## 3 Materials and Methods
### Tests and Methods  
In this study, three different lignin contents were selected based on the quantities suggested by the supplier; 0.25%, 0.40%, and 0.55% of lignin were added to the clayey soil (resultant mix S12 = S1 + S2) as a percentage of the optimum water content determined before (see Table 2). The type of lignin used in the experiment required dilution before mixing as it was in a concentrated condition. Part of the required optimum water content was used to ensure uniform mixing. The optimum content of lignin was selected based on the additive quantity that allowed recording both the maximum dry unit weight and the highest California bearing ratio (CBR) value. For the preparation of specimens, the soil was mixed with lignin at specific percentages and optimum water content. The mixture was allowed to cure for 12 h to ensure better interaction between water, lignin, and soil materials. The samples for testing were prepared using the standard Proctor compaction test. Each test was carried out on three compacted specimens in accordance with the European Standard (EN 13286-47:2012) [76]. As reported above, these test findings allowed defining the optimum lignin content. The lignin content which optimises CBR values tested after 0 days was 0.40% by optimum moisture content. The sequence of planned tests was carried out on the soil mixture treated with this additive rate. The mechanical performance of the 0.40% lignin-treated soil was assessed by the California bearing ratio (CBR). Two different conditioning procedures were adopted before the CBR test was performed. The air drying curing time was set at 7 days for both procedures, but the second test implied a wet test procedure: the specimens were completely immersed in a water bath for additional 4 days. Water sensitivity ratio (WSR\({}_{\text{ar}}\)) is defined as follows:  
\[\text{WSR}_{\text{ar}}\left[\%\right]=\frac{\left(\text{CBR}_{\text{ad}+\text {s}}\right)_{\text{ar}}}{\left(\text{CBR}_{\text{ad}}\right)_{\text{ar}}} \tag{1}\]  
where CBR refers to different curing periods: CBR\({}_{\text{ad}+\text{s}}\) is the CBR value obtained after a curing period involving both dry and wet conditioning (7 days of air drying and 4 additional days of soaking in this study); CBR\({}_{\text{ad}}\) refers to the test results obtained after only dry conditioning (7 days). The subscript \"ar\" after \"WSR\" indicates the specific additive rate of the tested soil specimens. This index allows a mechanical evaluation of the potential loss of bearing of the soil due to moisture conditions. Figure 4: LDI-MS of protonated lignin oligomers. The full saturation test allowed recording useful information about the swelling behaviour of the lignin-treated soil as established by the reference standard EN 13286-47:2012 [76]. The swelling strain \(\varepsilon\) (%) was estimated as follows [77]:  
\[\varepsilon= \frac{100\Delta\mathrm{h}}{\mathrm{h}_{1}} \tag{2}\]  
where \(\Delta\mathrm{h}\) is the change in specimen height due to absorption of water (mm) and \(\mathrm{h}_{1}\) is the initial specimen height (mm). Unconfined compressive strength (UCS) (EN 13286-41:2006) [78] and indirect tensile strength tests (ITS) (EN 13286-42:2006) [79] were carried out to measure the effects of lignin on the compressive and tensile strength of the claye soil. The specimen dimensions were 10.00 cm in diameter and 11.66 cm in height and they were compacted by the standard Proctor compaction method for both UCS and ITS tests. Three specimens were prepared for each test and the average was obtained. The curing time was fixed at 7 days for both tests. The indirect tensile strength is calculated as follows [79]:  
\[\mathrm{ITS}=\frac{2\mathrm{P}}{\pi\mathrm{DH}} \tag{3}\]  
where P is the peak load (daN), D and H are the diameter (cm) and the height (cm) of the specimen, respectively. Afterberg limits (EN ISO 17892-12:2018) [80] were examined for the treated soil and the results were compared with the values obtained for the untreated soil.