## 4 Main Conclusions  
Considering the experimental results of the unconfined compressive strength (UCS) tests and one-dimensional compression (oedometer) tests concerning the consequences of changes in the organic matter (OM) content (from 1.5% to 7.7%) on the behavior of  
Figure 8: Oedometer tests. Effect of organic matter content on the compressibility indices, Cc and Cs. Figure 9: Oedometer tests. Effect of organic matter content on the coefficient of consolidation (Cv) during the increments in compression stress (405 to 900 kPa). unstabilized and stabilized soils with a content of 1% of the biopolymer xanthan gum during 14 days of curing time, the following conclusions were reached:  
1. For an OM content in the range of 1.5-5.5%, there is a positive impact resulting from the stabilization with XG on the mechanical properties, with a significant increase in strength (from 130% to 270%) and stiffness (from 109% to 204%). Moreover, the stabilization with XG induces a decrease in the ductility of the stress-strain behavior characterized by a loss of strength after the peak;
2. There is a detrimental impact of the stabilization with XG in terms of mechanical properties for an OM content of 7.7%, while the stress-strain behavior remains ductile. These results seem to indicate that this OM content is sufficient to coat the soil particles, which prevents bonds (i.e., a micro-reinforcement network) being established between the soil particles and the XG, and simultaneously the hydration of the biopolymer hydrogels tends to minimize the friction between the soil particles;
3. The stabilization of the soil with XG induces an increase in the initial void ratio; this effect is enhanced by the increase in the OM content, which reflects the effect of two cumulative factors, the ability of the organic matter to retain water, and the hydration of the hydrogels of the XG in the presence of water;
4. The stabilization with XG increases the change in the void ratio both in compression (i.e., Cc) and swelling (i.e., Cs), with more significant effects in terms of compression; this behavior is amplified with the increase in the OM content;
5. In compression, the increase in the OM content induces an increase in the change in the void ratio (i.e., the Cc). The void ratio variation (and the Cs) is negligible for the swelling relative to the change in OM content;
6. The coefficient of consolidation (Cv) of the unstabilized soil decreases with the increase in OM content, which is due to the significant ability of the OM to absorb water;
7. The stabilization process induces a significant decrease in the Cv, which is related to the considerable capacity of the biopolymers to retain water, due to the hydration of the hydrogels. Considering the results obtained in this work, it is expected that this technique can be used to improve the properties of the soils with low/medium organic matter content (lower than 5%). Considering that biodegradability is inherent in the use of these biobased materials, it is expected that this methodology is suitable to apply in provisory works, mainly when a high level of sustainability is required. In this type of work, the biodegradability of the materials used can be a favorable issue, since it annuls all the traces of the work in long term. Conceptualization, P.J.V.O. ; Investigation: M.J.F.C.C.R. ; writing--original draft preparation, M.J.F.C.C.R. and P.J.V.O. ; writing--review and editing, P.J.V.O. ; supervision, P.J.V.O. ; funding acquisition, P.J.V.O. All authors have read and agreed to the published version of the manuscript. This work was partly financed by FCT/MCTES (project PTDC/ECI-CON/28382/2017) and through national funds (PIDDAC) under the R&D Unit Institute for Sustainability and Innovation in Structural Engineering (ISISE), under reference UIDB/04029/2020, and under the Associate Laboratory Advanced Production and Intelligent Systems ARISE under reference LA/P/0112/2020. Not applicable. Not applicable to this article.