Effect of Freezing and Thawing on the Unconfined Compressive Strength of Soft Soil Stabilized with Waste-Derived Biochars

The stabilization of soft soils using nontraditional, environmentally friendly materials has gained considerable attention in geoenvironmental engineering. Among such materials, biochar, a carbon-negative by-product of pyrolysis of biological feedstocks, has shown promising potential. However, the mechanical performance of biochar-treated soils under adverse environmental conditions remains a key area of investigation for its broader adoption. This study evaluates the freeze-thaw (FT) durability of soft soil specimens stabilized with various biochars derived from sludge and biomass. Four distinct biochar types (BCAS, BC75∶25, BC50∶50, and BC25∶75) derived from copyrolysis of sludge and biomass were utilized to enhance the unconfined compressive strength (UCS) of soft soil. Specimens were prepared with varying biochar contents (0, 5, 7.5, 10, and 12.5%) and cured for 28 days before being subjected to FT cycles (0, 1, 2, and 3 cycles). The results indicated that, on average, the UCS decreased by 30% after 3 freeze-thaw cycles in untreated specimens, while specimens with 7.5% biochar exhibited only an 11% reduction in UCS across all biochar types. Notably, a compressive strength ratio (CSR) of 7 was achieved in specimens with 10% BCAS after 3 FT cycles. Additionally, biochar stabilization improved the height ratio and density ratio of the treated soils. Microstructural analysis revealed crack formation induced by FT cycles, contributing to the observed strength reduction. The findings offer valuable insights into the FT durability of biochar as a sustainable soil stabilizer, highlighting its potential application in cold regions and challenging climatic conditions.