## 4 Test Results Analysis
### Analysis of the Results of Lignin Fiber-Improved Soil
#### 4.1.4 Microstructure Investigations  
As shown in Figure 7, treatment with lignin fibers significantly increases the total mercury intrusion porosity. As the lignin fiber content increases, the cumulative void ratio of the fiber-improved soil continues to increase. Figure 8 presents the pore size distribution curve of the expansive soil and the lignin fiber-improved soil. As shown in Figure 8, the pore size distribution of the fiber-improved soil exhibits a unimodal structure. After the addition of fibers, the peak value of the modal size shifts to the left, from 3.76 to 2.48 \(\upmu\)m, gradually. However, the quantities of these pore families with peak values increased due to the lignin fiber treatment.  
To further analyze the effect of the lignin fiber content on the pore distribution of the amended expansive soils, these pores were classified into two categories: micropores (<1 \(\upmu\)m) and macropores (>1 \(\upmu\)m).  
Figure 9 demonstrates that the proportions of macropores and micropores in the untreated soil were 27.2% and 72.8%, respectively. With the increase in fiber content, the proportion of macropores in the 2% lignin fiber-improved soil increased to 33.1%, while  
Figure 6: The relationship curve of the hydraulic conductivity and lignin fiber contents of the expansive soil improved by lignin fibers.  
the proportion of micropores decreased to 66.9%. The proportions of macroopes and micropores in the 8% lignin fiber-improved soil stood at 37.8% and 62.2%, respectively. The proportion of macroopes in the improved soil significantly increased by 5.9% to 10.6% compared with the untreated soil. This increase in macroopes is primarily attributed to the physical filling of lignin fibers in the micropores of the samples. In addition, lignin fibers can form aggregates, creating more macroopes. As a result, the increasing macroopes can increase the permeability of the treated soil. This is consistent with the results of the hydraulic conductivity tests above.  
Figure 10 presents a scanning electron microscope image of lignin fiber-improved soil at various dosages. As shown in Figure 10b, with a lignin fiber content of 2%, the fibers predominantly exist in the soil individually and connect with the soil particles. In this case, the tensile stress of individual fibers contributes to the increasing strength of the treated soil. As shown in Figure 10c, at a fiber content of 8%, the fibers intertwine in the  
Figure 8: The pore density distribution curves of expansive soil improved by different contents of lignin fibers.  
Figure 7: Cumulative pore distribution curves of expansive soil improved by different contents of lignin fibers.  
soil, forming a grid structure and exhibiting a bridging effect. As shown in Figure 10d, the fibers effectively fill the pores. Consequently, besides the effect of the tensile strength of the fibers, a three-dimensional enhancement effect is also superimposed. That is, lignin fibers not only fill the pores but also make the soil denser, thereby enhancing its strength and stability, leading to a largely improved shear strength in the case of the samples treated with a higher content of fibers. Note that, when an excessive amount of fibers is added, agglomeration among the fibers may occur, altering the particle connection structure and reducing the soil strength.  
Figure 10: Scanning electron microscopy of lignin fiber-improved soil: (**a**) lignin fiber content of 0% (500 times), (**b**) lignin fiber content of 2% (500 times), (**c**) lignin fiber content of 8% (500 times), and (**d**) lignin fiber content of 8% (300 times) (different magnifications for subfigure (**d**)).  
Figure 9: The percentage change in various types of pores in expansive soil improved by different contents of lignin fibers.