Mechanical and microstructural improvements in expansive soils treated with nano-modified biochar

Expansive clays cause cracking and heaving in pavements. This study employs a novel combination of bagasse biochar and nano-silica to increase strength and mitigate the swelling behaviour of expansive soil. Blends were prepared using combinations of bagasse biochar (0–12%) and nano-silica (0–2%) to stabilise the soil. The optimal blend, 1.33% nano-silica with 8% bagasse biochar, enhanced unconfined compressive strength from 300 kPa to 920.8 kPa and reduced the 24 h swell index from 5.55% to 1.65%. Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) patterns provided supportive evidence of attenuation in peaks associated with montmorillonite due to the addition of nano-modified biochar. Scanning electron microscopy (SEM) imagery illustrated a denser gel-bridged structure with fewer shrinkage cracks. Factorial ANOVA with Tukey–Kramer confirmed notable interactions among soil, nano-silica, and biochar. Pavement design analysis showed that the treated subgrades could support thinner sections while meeting the adopted design criteria.