Influence of bio-cementation on gas permeability of unsaturated soils in landfill cover system

Landfill cover systems should exhibit low gas permeability to minimize the overflow of greenhouse gases and subsequent air pollution. Microbially induced carbonate precipitation (MICP), a biocementation technique, has been applied for subsurface soil stabilization by improving the shear strength of the soil. However, the impact of MICP on the gas permeability of unsaturated soils remains unknown. Considering the role of biocementation in the modification of soil interpores, this study investigated the feasibility of using the MICP technique to reduce the gas permeability of granite residual soils in response to unsaturated conditions. The biocemented soil samples were prepared by mixing soils with MICP chemical solutions at different chemical concentrations. Water retention tests and measurements of gas permeability were performed, in which suction, volumetric water content and gas permeability were continuously monitored. Additionally, energy-dispersive X-ray spectroscopy and X-ray diffraction analyses were performed to investigate the formation of CaCO3 precipitates; scanning electron microscopy was used to study the impact of MICP on the soil interpore structure, and the acid washing method was used to determine the CaCO3 content. The results showed that soils treated with higher concentrations of MICP chemical solutions had higher air entry pressures and residual water contents. This indicates the improvement of water retention due to the presence of MICP, which increases the microstructural porosity and enhances the capillarity, as observed via microscopy. Furthermore, the results revealed that biocementation significantly reduced the gas permeability of the soil and that the change in the maximum gas permeability strongly correlated with the MICP chemical solution concentration and the CaCO3 content. This study highlights the role of MICP in soil-water-air interface studies and the potential application of this biocementation technique to minimizing gas emission issues in landfill cover systems.