Micro-scale processes in microbially induced carbonate precipitation

The concept of using bacteria to control the precipitation of calcium carbonate for engineering purposes, such as increasing soil strength and decreasing permeability, is well established in lab-scale experiments. What is not so clear is how to transition from these experiments to a practical field-scale ground improvement tool. One difficulty is that soil properties are highly site specific and also vary spatially within a site (e.g. porosity, permeability, particle shape and size distribution, mineralogical composition). Meanwhile, microbially induced carbonate precipitation relies on complex interactions between pore structure, fluid flow pathways, and injection strategies which all influence where the injected bacteria will attach, where CaCO3 will precipitate, and which evolve over time as CaCO3 is progressively precipitated and the pore structure is altered. To unpick these processes and optimise MICP treatment, we use light microscopy in microfluidic devices offering high time-resolution observations of bacterial attachment and CaCO3 crystal nucleation and growth in 2D systems. This is followed by X-ray μCT of sand packed columns offering more complex and realistic flow conditions in which we observe the evolving pore structure and relate this to changes in the flow fields through reactive-transport modelling with the software OpenFOAM. Results show that the processes of bacterial attachment and crystal growth are complex and highly dependent on micro-scale conditions, however feedback mechanisms, repeated treatment cycles, and operator controlled parameters such as flow velocity can act to minimise these local variations across a range of soil types.