Crackling noise and bio-cementation

Microbially induced calcite precipitation (MICP) is used for bio-cementation of calcareous sand using microbial metabolism to generated calcite precipitation. This technology has been proposed for sealing damage in geological reservoirs, repair of cracks in stone buildings, and strengthening of foundations in coastal engineering. To test the stability and crash dynamics, we compress bio-cementation samples under uniaxial stress and investigate their collapse mechanism by measuring their acoustic emission, AE. AE of the bio-cementation sample shows that the collapse commences even under very modest stress. The collapse proceeds via avalanches where a local collapse triggers other damage in the sample. All local collapse processes are highly correlated. The characteristic avalanche parameters (energy, amplitude, interevent time, etc.) are power law distributed, e.g. the energy exponents are ε = 1.35–1.6, depending on the degree of the collapse. Sands without cementation show equally avalanches with energy exponents near ε = 1.7 and very low strength. In contrast, compressed sand grains have high strength and energy exponents near ε = 1.4. Correlations between AE spectra show that compressed bio-cementation samples immediately reconvert to sand with some larger grains resisting. The distribution of low AE energies in bio-cementation samples has the same avalanche exponents as sand. This demonstrates that some parts of the bio-cementation sample behave exactly as sand with some consolidating, bio-cemented structures in between. These structures are destroyed and local grains collapse under further compression. The distribution of other avalanche characteristics like inter-event times, the Omori's law, and the Båth's law, are identical for bio-cementation samples, sands and grain.