Effect of fine-grained wood biochar on the geotechnical and microstructural behaviour of expansive clay as pavement subgrade

Biochar, a solid carbonaceous material produced by heating biomass in oxygen-free or low-oxygen conditions (pyrolysis), has been used in various applications, including wastewater treatment, carbon sequestration, and improving soil fertility. However, very limited research has been performed to explore its feasibility to improve the expansive clay (EC) subgrade. In this study, fine-grained wood biochar derived from wood waste was used to stabilise and enhance the mechanical performance of the EC as road subgrade. A comprehensive series of geotechnical tests, including unconfined compressive strength (UCS), California bearing ratio (CBR), repeated load triaxial (RLT), and swelling-shrinkage tests, were conducted to investigate the engineering properties of expansive clay mixed with different contents of the fine-grained biochar (FGB) (i.e. 0, 1%, 2%, 3%, and 4% by weight of dry soil). Furthermore, X-ray diffraction (XRD), X-ray fluorescence (XRF), X-ray micro-CT, and thermogravimetric analysis (TGA) analyses were performed to study the microchemical modification of the EC-FGB mixtures. The results showed that adding FGB reduced the swelling and shrinkage potential while enhancing the mechanical properties of the EC. The micro-level analysis also supported the enhancement of the geotechnical performance of the EC resulting from the incorporation of FGB. According to the test results, 2% FGB was considered the optimum content, increasing UCS, CBR, and resilient modulus by 31.1%, 24.1%, and 31.5%, respectively, and decreasing the swell-shrinkage index by 7%.