Unsaturated Behavior of Coal Gangue–Soil Mixtures Improved by Microbially Induced Calcium Carbonate Precipitation

Abstract

Coal gangue–soil mixtures are increasingly considered for engineered fills, slope restoration, and mine-site reclamation, where they commonly remain unsaturated and experience infiltration, evaporation, and repeated wetting–drying. Microbially induced calcium carbonate precipitation (MICP) can improve particle bonding and modify pore geometry, but its benefit cannot be evaluated from carbonate content or dry strength alone. This review synthesizes 40 recent and foundational studies, including 16 studies on water retention, high suction, suction-controlled strength, consolidation, multiphysics coupling, and state-dependent engineering response. The evidence shows that capillary storage and pore-throat drainage dominate at low to intermediate suction, whereas adsorbed water becomes increasingly important at high suction; consequently, the influence of void ratio depends on the suction range and the selected water-content variable. Suction-induced strength is material- and path-dependent and may increase, plateau, or become nonmonotonic. Mineralization, flushing, deformation, and cyclic damage can simultaneously alter retention, hydraulic conductivity, compressibility, and failure mode. Analytical consolidation, slope, earth-pressure, and wave-propagation studies confirm that air–water pressure, hydraulic boundaries, saturation, and load history affect engineering performance, although they do not directly demonstrate MICP benefits. A research framework is therefore proposed that preserves cemented fabric, connects full-range retention and unsaturated conductivity with suction-controlled shear and tensile tests, separates capillary and cementation contributions, and validates hydraulic–mechanical models against independent time-series data.