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Study on healing technique for weak interlayer and related mechanical properties based on microbially-induced calcium carbonate precipitation.

ABSTRACT: The weak interlayer refers to the filling material in shear belts or large-scale structural planes, which is usually composed of soil, fine sand and gravels. It is prone to argillization when encountering water and its mechanical strength and stiffness are generally low, which has adverse effects on the stability of underground structures. In recent years, research on reinforcement techniques for weak interlayers has been a hot topic in geotechnical field. As a new reinforcement method for structural planes, the microbial healing technique has attracted a lot of attention. In this paper, a study on the healing technique for weak interlayer based on microbially induced calcium carbonate precipitation (MICP) and related mechanical properties was conducted for the interlayer shear belt at Baihetan Hydropower Station in China. First, Sporosarcina pasteurii was activated in laboratory. Reinforcement of the weak interlayer was realized by utilizing calcium carbonate precipitation on the weak interlayer. Continuous monitoring of the precipitates on the weak interlayer by XRD and SEM indicated that the precipitates on the weak layer were microbially induced calcium carbonate. Its crystals were irregular fish scale-shaped cubes with size in the range of 5~20μm. With favorable crystal growth, the crystals and the particles of the weak interlayer were cemented together. Finally, the mechanical properties of the healed weak interlayer were tested and the variations of uniaxial compressive strength, shear strength and triaxial compressive strength with bacteria concentration were discussed. The test results indicated that the maximum uniaxial compressive strength, peak shear strength and triaxial compressive strength can be increased by 149%, 162% and 119%, respectively, which subsequently improve the overall strength of the shear zone or structural plane. This can provide a new idea for soft ground reinforcement in underground projects.

SUBMITTER: Jin C

PROVIDER: S-EPMC6136771 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Simple plugging of the high-permeability "thief zones" of oil reservoirs is the most plausible and also the most straightforwardly achievable approach to enhance sweep efficiency and oil recovery. Sporosarcina pasteurii is a representative microorganism with the ability to precipitate calcium carbonate (CaCO3) via enzymatic hydrolysis of urea in the presence of calcium ions. Microbially induced calcium carbonate precipitation (MICP) can cement and seal the granular and fractured media and thus can be used as a potential microbial plugging agent for the high-permeability zones of oil reservoirs. The following investigated the microscopic characteristics of MICP plugging and its efficacy in permeability reduction. The columns of near-spherical silica sand and angular silica sand with three separate granularities (40/60, 60/80, and 80/120 mesh) were used as artificial rock cores representing distinct pore sizes and pore characteristics to investigate the efficacy and microprocess of MICP plugging with different biotreatment periods. The results indicated that permeability is reduced significantly after only short periods of biotreatment. After eight cycles of MICP treatments, the permeability for each type of cores dropped by 54-90% of individual initial permeabilities. The measured CaCO3 content indicated that the decreasing rate in permeability with the increasing CaCO3 content experiences three contrasting stages, namely, slow decline, speedy decline, and plateauing. X-ray diffraction indicated that most of the generated CaCO3 crystals occur as vaterite with only a small amount of calcite. Imaging by scanning electron microscopy further revealed the microprocess of MICP plugging. Microorganisms first concentrate on the pore wall to secrete CaCO3, forming a thin and large uniform layer of CaCO3. Then, some nucleation sites of CaCO3 crystals will experience further preferential growth, resulting in large, dominant crystals that act as a plugging agent within the pore space. Compared to extracellular polymeric substances, which are currently the primary microbial plugging agent used to enhance sweep efficiency of oil reservoirs, bio-CaCO3 appears more effective in plugging in terms of its morphology, size, and growth characteristics.

Project description:Microbially induced calcium carbonate precipitation (MICP) process utilising the biogeochemical reactions for low energy cementation has recently emerged as a potential technology for numerous engineering applications. The design and development of an efficient MICP process depends upon several physicochemical and biological variables; amongst which the initial bacterial cell concentration is a major factor. The goal of this study is to assess the impact of initial bacterial cell concentration on ureolysis and carbonate precipitation kinetics along with its influence on the calcium carbonate crystal properties; as all these factors determine the efficacy of this process for specific engineering applications. We have also investigated the role of subsequent cell recharge in calcium carbonate precipitation kinetics for the first time. Experimental results showed that the kinetics of ureolysis and calcium carbonate precipitation are well-fitted by an exponential logistic equation for cell concentrations between optical density range of 0.1 OD to 0.4 OD. This equation is highly applicable for designing the optimal processes for microbially cemented soil stabilization applications using native or augmented bacterial cultures. Multiple recharge kinetics study revealed that the addition of fresh bacterial cells is an essential step to keep the fast rate of precipitation, as desirable in certain applications. Our results of calcium carbonate crystal morphology and mineralogy via scanning electron micrography, energy dispersive X-ray spectroscopy and X-ray diffraction analysis exhibited a notable impact of cell number and extracellular urease concentration on the properties of carbonate crystals. Lower cell numbers led to formation of larger crystals compared to high cell numbers and these crystals transform from vaterite phase to the calcite phase over time. This study has demonstrated the significance of kinetic models for designing large-scale MICP applications.

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Study on healing technique for weak interlayer and related mechanical properties based on microbially-induced calcium carbonate precipitation.

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