A comparative study of the shear strength of a poorly graded soil using Bacillus subtilis, vetiver fibre and bamboo fibre

Biological soil improvement utilizing chemical and biological processes provides a potentially sustainable solution for enhancing soil stability. This study evaluates the impact of Bacillus subtilis , vetiver fibre and bamboo fibre on the shear strength of poorly graded sandy silt soil. Direct shear tests were conducted under normal stresses to analyze the mechanical performance of the treated soil. The shear strength of untreated soil (13 kPa ) significantly improved with treatments. Incorporating Bacillus subtilis and a cementation solution enhanced cohesion. The addition of vetiver fibre further increased cohesion, while bamboo fibre delivered superior results, achieving a shear strength of 42.8 kPa . The synergy between the bamboo fibre and the microbial treatment facilitated a 3.3-fold improvement in cohesion over untreated soil, attributed to calcite precipitation and enhanced interparticle bonding. These findings highlight the potential of combining natural fibres with microbial treatments for sustainable soil stabilization. • Bamboo fiber with MICP improved cohesion by 3.5 times (15.2 kPa). • Vetiver fiber also enhanced cohesion but was less effective than bamboo. • MICP treatment formed calcite bridges, enhancing soil stability. • Bamboo fiber's rough texture provided better mechanical interlocking. • Synergistic effect of fibers and MICP offers sustainable soil stabilization.