Study on a macro–micro damage evolution model for plant-reinforced microplastic-contaminated tailings in seasonally frozen regions

To investigate the damage evolution of plant-reinforced microplastic-contaminated tailings under freeze–thaw cycles, this study designed four test conditions: pure tailings, microplastic–tailings, root exudate–tailings, and microplastics-contaminated tailings stabilized with root exudates. Freeze–thaw cycle tests, direct shear tests, and scanning electron microscope (SEM) image-based porosity analyses were conducted to establish an empirical freeze–thaw damage evolution equation under the tested microplastic content, root exudate condition, and freeze–thaw cycle range. At the macroscopic level, the results indicate that with increasing freeze–thaw cycles, the overall damage of tailings increases under all conditions; root exudates can partially delay damage development, whereas microplastics exacerbate cumulative degradation induced by freeze–thaw cycles. At the microscopic level, a pore structure disturbance index was constructed based on porosity changes, showing that freeze–thaw cycles cause pore rearrangement, expansion, and crack development in the tailings. Although microplastic addition results in relatively smaller pore structure disturbances, it weakens particle contact and skeletal continuity, whereas root exudates influence pore reconstruction through particle coating, bridging, and cementation. At the macro–micro level, a mapping relationship between macroscopic shear strength damage and microscopic pore structure disturbance was established, and the macro–micro damage influence coefficient β was derived for each condition. The results indicate that microplastics amplify the conversion of pore structure disturbances into shear strength damage, while root exudates mitigate the effect of pore disturbance on strength degradation, though they cannot completely offset the strength reduction caused by the combined effects of microplastics and freeze–thaw cycles. These findings provide a theoretical reference for ecological reinforcement and freeze–thaw damage assessment of microplastic-contaminated tailings in cold regions.