Microbially induced carbonate precipitation for soil improvement: Insights from a meter-scale radial grouting trial

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Despite the growing interest in microbially induced carbonate precipitation (MICP) for geotechnical applications, reports on meter-scale MICP trials for soil improvement remain limited, and controlling and predicting cementation efficiency on a large-scale is even more scarce. This study presented a meter-scale improvement of a poorly-graded sand (initial dry density: 1581 kg/m 3 , porosity: 40%) through MICP in a cylindrical cell (diameter: 1 m; thickness: 15 cm) using a radial flow injection strategy, which involves injecting fluids radially from a single well located at the center while maintaining a constant hydraulic head at the outer boundary. Nine cycles of a two-phase MICP treatment were applied: Phase 1- injection of 0.7 pore volumes (PVs) of bacterial solution and 1-L water pulse; Phase 2- injection of 1.4 PVs of 0.5 M cementing solution in two stages (i) 0.7 PV injection two hours after the bacteria were injected, and (ii) a further 0.7 PV injection the following morning after an overnight static reaction period. We observed non-uniform CaCO 3 precipitations along the distance from the central well and over the depth, which was induced by the decreasing flux towards the outer boundary under the radial flow pattern, along with influences from layered soil packing and hydraulically induced flow channels. CaCO 3 precipitation with distance from the central well follows a symmetric Gaussian-type distribution, with sufficient cementation to retrieve full-length cores occurring near the midpoint between the central well and the outer boundary. The unconfined compressive strengths of the full-length cores were in the range of 1.2-6.8 MPa with CaCO 3 contents of 0.08-0.17. Our study suggests that cementation level under radial flow conditions is controllable on a large scale and highly dependent on the injection volume of both bacteria and rinsing water pulse. The study provides a solid baseline for predicting and controlling CaCO 3 distribution in large-scale MICP soil improvement using a two-phase radial injection approach.