Hydrological properties of biochar-amended expansive soil under dynamic water environment and biochar-amended soil’s application in green roofs

Biochar is widely used as a soil amendment in green roofs because it can increase the soil’s water-holding capacity, inhibit cracking, and provide nutrients for vegetation. The interaction mechanism between biochar and expansive soil is a significant factor in the performance of green roofs. However, research mainly focused on the hydrological scale (i.e. runoff regulation and rainwater storage), and there is a lack of research on the hydrological properties (i.e. hysteresis of soil–water characteristic curves, changes in soil pore size and pore distribution, formation and development of cracks and swelling–shrinking characteristics) of biochar-amended expansive soils under dynamic water environment. This test was conducted out on soil columns with expansive soil and 5% (by mass) biochar-amended under 5 drying–wetting cycles. The results showed that within the range of water content suitable for plant growth (i.e. between the anaerobic point and wilting point), the hysteresis area of soil–water characteristic curves of the amended soil is smaller than the bare soil at each drying–wetting cycle. The addition of biochar changes the pore size distribution of the soil, reduces the pore size and swelling–shrinking characteristics of the expansive soil, and inhibits the development of cracks. Therefore, the addition of biochar significantly attenuates the hysteresis characteristics of the soil–water characteristic curves of expansive soils, which may improve the stability of the hydraulic performance of green roofs. The results contribute to the understanding of the physicochemical and water properties of biochar-amended expansive soil and provide theoretical support for the application of biochar to green roofs.