Durability mechanisms of MICP-treated granite residual soil under wetting–drying cycles

Biogeotechnics provides a sustainable engineering alternative for soil reinforcement, yet the long-term durability of microbially induced calcium carbonate precipitation (MICP) in granite residual soil remains insufficiently understood. This study investigates the influence of wetting–drying (W–D) cycles and curing times on the mechanical behaviour and microstructural evolution of MICP-treated granite residual soil through integrated mechanical, mineralogical, and pore-scale analyses. The results show that W–D cycles progressively reduce strength, with unconfined compressive strength (UCS) and peak deviatoric stress decreasing by 10.3% and 12.9% every 4 cycles, respectively, while the secant and shear moduli undergo the most severe deterioration. Nevertheless, after 20 cycles, the treated specimens still exhibited significantly higher UCS and peak deviatoric stress than the untreated soil. Longer curing substantially improves durability, with strength tending to stabilise after 9–12 days due to sustained calcite precipitation, which strengthens interparticle bonding and improves resistance to W–D-induced degradation. Microstructural characterisation suggests that pore enlargement, calcite dissolution, and feldspar disintegration are major contributors to cementation loss and strength decline. These findings help clarify the degradation mechanisms of biocemented granite residual soil and provide a useful reference for the design and long-term performance assessment of MICP-based soil improvement strategies from a sustainable engineering perspective.