Experimental Study of the Mechanical and Durability Properties of a Collapsible Soil Treated with Biopolymers

Near-surface weak or collapsible soils pose significant challenges in construction and mobility. Conventional stabilization methods using cement and lime have a high carbon footprint, prompting the exploration of sustainable alternatives such as biopolymers. This study investigated the effects of xanthan gum (XG) and chitosan (CH) on the mechanical properties of a collapsible silty sand (SM). Unconfined compressive strengths (UCS) of SM-polymer mixtures were measured at varying remaining water contents (RWC) for two polymer concentrations (0.5% and 1% by the mass of dry soil). Additionally, durability under wetting-drying (W-D) cycles was assessed to address susceptibility to wetting-induced degradation, a critical concern for biopolymer-treated soils. Scanning electron microscopy (SEM) was used to observe microstructural changes. Results indicated that both XG and CH enhanced the UCS of SM-polymer mixtures with decreasing RWC. CH provided similar or greater strength gains than XG at RWC ≥ 4%, while XG outperformed CH at lower RWCs, achieving 72% higher strength at the lowest measured RWC. The enhanced strength of SM-CH mixtures at higher RWCs could result from electrostatic attraction between the positively charged CH and negatively charged silica soil particles. XG’s superior performance at lower RWCs showed its ability to aggregate fines by polymer filaments, overcoming the electrostatic repulsion between its negatively charged chains and the silty sand. Under unconfined W-D cycles, SM-XG and SM-CH mixtures lost 53% and 70%, respectively, of their initial strength after the first cycle due to polymer filament degradation during wetting and limited filament regeneration during drying. While SM-XG mixtures could not endure more than three unconfined W-D cycles, SM-CH mixtures resisted up to six cycles, maintaining a UCS of 180 kPa. These findings suggest that while XG offered higher strength at low RWC, CH demonstrated superior durability and resistance to W-D cycles, making it more suitable for short-term, near-surface applications with silty sand.