Mechanical response of biochar-amended saline soil under freeze-thaw conditions

Biochar is widely applied in the amelioration of saline soils; however, few studies have focused on the mechanical properties of biochar-amended saline soils. To evaluate the mechanical properties of biochar-amended saline soil in seasonally frozen regions, specimens with different salt and biochar contents were first subjected to unconfined compressive strength (UCS) tests, and the optimum biochar content was determined. Freeze–thaw cycling, UCS, NMR and SEM tests were then performed at the optimum biochar content to examine the strength attenuation, pore-structure evolution and microstructural damage mechanism of the biochar-amended saline soil. The results indicate that biochar effectiveness depends on both salt and biochar contents. Considering strength enhancement, applicability across different salt contents and material utilization efficiency, 5% biochar was identified as the optimum biochar content in this study. At this content, the peak strength declined continuously as freeze–thaw cycles increased, and the strength loss was generally greater at moderate-to-high salt contents. The maximum peak strength loss ratio reached 90.19% after 12 freeze–thaw cycles. The NMR and SEM results suggest that freeze–thaw cycling promoted the transformation of small pores into medium and large pores, together with crack propagation, aggregate loosening and interparticle separation. Therefore, the strength degradation of saline soil amended with 5% biochar under freeze–thaw cycles was closely associated with pore-structure coarsening and deterioration of microstructural integrity. These findings provide an experimental basis for the low-carbon stabilization of saline soil, freeze–thaw durability evaluation and the design of engineering fill materials in seasonally frozen regions.