The coupling effects of wet-dry and freeze–thaw cycles on the mechanical properties of saline soil synergistically solidified with sulfur-free lignin, basalt fiber and hydrophobic polymer

Saline soil in semi-arid and seasonally frozen areas is highly susceptible to periodic evaporative freezing and thawing, leading to impacts on the physical properties and structure of the soil. This can have a significantly negative impact on engineering construction. Renewable and environmentally friendly composite materials (sulfur-free lignin, basalt fiber and hydrophobic polymer) were proposed to improve the physicochemical properties of saline soil. Based on the periodic changes in water and temperature in these regions, the coupling effects of wet-dry and freeze–thaw (WDFT) on saline soil was simulated by the experimental setup; consolidated undrained triaxial compression tests were carried out to further investigate the changing pattern of the strength characteristics of the untreated soil and the composite solidified soil. The results show that WDFT cycles have a deterioration effect on the mechanical properties of the soil; the deterioration stage mainly occurs in the first five cycles, while the trend of strength deterioration slows down after 10 cycles. The composite solidified material can significantly improve the strength of the saline soil, compared with the untreated soil. The rate of strength increase can reach up to 85.27% and the residual ratio of the failure strength is higher than that of the untreated soil. It also has a high resistance to coupling deterioration, with the solidification effect coefficient of the composite solidified soil being greater than 1 for different WDFT cycles. In addition, based on the hyperbolic model parameter fitting method, the effects of WDFT cycles and the confining pressure on the corresponding parameters were analyzed and the corresponding expressions were established. The stress–strain relationship can be better predicted by adopting the ultimate deviatoric stress σ1-σ3ult as the normalization factor.