## 5 Conclusions  
In this study, a series of laboratory tests were carried out on untreated and lignin-treated specimens in order to obtain information on the potential improvement of the performance of the stabilised clayey soil. It is necessary to underline that the soil used in this investigation was obtained as a laboratory mix of two soils with different properties: a fine sand (A-3) and a clayey soil (A-7-6), respectively. The resultant mix was classified as A-6 (clayey soil). For the physical and mechanical characterization of both the natural and lignin-treated soils, a series of tests such as Atterberg limits, swelling behaviour, CBR, UCS, and ITS tests were carried out according to the standards. The test results were discussed and compared with previous studies. Furthermore, tests were carried out on the same clayey soil improved by the addition of lime, for comparison purposes. The comprehensive analysis of the various tests findings allowed us to draw the following main conclusions:  
* CBR tests carried out on specimens with three different types of curing showed that the addition of lignin improved the bearing capacity of the case study soil. In particular, after 7 days of air drying, the lignin-treated soil allowed recording an increase in the bearing of about 38% compared to the untreated soil. When the curing time included a soaking period of additional 4 days, the CBR values of the treated soil and untreated soil were approximately comparable. This latest result suggested that moisture conditions had an impact on the mechanical properties of the stabilised soil which lost the effect of lignin; thus, CBR values drastically decreased. * The comparison between lignin-treated soil and lime-treated soil highlighted that the highest values of CBR were observed for the 2.5% lime-treated soil; water immersion caused a reduction in bearing capacity for both treated soils but the lime treatment allowed recording the lowest decrease. This result translated into the significantly lower water sensitivity of the lime-stabilised soil compared to the lignin-stabilised soil. * The analysis of CII values recorded for the cured and soaked specimens suggested that lime addition continued to strongly impact the bearing capacity of the soil. 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. For this reason, the use of lignin must be carefully considered when wet soil conditions are present  
Figure 8: Comparison between the results of the ITS (daN/cm\({}^{2}\)) and UCS (N/mm\({}^{2}\)) tests carried out on the untreated soil, the lignin-treated soil, and the soil treated with lime. (e.g., in the presence of an aquifer, water infiltration into the pavement structure, wet environments). * The immersion in water as part of the curing of the samples for the CBR test allowed investigating the swelling behaviour of the stabilised soil. The test showed that the presence of lignin helped in controlling the swelling potential of this type of soil. In particular, the values of the swelling strain were lower than the values recorded for the lime-treated soil. For a comprehensive analysis, Atterberg limits, ITS, and UCS test results are reported, too. The approach proposed in this study contributes to highlighting the limits and effectiveness of the use of lignin as a non-traditional stabilizing agent of clayey soil road subgrades. These findings underline that the selection of a stabilizing agent and treatment cannot be generalised, given the dependency on site-specific conditions. For this reason, in future research, attention will be focused on:  
* the application of lignin with different types of extraction processes and sources;
* the application of clayey soils with different properties;
* other types of tests for the characterization of the stabilised soil.