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The silt deposited in the Yellow River floodplains in China is weak and widely dispersed. In engineering practice, chemically modified agents such as lime and fly ash are frequently employed to enhance the hydro-mechanical behaviour of soils, which however generates significant amounts of CO 2. As an environmentally friendly soil improvement material, Xanthan gum has broad application prospects and is therefore being considered as a solidified material to improve silty soil, which meets the national strategy requirements of ecological protection and high-quality development in the Yellow River Basin. In this study, scanning electron microscope, WP4C apparatus, and an automatic direct shear test system were used to perform several microscopic, water-retention behaviour, and direct shear tests on Xanthan gum-treated silt in drying and wetting paths. According to test results, the differences in shear strength behavior were observed in drying and wetting paths. The average skeleton stress of untreated silt in the drying path is greater than that in the wetting path due to the hysteresis effect, and thus the shear strength in the drying path is higher than that in the wetting path. Xanthan gum-treated silt specimens in the wetting path first were cured at a relative humidity of 47% and temperature of 24°C for seven days, and then were wetted to different water contents. The microstructure of the Xanthan gum-treated silt in the wetting path became firmer, and thus the shear strength was higher than that in the drying path. There is also the hysteretic effect of Xanthan gum-treated silt. However, the curing age is the most important factor affecting this phenomenon. Our findings can serve as a scientific foundation for designing, constructing, and maintaining relevant geotechnical structures in the Yellow River flooded area. • The microstructure of Xanthan-treated silt is firmer than that of untreated silt. • Water retention characteristics of Xanthan-treated silt was obtained in wide suction range. • Shear strength of treated silt in drying and wetting paths was studied quantitatively. • Shear strength of untreated silt in drying path is higher than that in wetting path. • Shear strength of treated silt in wetting path is higher than that in drying path. [ABSTRACT FROM AUTHOR]

• Water retention characteristics of biopolymer-treated sandy soil in wide suction range. • The microscopic mechanism was explained. • The water retention capacity of treated sandy soil increases as the biopolymer ratio increases. • The water retention capacity of the Gellan gum-treated sandy soil is better than that of Xanthan gum-treated sandy soil. • Based on Fredlund-Xing model, a semi-empirical model was proposed for predicting the soil–water retention curve of biopolymer- treated sandy soil. Soil erosion in the Yellow River basin in China has seriously affected the stability of the foundations of civil engineering structures. Traditional treated materials used for preventing the soil erosion are often harmful to the environment. Biopolymers have been widely used for soil improvement because of their low cost, environmental friendliness, and feasible application. However, most studies on the water-retention behaviour of treated soils conducted to date have focused on the low or high suction range, while few have examined the behaviour over a wide suction range. Converting the saturated state of treated soils into unsaturated state until it is completely dry will lead to changes in the water retention behaviour of biopolymer-treated soils. Therefore, this study investigated variations in the water-retention behaviour and micromechanism of biopolymer-treated sandy soils over a wide range of suction. Specifically, the water-retention behaviour of treated soil was tested using a pressure plate apparatus in the low suction range and a WP4C apparatus in the high suction range. Scanning electron microscopy was also used to evaluate water-retention mechanism of soils treated with different ratios of biopolymer. Biopolymers were found to absorb water to form hydrogels and biofilms, which reduced the pore spaces between soil particles and improved the water storage ability of sandy soils. The water-retention capacity of both Xanthan gum and Gellan gum-treated sandy soils was higher than that of untreated sandy soils, and increased as the biopolymer ratio increased. The water-retention behaviour of the Gellan gum-treated sandy soil were better than that of the Xanthan gum-treated sandy soil. Finally, a prediction model of the soil–water retention curve that considered the biopolymer ratio was proposed based on the Fredlund-Xing model. This study provides a scientific basis for the application of biopolymers to control soil erosion in the Yellow River basin in China. [ABSTRACT FROM AUTHOR]