## 4 Results and Discussion
### Comparison between the Lignin-Treated Soil and the Lime-Treated Soil  
In this section, the results of tests carried out on the lignin-treated soil are compared with those obtained when the same soil was treated with lime. Table 6 shows the data relating to CBR values obtained for each different type of stabilisation treatment. Comparing the lignin-treated soil with the natural soil, as noted in Section 4.1, the lignin treatment allowed recording an improvement in bearing characteristics in every type of curing conditions. However, when lignin effects on bearing are compared with those recorded for the lime treatment, the highest values of CBR were observed for the 2.5% lime-treated soil; the measures were significant for CBR tested after 7 days of air drying (CBR = 39.1%). Water immersion caused a reduction in the bearing capacity for both treated soils, but the lime treatment allowed recording the lowest decrease. This result is confirmed by the WSR\({}_{\text{ar}}\) values: WSR\({}_{2.5\%}\) = 57.5% for the 2.5% lime-treated soil whereas WSR\({}_{0.4\%}\) = 13.5% for the 0.4% lignin-treated soil; this result translated in the significantly lower water sensitivity of the lime-stabilised soil compared to the lignin-stabilised soil. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Lignin Content (\%)**} & \multicolumn{3}{c}{**Atterberg Limits**} \\ \cline{2-4}  & **Liquid Limit (LL)** & **Plastic Limit (PL)** & **Plasticity Index (PI)** \\ \hline
0\% (untreated soil) & 38 & 20 & 18 \\
0.40\% & 41 & 21 & 20 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Afterberg limits recorded for the untreated soil and the 0.40% lignin-treated soil. Considerations on the CII and \(\Delta\)CII values must be shown for each set of results in relative terms, taking into account the particular conditions determined by different curing procedures. Table 6 reports the CBR, CII, and \(\Delta\)CII values related to two confined cases. In terms of moisture content, the first case (0 days of curing) refers to the optimum moisture content (\(w_{\text{opt}}\)); CII values obtained for this case suggest that both lignin and lime contributed to the increase in bearing capacity of the untreated soil in a measure of about 60% and 80%, respectively. The difference in the CBR increase between the lime effect and the lignin effect was about 20%. Comparing the first case with the second one (7 days of air drying + 4 days of soaking), the second case was considered to be the worst condition. Water absorption rate depends on the type of clay. For the case study soil, illite, the predominant clay mineral, is characterised by a water absorption rate that ranges between 70% and 150% [81]. As a consequence, full saturation may represent an issue in clayey soil road subgrades. Analysis of the CII values recorded for the cured and soaked specimens suggested that lime addition continued to strongly impact the load-bearing capacity of the soil: CII increased from 82% to 90%. CII obtained for the cured and soaked lignin-treated soil specimens was about two times lower than CII obtained for the first case: these findings underline that when lignin-treated clayey soil is exposed to wet conditions, its effect on increasing the bearing capacity dramatically decreases. In the second case, the value of \(\Delta\)CII, which was more than 50%, reflected the poor ability of lignin to upgrade the bearing capacity of the soil in wet conditions compared to lime. Furthermore, the values of the swelling strain \(\varepsilon\) show that lignin seems to be the most adequate stabilizing agent for the control of volume changes of the investigated soil. The \(\varepsilon\) value recorded for the soil treated with lime on the fourth day was 1.6% against the value of 1.4% observed for the lignin-treated specimens. Finally, for a comprehensive analysis, Figure 8 reports the comparison between each different soil treatment for ITS and UCS tests. It is easy to note that the values were comparable for both tests and for each type of treatment.