## 4 Test Results Analysis  
### Analysis of the Results of Lignin Fiber-Improved Soil  
#### 4.1.1 Unconfined Compressive Strength Test  
Figure 3 demonstrates that the addition of an appropriate amount of lignin fibers can increase the compressive strength of expansive soil. The unconfined compressive strength of the improved soil initially increases with the increasing lignin fiber content, reaching a maximum at 8%. The addition of 2% to 10% lignin fibers increased the unconfined compressive strength by 87 kPa, 127 kPa, 157 kPa, 191 kPa, and 163 kPa, respectively. As the lignin fiber content increases, the slope of the stress-strain curve also gradually increases, indicating that lignin fibers contribute to the improvement of the elastic modulus of expansive soil. After the addition of lignin fibers, the stress-strain curve of the improved soil exhibits stress softening, indicating that it does not alter the stress-strain characteristics of the expansive soil. At 2% lignin fiber content, lignin fibers are mainly present in the soil alone, and the effect on compressive strength is limited. As the lignin fiber content increases, the lignin fibers not only weave together to form a network structure but also effectively fill the pores between the soil particles. When the lignin fiber content reaches 10%, there are too many lignin fibers that cannot be evenly distributed in the soil and tend to agglomerate. As a result, a significant reduction was observed in soil strength and stability. This behavior is aligned with findings by [PERSON] et al. (2020), who reported that relative humidity impacts the Young's modulus of lignocellulosic polymers and their adhesion with cellulose nanocrystals at the nanoscale, providing a microstructural explanation for the variance in performance observed at different lignin fiber concentrations [36].  
Figure 3: The relationship curves between the axial stress and axial strain of the expansive soil improved by different contents of lignin fibers.  
\begin{table}
\begin{tabular}{l l l l} \hline
**Soil Type** & **Lignin Fiber (LF)/(\%)** & **Lime/(\%)** & **Curing Age/(d)** \\ \hline Prime expansive soil & - & - & - \\ Lignin fiber-improved soil & 2/4/6/8/10 & - & - \\ Lime-improved soil & - & 2/4 & 1/3/7/14 \\ Composite-improved soil & Optimum dosage of & 2 & 1/3/7/14 \\  & lignin fiber & & \\ \hline \end{tabular}
\end{table}
Table 3: Basic information on soil samples.  
As shown in Figure 3, the strain at the maximum stress initially increases and then decreases with the increase in lignin fiber content, peaking at 8%. The strain at the maximum stress for the other doped soil samples exceeds that of the untreated soil, indicating that lignin fibers enhance the ductility of expansive soils. A moderate fiber content improves the toughness and strength of the soil by providing a physical bridging effect, which increases soil cohesion and crack resistance. [PERSON] and [PERSON] [37] also observed that the enhancement of unconfined compressive strength in clay soils through basalt fiber (BF) additions is effective only to a certain limit. Specifically, a 4% BF reinforcement raised the qu value by 29.35% compared to unreinforced clay, but a further increase to 5% BF resulted in a slight reduction of 2.02%. This indicates that more fibers can lead to a greater fiber concentration, which results in the formation of aggregates, as also observed by [PERSON] et al. in their study on the shear strength of loess improved with lignin fibers, where a high fiber content led to fiber agglomeration [38]. Similarly, this decline is attributed to fiber aggregation caused by electrostatic interactions at higher fiber concentrations, which prevents even dispersion and diminishes the interface effectiveness between the fibers and the soil [39].