Microbially induced calcite precipitation for near-surface soil stabilization: Influence of injection strategy and treatment configuration across scales

This study evaluates the effectiveness of microbially-induced calcite precipitation (MICP) as a bio-cementation strategy for soil stabilization in shallow soils without direct access to the soil surface (e.g. beneath pavements or other surface-impermeable infrastructure), with emphasis on injection method and treatment scale-up. The objective was to assess how different delivery strategies and solution formulations influence calcium carbonate distribution and mechanical improvement in treated soils. Experimental testing was conducted at two scales: small-scale (15-cm diameter) columns to investigate injection layout and media chemistries, and an intermediate-scale (30-cm-by-30-cm) specimen to explore spatial uniformity and scale-up effects. Two injection methods, top-down (TD) and injection well (IW), were applied using both laboratory-grade and commercially sourced nutrient recipes. Strength was assessed through California bearing ratio (CBR) testing, and calcium carbonate precipitation was quantified via acid digestion across multiple depths. The CBR test was selected as a practical and minimally invasive method to evaluate strength improvements across multiple depths and locations, and it correlates well with dynamic cone penetration (DCP) test results for assessing near-surface strength in confined systems. Results demonstrated that the IW method improved deeper treatment while TD method enhanced surficial bonding. It was observed that commercial yeast-extract-based media produced comparable or greater strength improvements than traditional lab-grade solutions. CBR values increased from 11% to as high as 188%, indicating the potential of MICP for in situ strengthening of soil. While findings support the viability of MICP for scalable ground improvement, limitations related to sample replication and uniform fluid distribution in larger specimens warrant consideration.