury intrusion porosity significantly increases. This indicates that a large population of pores primarily concentrated in this range occurs in the improved soils. The total intruded void ratio of the 2% lime-improved soil is the largest, followed by the composite-improved soil, with the 4% lime-improved soil having the smallest value. This indicates that the composite treatment of 8% lignin fibers and 2% lime actually reduces the porosity compared to the soil solely improved with 2% lime. A decrease in porosity also leads to reduced hydraulic conductivity, thereby impacting the permeability of the improved soil. This outcome is consistent with the observed reduction in hydraulic conductivity in the composite-improved soils, as presented in Figure 17. This interpretation is corroborated by the findings of [PERSON] and [PERSON] [43], who reported that under various freeze-thaw cycles, the porosity volume of fiber-lime composite-improved soils significantly diminishes relative to that of soils treated with lime alone across different dry densities.  
Figure 17: The relationship between the hydraulic conductivity and the curing days of the improved soils with different treatments.  
Figure 18: Cumulative pore distribution curves of the improved soils with different treatments.  
As illustrated in Figure 19, the average pore diameters of the improved soils are much smaller than that of the untreated soil. The peak values of the pore distribution curves shift to the left, with corresponding pore sizes of 2.49, 2.46, and 2.04 \(\upmu\)m, respectively. The pore size distribution densities at these peaks are 0.142, 0.179, and 0.218 mL/g, respectively.  
Figure 20 shows the proportions of various pores in the soils improved with the different treatments. In comparison with the untreated soil, the macropores in the 2% lime-improved soil and the 4% lime-improved soil increased by 17.5% and 18.1%, respectively, whereas those in the composite-improved soil improved soil increased by only 6.5%. In the case of the lime-treated expansive soils, the ion exchange reaction and flocculation process of the soil particles take place, leading to the generation of aggregates and large pores. As for the lignin fiber- and lime-improved expansive soils, the lignin fibers fill the pores, leading to a reduction in the quantity of macropores. This result is also consistent with the lower permeability observed in the composite-improved soil compared with that of the lime-improved soil.  
Figure 19: The pore density distribution curves of the improved soils with different treatments.  
Figure 20: Changes in the percentage of various pore types in the expansive soil improved by different treatments.  
As illustrated in Figure 21, the SEM observations reveal that the content of lignin fibers has an essential role in the microstructure of the treated samples. As for the soil treated with 2% lime and 2% lignin fibers, a more compact microstructure can be observed (as shown in Figure 21a), in comparison with the soil only treated with 2% lime (as shown in Figure 21a). That is, the appearance of lignin fibers provides more junctures but less porosity between particles. In the case of the sample treated with 2% lime and 8% lignin fibers, as illustrated in Figure 21c, more lignin fibers form a much denser microstructure.