Bio-Mediated Soil Improvement: Load Transfer Mechanisms at the Micro- and Macro- Scales

New opportunities for utilizing biological processes to modify the engineering properties of the subsurface have recently emerged at the interface of microbiology, geochemistry, and civil engineering. This paper presents an overview of bio-mediated soil improvement systems in the context bio-mediated calcite precipitation of sands. Micro-scale and macro-scale investigations of microbial-induced calcite precipitation (MICP) identify fundamental material properties and mechanical characteristics of bio-cementation. Scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) techniques reveal micro-scale calcite formation and degradation characteristics. Calcite minerals are found predominantly near the silica sand grain contacts, strengthening particle bonds. Fracturing of the cemented matrix is found to occur within the calcite phase. A 1-g scaled shallow foundation model is developed to evaluate how MICP treated sands may improve the performance of geosystems at the macro-scale. MICP treatment targets a passive treatment zone of loose sand directly beneath a footing. The spatial distribution of calcite within a sand matrix at the micro-scale translates to a reduction in settlement (or increase in load capacity) at the macro-scale. Comparison of load tests on the footing for MICP treated and untreated experiments reveals up to a five-fold reduction in foundation settlement.