## 4 Results and Discussion  
### Effects of Lignin on Soil Mix S12  
#### 4.1.1 California Bearing Ratio (CBR) Test with Dry and Wet Conditioning  
As discussed in the previous section, CBR was first assessed without storage conditions between manufacturing and testing of the specimens. The test results reported in Figure 6 allow assessing how different lignin contents affect soil strength. Each bar in the graph indicates the CBR value (%) obtained by the mean of the values of two specimens prepared with the same additive dosage. The horizontal axis indicates the corresponding percentage of lignin content (0%, 0.25%, 0.40%, and 0.55%). Comparing the treated soil with the untreated soil, it is evident that the lignin addition increased the CBR value by more than 70%. Moreover, the findings highlight that the highest value of CBR tested immediately could be recorded for the 0.40% lignin-treated soil. An increase in the amount of the additive (0.55%) translated into a decrease in mechanical properties. For this reason, the additive rate of 0.40% was selected as the optimum in this study.  
Figure 5: Summary of the experiment design and main activities.  
In addition, CBR was tested after air-drying the material for 7 days to investigate the effects of the curing time on mechanical performance. CBR data were collected for specimens tested after dry and wet conditioning. Table 3 reports CBR values for dry and wet tests, respectively. The wet test refers to values obtained for specimens submerged in water for 4 days at the end of the seventh air drying day and before testing.  
It can be seen from Table 3 that the highest CBR value was recorded for the lignin-treated soil after 7 days of curing; this implies that the addition of 0.40% lignin contributed to upgrading the mechanical performance of the soil. Compared to the untreated soil, the presence of a stabilizing agent allowed obtaining an increase of about 38% in the average value of CBR after 7 days of curing. Moreover, the increase in CBR values is noticeable comparing the results obtained after curing for 7 days with the above-discussed CBR values. These results confirm the literature findings [58; 60].  
When the curing time includes a soaking period of additional 4 days, the CBR values of the treated soil and untreated soil are approximately comparable. This last result suggests that 4 days after soaking, the stabilised soil lost the effect of lignin on its mechanical properties; thus, the CBR values drastically decreased.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**CBR (\%)**} \\ \cline{2-4}
**Lignin Content (\%)** & \multicolumn{3}{c}{**Curing Time**} & **Water Sensitivity** \\ \cline{2-4}  & **7 Days of Air Drying** &
\begin{tabular}{c} **7 Days of Air Drying** \\ **+ 4 Days of Soaking** \\ \end{tabular} & **Ratio (\%)** \\ \hline
0\% (untreated soil) & 18.2 & 2.2 & 12.1 \\
0.40\% & 25.1 & 3.4 & 13.5 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Effect of curing time on CBR values for the untreated soil and the lignin-treated soil, respectively.  
Figure 6: CBR values recorded for the untreated soil and the lignin-treated soil, respectively.  
The values of the water sensitivity ratio suggest that, for the specific case study, the untreated soil and the lignin-treated soil had similar moisture susceptibility (water sensitivity ratio was about 12-13% for both conditions).  
This result confirms the inadequate contribution in terms of bearing provided by lignin in wet conditions.