Abstract
Artificially bio-cemented sands treated with microbially induced calcite precipitation are weakly cemented rocks representing intermediate materials between locked and carbonate sands. Variations in cementation significantly affect the strength of sample, particularly tensile stregth. The modes of fracture and the surface characteristics resulting from the indirect tensile strength tests (Brazilian tests) are strongly correlated with the specimen strength and consequently the degree of cementation. This study examines the tensile strength of bio-cemented fine and coarse sands (average particle diameter 0.18 and 1.82 mm, respectively) and investigates failure modes by recording fracture evolution at both sides of specimen and surface characteristics of the reconstructed surfaces. The dimensionless slope parameter provided the best fit with respect to tensile strength while the power spectral density was a good indicator of surface anisotropy. Finally, wavelet decomposition allowed for comparison of fracture surface characteristics of the two sands ignoring the grain size effects.
Highlights
Reproduction of rock specimens has received great attention in the literature [1,2,3,4], due to difficulties often experienced with conventional sampling
At a cementation level of 3%, the tensile strength of fine sands was around 50 kPa, and this increased to about 700 kPa at 11% cementation, the highest cementation level reached with this microbially induced carbonate precipitation (MICP) protocol
Coarse sands showed a tensile strength of about 25 kPa at 4% cement by weight, which increased to 500 kPa at 11% cementation
Summary
Reproduction of rock specimens has received great attention in the literature [1,2,3,4], due to difficulties often experienced with conventional sampling. Synthetic material preparation allows structural parameters to be varied independently, and for their effects to be isolated [5]. One of the methods used to create rock-like material and biocemented sands, is microbially induced carbonate precipitation (MICP) [6]. This is a bio-cementation technique where calcium carbonate precipitates and acts as a cementation agent across sand particles. Bacteria hydrolise urea according to the first chemical equation and, in the presence of a calcium source, calcium carbonate precipitates into solid form (second chemical equation) [7,8]. CO(NH2 )2 + 2 H2 O −−→ 2 NH4 + + CO3 2− iations.
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