# Application of Lignin for Slope Bioengineering: Effect on Soil Improvement and Plant Growth
## 2 Literature Review  
Effect of lignin on soil properties. Several studies have been conducted to understand the effect of lignin on the engineering properties of soil, including its strength. Table 1 summarizes the insights from these studies with a focus on the soil type, lignin concentration and the type of testing. A few well-documented studies are briefly discussed below. [PERSON] and [PERSON] [9] used lignin to stabilize clay soil and reported that the unconfined compressive strength (UCS) of lignin-treated clay improved significantly at a 5% lignin concentration. [PERSON] et al. [10] used lignin sulfonate and cement (0.6%, 1%, 2%) to enhance the strength properties of clays. The investigators noted that the UCS increased with lignin content while increasing cement content more than a certain point decreased UCS. [PERSON] et al. [11] showed that 3% of lignin increased the UCS of loess (silty soil) by about 180%. [PERSON] et al. [12] reported that the addition of lignin (1-2%) increased the mechanical strength of quarry sand by more than 60%. [PERSON] et al. [13] found that the California bearing ratio (CBR) of lignin-treated clay significantly increased after 7 days of curing age. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
\begin{tabular}{c} Sandy silt \\  & 0.5, 1, 2, 3, 4 \\ \end{tabular} 7–28 days & 7–28 days & TT & \(\bullet\) & UCS and E increase significantly with Lignin at 2\%. & Post-peak ductility of lignin-treated soil remained unchanged compared to untreated soil. & [PERSON] et al. [16] \\  & & & & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\ \end{tabular}
\end{table} 
Table 1: Summary of studies on the effect of lignin on soil properties. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Soil Type** & **Lignin Type** & \begin{tabular}{c} **Additive** \\ **Dosage (\%)** \\ \end{tabular}  & \begin{tabular}{c} **Curing** \\ **Period** \\ \end{tabular}  & **Test Method** & **Key Findings** & **Reference** \\ \hline Silty soil & \begin{tabular}{c} Lignin, quick \\ lime \\ \end{tabular}  & 2, 5, 8, 12, 15 & 0–60 days & UCT, CBR, Vs & \begin{tabular}{c} Lignin-treated silt soil \\ achieved optimum \\ performance at 12\% \\ lignin content and a \\ curing age of 28 days. \\ \end{tabular}  & \begin{tabular}{c} [PERSON] et al. \\ [PERSON] et al. \\ [PERSON] et al. \\ \end{tabular}  \\ \hline Silt soil & \begin{tabular}{c} Lignin, lime, \\ cement, fly ash \\ \end{tabular}  & 7, 10, 12, 14, 16 & 1–28 days & UCT & \begin{tabular}{c} UCS of lignin-treated \\ silt increased at a peak \\ of 12\% lignin and 28 days curing age. \\ \end{tabular}  & \begin{tabular}{c} [PERSON] et al. \\ [PERSON] et al. \\ [PERSON] et al. \\ days curing age. \\ \end{tabular}  \\ \hline Silty soil & \begin{tabular}{c} Sulfur-free lignin \\ \end{tabular}  & 3, 7, 10, 12, 15 & 1–60 days & UCT & \ \begin{tabular}{c} UCS increased five \\ times at 10\% lignin \\ content compared to \\ untreated soil after \\ 60 days. \\ \end{tabular}  &
\begin{tabular}{c} [PERSON] et al. \\ [PERSON] et al. In summary, the previous studies primarily dealt with the engineering properties of plastic clay or silt, while limited research was conducted on cohesionless materials such as sand and gravel. The majority of studies employed a series of UCS tests in which cylindrical specimens of soil were treated with lignin to obtain UCS. In these studies, the soil was thoroughly mixed with lignin solutions with a relatively low concentration of 1-5% and cured at a different time (mostly 7-10 days). Effect of biopolymers on vegetation growth. Vegetation is an essential part of the bioengineering of slopes as it creates a mesh structure with interconnected roots, forming a root-reinforced soil mass. Several studies, summarized in Table 2, have been conducted on vegetation growth in relation to slope stability [30; 31; 32], erosion control [33], dusting [30; 34; 35], microbially induced calcium carbonate precipitation [36] and vegetation [37; 38; 39; 34]. [PERSON] et al.