Complex conductivity signatures of microbial induced calcite precipitation, field and laboratory scales

Soil stabilization processes aim at enhancing soil's engineering properties. Although the concept is straightforward, it involves physical and chemical changes to the subsurface that could result in local environmental changes. Compared to conventional soil stabilization methods (such as cement grouting), bio-mediated soil stabilization, such as microbial-induced calcite precipitation (MICP), offers the opportunity to minimize environmental impact, but the underlying processes need to be well understood for proper applications. Accurate characterization and long-term monitoring arc paramount for the success of soil improvement, especially MICP treatments. Spectral induced polarization (SIP), an established geophysical method, has shown to be sensitive to MICP processes and products (e.g. calcite). In this work, we performed a two-phase study to explore SIP's suitability as a monitoring tool. Phase 1 involved a laboratory scale MICP study under controlled conditions and phase 2 a pilot field scale study. In the laboratory, MICP was induced through the introduction of ureolytic microorganisms, while in the field, indigenous soil microbes were stimulated to promote ureolysis. In both cases, traditional geochemical monitoring, along with spatiotemporally dense SIP monitoring, were performed. Over the course of the laboratory study, SIP successfully tracked the MICP progress as well as the calcite precipitation behaviour. Similarly, the SIP results of the field scale study showed to be sensitive to the subsurface changes in response to MICP. SIP offered spatiotemporally rich information on the MICP progress and process status. The similarity between observed signal trends in the laboratory and field in this study clearly proved that SIP signals from MICP in controlled laboratory environments can be successfully used to study field MICP applications despite scale and complexity differences.