Influence of Bacillus subtilis on the Stabilization of Expansive Clay Soils in Chiclayo, Peru

Expansive clay soils exhibit significant shrink-swell behavior that affects the performance of shallow foundations and motivates the search for lower-carbon stabilization alternatives to lime and cement. The present study assessed the effect of Bacillus subtilis on stabilizing expansive clay soils from Monsefu and La Victoria Districts in Chiclayo, Lambayeque, Peru, intended as foundation soil. A non-ureolytic bacterial broth was inoculated into natural soil at 4%, 5%, 6%, and 7% by weight for comparison between natural and treated specimens through Atterberg limits, USCS classification, free-swell index, consolidated-drained direct shear, one-dimensional consolidation, X-ray fluorescence, X-ray diffraction (XRD), and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS). The optimal dosage was 5%, designated as SN5BBS. The mean plasticity index reduced from 34.6% to 14.8%, while the mean free swell index decreased from 108.9 to 62.7, moving the soil from a high expansion class to a medium expansion class with highly significant results (one-way ANOVA F=299.9, p<0.001). This treatment increased effective cohesion, friction angle, and unconfined compressive strength, while decreasing compression and swelling indices by increasing pre-consolidation pressure and overconsolidation ratio, indicating a stiffer and less compressible foundation soil. XRD showed a calcite phase (17.4%) in treated soil that was not present in natural soil, which is consistent with literature on microbial-induced calcite precipitation reported previously. Results support Bacillus subtilis biostabilization for local expansive foundation soils.