NBPT controlled microbially induced calcite precipitation for sustainable soil stabilization in transportation infrastructure

Microbially induced calcium carbonate precipitation (MICP) faces persistent obstacles in transportation subgrade stabilization because rapid ureolysis can trigger premature clogging and pronounced spatial heterogeneity. This study develops a construction-oriented biocementation strategy that uses the urease inhibitor N-(n-butyl)-thiophosphoric triamide (NBPT) to regulate reaction kinetics and improve treatment uniformity under groundwater-relevant conditions. A multivariate optimization framework was established by jointly evaluating inhibitor dosage, cementation solution concentration, biological-to-chemical ratio, ambient temperature, and pH buffering. The optimal formulation was achieved at 0.1% NBPT, which delivered an unconfined compressive strength of 2.53 MPa compared with 2.72 MPa for the inhibitor-free control, indicating that strength was largely preserved while reaction kinetics were moderated. Calcium carbonate deposition proceeded steadily over 72 h and reached a carbonate content of 11.2 kg/m³ under the optimized protocol. This mineral accumulation reduced hydraulic conductivity from 1.7 × 10⁻³ m/s to 6.4 × 10⁻⁵ m/s, corresponding to a 96.3% decrease, while avoiding localized pore occlusion. Under simulated AASHTO T307 cyclic loading, the optimized treatment achieved a resilient modulus of 152 MPa and limited residual deformation to 0.32 mm per 1000 load cycles at an 80 kN axle load. The proposed NBPT-controlled MICP framework provides an operational window that balances strength gain, hydraulic functionality, and field implementability for subgrade applications.