Mining-induced fissure types reorganize pore architecture and regulate preferential flow in loessal subsidence-affected soils

Underground coal mining can induce surface subsidence and fissure formation, altering soil architecture and water-flow pathways in loessal landscapes. However, how distinct fissure types reorganize pore networks and influence preferential flow remains insufficiently understood. This study compared tensile fissure, step-type fissure, and undisturbed soils in a mining subsidence area on the Loess Plateau of China. Field infiltration measurements, dye tracing, X-ray micro-CT, XGBoost modeling, segmented regression, and correlation analysis were integrated to link fissure type, matrix-scale pore architecture, and plot-scale hydrological response. Step-type fissure soils showed the strongest pore-network reorganization, with greater porosity, larger mean throat diameter, stronger connectivity, and more pronounced macropore development; tensile fissure soils showed an intermediate structural state, and undisturbed soils retained the most compact pore system. Preferential flow was strongest in step-type fissure soils, weaker but evident in tensile fissure soils, and weakest in undisturbed soils. XGBoost indicated that coordination number, average path length, node degree, and mean throat diameter explained hydrological variation better than bulk porosity. An empirical, site-specific breakpoint near 0.3 mm mean throat diameter was identified for the 0–20 cm surface loess soils, marking a shift toward stronger preferential flow rather than a universal threshold. These findings indicate that hydrological regulation in subsidence soils is topology-dominated rather than porosity-centered and support fissure-type-specific restoration and water-management strategies.