Abstract: In recent years, biochar has been widely applied as a green and environmentally friendly soil modification material. After the addition to the soil, the physicochemical properties of biochar would be changed by the chemical environment of the soil, thus affecting the volume deformation characteristics of the modified soil. In order to explore the effects of chemically aged biochar on the swelling and compression characteristics of expansive soil, woody biochar was soaking into citric acid solution to simulate the chemical oxidation by plant organic acids, and its mechanism was explained by microstructural analysis (Scanning electron microscopy and Fourier transform infrared spectroscopy). The results show that: (1) The effect of aged biochar on soil swelling behavior is closely related to its particle size and dosage. When the particle size of biochar was fine (<0.25 mm), the enhanced promotion effect of aged biochar on the swelling of expansive soil was only observed in the presence of more biochar; while the enhanced promotion effect was occurred at low dosage with the increase of biochar particle size (0.25-1 mm); (2) The compressibility of biochar amended expansive soil is independent of the chemical aging of biochar, but related to the biochar dosage and particle size. Biochar could reduce the compressibility of expansive soil significantly, and this reduction becomes more pronounced with increasing biochar dosage and decreasing particle size; (3) Chemical aging affects the hydrophilicity of the expansive soil by changing the specific surface area and number of functional groups of biochar, thus the changed swelling properties of the soil; by contrast, the compressibility of the soil is more sensitive to changes in pore structure.

Key words: biochar, chemical aging, expansive soil, swelling behavior, com pression behavior