Effect of biochar and sisal fiber on compressive strength of expansive soil

The poor strength stability of expansive soils presents a significant threat to the integrity of engineering structures. This study proposes an eco-friendly stabilization approach combining biochar and sisal fiber composites as an alternative to conventional cement-lime treatments that carry environmental concerns. A systematic investigation was conducted through unconfined compressive strength (UCS) tests analyzed via response surface methodology (RSM), complemented by microstructural characterization using scanning electron microscopy (SEM). Three key factors influencing UCS performance were identified: biochar content, sisal fiber dosage, and fiber length. The RSM analysis revealed their relative significance in descending order as fiber content > biochar content > fiber length, with notable interactions between biochar and fiber dosage. The mechanical enhancement demonstrated gradual improvement with increasing biochar content, while optimal reinforcement occurred at 6‰ fiber dosage and 20 mm fiber length. Microstructural observations revealed two primary reinforcement mechanisms: fiber bridging effects and biochar-induced interfacial friction. Failure patterns predominantly correlated with fiber content variations. To model the mechanical behavior, a two-phase constitutive model was developed based on stress-strain relationships, effectively predicting deformation characteristics under loading conditions.