In the last decade, increasing attention has been paid to the study of biopolymers as an alternative for soil treatment. Compared to conventional binders such as cement and lime, the main advantages of biopolymers are their lower cost and significantly smaller carbon footprint. One of the most extensively studied biopolymers is xanthan gum, primarily due to its low price, availability and proven effectiveness. When added to soils, xanthan gum alerts their mechanical and hydraulic properties, perticularly enhancing their ability to retain water during periods of rainfall and drought. However, the main challenge associated with xanthan gum is its sensitivity to moisture. At low moisture contents, it acts as a binder between soil particles, whereas at higher moisture levels it forms a hydrogel, which reduces interparticle contact and may consequently decrease shear strength. The aim of this thesis was to investigate the effect of xanthan gum-based soil conditioners on the shear strength of soils, to determine how soil properties influence the effectiveness of the soil conditioner and to evaluate the influence of moisture on the drained shear strength of treated and untreated soils. Laboratory investigations were carried out on two different soils, one originating from Heldburg in Germany and the other from Catalonia in Spain. DDirect shear tests were performed on specimens prepared from untreated and treated soil at different moisture contents. The results presented in the tables and graphs show that moisture content has a significant influence on the effectiveness of biopolymers; under saturated conditions, the drained shear strength of the treated soils is lower than that of the untreated soils. The results were further analysed in detail and compared with findings from other studies and the available literature.
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