Wetting–drying effects on the mechanical performance of xanthan gum biopolymer-stabilized soil

Xanthan gum biopolymers have gained increasing attention in geotechnical engineering due to the effectiveness and environmental-friendliness, and are proposed as a potential alternative to conventional materials for soil stabilization. Cyclic wetting and drying are the crucial factors that affect the behavior of surface soil, which are also a major challenge for biopolymer applications. This study aims to investigate the strength durability of xanthan gum-treated soil during wetting–drying cycles. The soil was treated with different contents of xanthan gum (0, 0.5, 1.5% by the mass of dry soil) and a total of 12 wetting–drying cycles were applied. Unconfined compression tests were performed to evaluate the changes in soil mechanical properties. The changes in microstructure were observed using nuclear magnetic resonance technology and scanning electrical microscopy. The results showed that soil mechanical properties decreased significantly in the first four cycles, and then tended to equilibrium. The compressive strength of soil treated with 1.5% xanthan gum could be approximately twice than that of non-treated soil after 12 cycles, and its strength reduction caused by wetting–drying cycling is about 20% less than that of the latter. When increasing the water content at drying stage, specimens subjected to wetting–drying cycles with less moisture change presented higher compressive strength, in which case the effectiveness of biopolymer treatment can be maximally retained. Xanthan gum treatment conferred great resistance to wetting–drying cycling due to its cementation and aggregation effects. The presence of xanthan gum leads to more inter-aggregate pores with a radius of about 0.1–1 μm and limits the development of macropores. The strengthening effect of xanthan gum depends on direct clay particle–biopolymer interactions and inter-particle connection formed by xanthan gum matrix. From the results, xanthan gum biopolymers can significantly improve the mechanical properties of soil at shallow depth even after wetting–drying cycles.