Enzyme-induced carbonate precipitation stabilisation of a moderately gypseous soil

This study assesses EICP treatment using varied urea calcium urease formulations combined with a staged water addition protocol in a sand dominant soil with 29 wt. % gypsum and explores, within a single experimental matrix, the relative influence of externally supplied calcium versus gypsum-derived calcium on carbonate mineral precipitation, collapse mitigation, and short-term undrained strength development. Specimens were characterised using the Jackson pressure calcimeter method for net CaCO3, single oedometer collapse testing and unconsolidated undrained triaxial compression test. The net CaCO3 was between 1.081 and 3.138 wt. %, this range represents a low to moderate but mechanically meaningful carbonate precipitation level for granular soil biocementation. For untreated soil, most of the wetting-induced strain developed after inundation. EICP decreased collapse in the first hour by 38.0–58.6% and reduced Cp24 to 0.290–0.575%, and the corresponding improvement was measured at 55.8–77.7%, with Mix J showing the lowest potential for collapse when analysing the first 24 h. The triaxial results indicated that, the untreated soil behaved primarily as a frictional material, while EICP stabilised samples demonstrated increased apparent cohesion with only minimal changes to friction angle. SEM-EDS and XRD results further supported the mechanism by showing Ca-C-O enrichment, calcite-related reflections, and precipitation-associated bonding.