Influence of Microbially Induced Calcite Precipitation on the Hydraulic and Unsaturated Behavior of Coal Mine Overburden Waste: Experimental Insights and Numerical Correlations

Microbially Induced Calcite Precipitation (MICP) has emerged as a sustainable ground improvement technique, however its application for enhancing the performance by improving the mechanical behavior of Coal Mine Overburden Waste (CMOW) materials is very limited. Further, the impact of MICP technique on unsaturated properties governing dump slope stability such as hydraulic conductivity, pore size distribution and Soil Water Characteristic Curves (SWCC) has not been explored. Hence, the present research works have aimed to investigate potential of MICP technique in improving the unsaturated properties CMOW material using three Bacillus family (Sporosarcina pasteurii) bacterial strains ie, SP77 (Bacillus sp.), SP78 (Bacillus sphaericus), and SP79 (Bacillus subtilis) through detailed experiments, with microstructural modifications confirmed through Field Emission Scanning Electron Microscopy (FESEM) and Mercury Intrusion Porosimetry (MIP). The SWCC response of untreated and treated CMOW is further interpreted using models by Brooks and Corey, Van Genuchten, Fredlund and Xing, and Durner, providing new insights into the role of MICP in slope stability assessment. The untreated CMOW has exhibited a relatively saturated hydraulic conductivity of 1.48 × 10−5 m/s, which is reduced to the order of 10−7 m/s after MICP treatment with S. pasteurii strains, indicating a significant improvement in hydraulic conductivity. Further, SWCC results have pronounced the bacteria strain and depth-dependent changes in water retention after MICP treatment, associated with the redistribution of pore-size domains and heterogeneous carbonate precipitation. Pore structure analysis through MIP reveals a shift from a unimodal to a bimodal distribution, corroborated by FESEM images that confirm calcite precipitation in the form of vaterite and rhombohedral crystals. Curve fitting further indicated that the Fredlund and Xing model has provided the best representation of untreated CMOW, while the Durner model effectively describes the bimodal pore system generated by MICP. The results confirm the potential of MICP as a viable approach for enhancing the stability of overburden dumps in unsaturated environments. Hence, the findings of the present study are expected to provide a scientific basis for extending the proposed approach to field-scale application.