## 3 Results and Discussion  
### UCS Tests  
The stress-strain curves of the UCS tests for the unstabilized and stabilized specimens for the various organic matter contents are compared in Figure 3. Despite some scattering, the results clearly indicate a beneficial impact concerning the stabilization with 1% of XG for an OM content in the range of 1.5-5.5%, manifested by a significant increase in strength and stiffness. On the other hand, for a higher OM content (7.7%), a detrimental consequence of the stabilization with XG was observed, indicated by the decrease in mechanical properties (strength and stiffness). Moreover, for the range of OM content of 1.5-5.5%, the results also show that the stabilization with XG tends to decrease the ductility of the stress-strain behavior; in fact, the stabilized soil shows a peak strength followed by a decrease, while a continuous increase in the strength is seen for the unstabilized soil, which is characteristic of ductile behavior. In contrast, stabilized samples with an OM content of 7.7% show clear ductile stress-strain behavior, both for the unstabilized and stabilized soils.  
Considering the ductile stress-strain behavior exhibited by the majority of the samples tested, a failure criterion that corresponds to an axial strain (\(\varepsilon_{\text{ax}}\)) of 20% is used to quantify the unconfined compressive strength (q\({}_{\text{u}}\)). The evolution of q\({}_{\text{u}}\) with the OM content, depicted in Figure 4a, highlights the features previously described, namely, a significant increase in the unconfined compressive strength with positive gains between +131% and +270%, for soils with an OM content of 1.5% and 5.5%, respectively. The opposite of this behavior is seen for the soil with an OM content of 7.7%, described by a loss of strength higher than 50% after stabilization with XG.  
Figure 3: UCS tests. Effect of organic matter content on the stress-strain behavior: (**a**) OM = 1.5%; (**b**) OM = 2.4%; (**c**) OM = 4.1%; (**d**) OM = 5.5%; (**e**) OM = 7.7%.  
The evolution of the secant modulus at 0.5 qu (E\({}_{\text{u}50}\)) with the OM content (Figure 4b) matches with the trend observed in Figure 4a for qu. In fact, the stabilization with XG induced a significant gain of stiffness (from about 109 to 204%) for OM contents in the range of 1.5-5.5%, while for an OM content of 7.7% a reduction of 29% was obtained.  
These results indicate that the process of stabilization with XG can be used successfully for soils with a low organic matter content (less than 5.5%), but is counterproductive for higher values of OM content (7.7%). Indeed, after a certain threshold value of OM content (7.7%), the OM that exists in the soil tends to coat the soil particle and partially fill the voids. This prevents the creation of a network of micro-fibers of XG that would create bonds between the soil particles. Simultaneously, the hydration of the biopolymer's hydrogels that exist in the soil's porous spaces seems to promote a reduction in the friction between the soil particles, inducing a decrease in the mechanical properties of the stabilized soil. The SEM image of the stabilized soil with an OM content of 7.7% (Figure 5b) confirms a greater coating of the soil particles with OM than that observed with a stabilized soil with an OM content of 1.5% (Figure 5a). Thus, the increase in the OM content makes it difficult to establish a network of bonds, similar to the micro-reinforcement observed in a stabilized non-organic silty sand (Figure 1).  
Figure 4: UCS tests. Effect of OM on the evolution of: (**a**) q\({}_{\text{u}}\); (**b**) E\({}_{\text{u}50}\).