Development of a Coupled BCHM-Model for Numerical Investigations of MICP treatment of Soil
Final Report Abstract
We have successfully developed a coupled bio-chemo-hydro-mechanical (BCHM) model for the MICP treatment in soil, which can reasonably capture the most relevant processes involved in MICP. The project has resulted in not only better understanding, estimation and prediction of the the MICP behaviour, but also extending the current modelling work for the coupled problems in porous media. In our model, we considered the decay, attachment and transport of bacteria; transport of aqueous chemical components by advection, diffusion and dispersion; mass transfer and decrease in the liquid phase due to the chemical reactions; mass increase in the solid phase caused by calcite precipitation. For the relevant coupling effects among the different fields, we considered the change of reaction rate due to the change of spatial distribution of bacterial and chemical concentration, porosity/permeability reduction, improvement of mechanical properties, variation in the advective field caused by porosity and permeability change, the change of effective stress on solid skeleton due to the change of pore pressure and the variation in pore pressure due to deformation. We have successfully validated our model against the experimental data from different literature sources. To ensure an acceptable computational efficiency the model was designed to be simple and with less parameters to be calibrated. Despite the simplicity, we proved that our model has a broad capability. The model has been implemented in the FE solver OGS. For the model flexibility, a semi-staggered numerical strategy is adopted to solve the coupled problems. To be specific, the reaction and mass transport (BC) are solved monolithically using the GI method. However, the BC problems are solved with pore flow (H) and deformation (M) processes in a staggered manner. Standard Galerkin method is applied for the spatial discretisation, whereas the time discretisation is via the implicit backward Euler method. In order to get reasonable prediction of the temporal and spatial evolution of permeability reduction during MICP treatment, we introduced an effective porosity concept and accordingly modified the Kozeny-Carman equation. In this way, we can capture the influence of the intrinsic pore structure on the permeability change. In our mechanical model, we successfully uncovered the non-linear correlation between the improvement of mechanical stiffness, strength and the calcium-carbonate contents. Using this mechanical model, we not only can well quantify the improvement of the mechanical stiffness and strength, but also addressed a typical mechanical features of the MICP treated soils, which however have been rarely studied. To be specific, the more pronounced dilative behaviour and the transition from the ductile to brittle material property with the increase of calcite contents. Besides, with this model we have extended our study on the relevant influence factors of MICP. Specifically, we found that temperature effects, microstructural of the soil and injection methods play an important role in the MICP performance. Although we faced some obstacles during the progress of the project due to pandemic, finally we have achieved the main research goals. Within the research project, we accomplished five journal articles (four published, one under review). We have reported our research findings at two international conferences (Interpore 2020 and WCCM 2021). Based on the findings at the current research stage, we also identified the future research directions. Among those, an important open question emerging from our project is how to quantify the uncertainties involved in MICP.
Publications
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A coupled bio-chemo-hydraulic model to predict porosity and permeability reduction during microbially induced calcite precipitation. Advances in Water Resources, 140, 103563.
Wang, Xuerui & Nackenhorst, Udo
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A comparative study of using two numerical strategies to simulate the biochemical processes in microbially induced calcite precipitation. Journal of Rock Mechanics and Geotechnical Engineering, 14(2), 592-602.
Feng, Dianlei; Wang, Xuerui; Nackenhorst, Udo; Zhang, Xuming & Pan, Pengzhi
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Micro-feature-motivated numerical analysis of the coupled bio-chemo-hydro-mechanical behaviour in MICP. Acta Geotechnica, 17(10), 4537-4553.
Wang, Xuerui & Nackenhorst, Udo
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Modellierung der Verbesserung von Bodeneigenschaften mittels MICP/Modelling of Soil Improvement via MICP. Bauingenieur, 98(01-02), 10-17.
Wang, Xuerui & Nackenhorst, Udo
