Exploring the pore structure of reconstructed soils and its effects on water and salt transport based on CT scanning

Coal gangue serves as a critical material for landfilling and reclamation in the subsidence areas of coal mines. However, due to its rough textures and poor water-holding capacity, soils reconstructed using coal gangue exhibit somewhat different pore structures and water-salt transport from undisturbed soils. To explore the changes in the pore structure of the reconstructed soils and their effects on water and salt transport, this study analyzed the differences in the pore structures between four samples from undisturbed soils, the overburden, mud-gangue mixtures, and gangue using CT scanning and image analysis. Furthermore, an indoor experimental device was designed to simulate the water and salt transport in reconstructed soil. The sensors arranged at different depths of the device allowed for the continuous recording of the water and salt transport along the profile of the reconstructed soil. The results are as follows: (1) Among the four samples, the gangue sample exhibited the highest porosity, reaching 8.299%, resulting in poor water-holding capacity. The mud-gangue mixture sample manifested a high proportion (58.73%) of small pores, low pore connectivity, and a lack of water transport pathways, leading to the formation of an interlayer barrier zone. The overburden and undisturbed soil samples displayed similar pore structures. (2) In the simulation experiment of a soil column, the salt content in the soils varied with water migration, presenting a trend of a first increase followed by a decrease across all soil layers. Post-infiltration, the topsoils were desalted, while the deep soils showed salt accumulation. (3) Within six days after water injection, the salt content in all soil layers first decreased and then increased, with the most significant changing amplitude, 38.34%, emerging at the 50-cm-deep soil layer. As the salt in the deep soils migrated upward under the action of capillary force, the salt content in the topsoils gradually increased. (4) The presence of the interlayer barrier zone hindered the upward transport of water and salt in the gangue layer, resulting in gradually decreasing contents. The results of this study can serve as a reference for the landfilling and reclamation of mining subsidence areas and the ecological restoration of mining areas.