Exploring implication of variation in biochar production on geotechnical properties of soil

Biochar produced from the pyrolysis of plant-based feedstock has been advocated as an alternative soil amendment for landfill cover. Previous literature indicated that the pyrolysis temperature influences the intra-pore distribution and surface functional groups (especially hydroxyl groups), resulting in “love-hate relationship” of the biochar-amended soil (BAS) with water. From the purview of geotechnical engineering, the effects of pyrolysis temperature on geotechnical properties are rarely investigated. In total, three biochar rates (0, 5, and 10%) were considered for a set of geotechnical experiments in sand clay mixture soil with biochar produced at 350°C and 550°C. Test results show that biochar addition in soil, in general regardless of pyrolysis temperature, increased the optimum moisture content (OMC), plasticity index, and soil water retention characteristics (SWRC) and decreased the maximum dry density (MDD), shear strength parameters (cohesion, friction), and erosion rates. Whilst comparing the pyrolysis temperature effects on two biochar-amended soils, only marginal effects (in terms of magnitude) on SWRC were observed. The most significant decrease of MDD (or increase of OMC) for 5% (w/w) and 10% (w/w) biochar additions occurred at pyrolysis temperatures of 550 °C and 350°C, respectively. In addition, biochar produced at lower pyrolysis temperature (350 °C) was more effective in reducing cracks and enhancing shrinkage area ratio. Ten percent of biochar addition with pyrolysis temperature of 350 °C was the optimum combination in resisting soil erosion. The study provides evidence that the geotechnical properties of biochar-amended soils for landfill cover soil applications could be tailor made by controlling the pyrolysis temperature.