Objective In the arid regions of western China, the ecological restoration of waste dumps within open-pit coal mines plays a key role in green mine construction, while creating a suitable soil profile structure serves as a prerequisite for the ecological restoration.

Methods  This study investigated the reconstructed soils with a three-layer sponge-like structure consisting of the topsoil, sandy-soil, and mudstone layers for the waste dump of the Hongshaquan open-pit coal mine in Xinjiang. Using computed tomography (CT) scanning, a three-dimensional (3D) pore network model (PNM), and water migration simulation, this study systematically analyzed the pore structures and water migration characteristics of various soil layers reconstructed and their transition zones.

Results and Conclusions The reconstructed soil profile consisting of a 40-cm-thick topsoil layer, a sandy-soil layer with a thickness of 60 cm, and a 20-cm-thick mudstone layer exhibited the highest water-holding capacity. In this profile, the sandy-soil layer showed a favorable porosity of 27.85% and a high pore connectivity of 99.88%, with a uniform pore distribution and high hydraulic conductivity. In contrast, the topsoil layer demonstrated roughly prolate pores, with pore throats showing higher vertical permeability. As a result, localized preferential flow was prone to form. The transition zones between various soil layers demonstrated significant barrier effects due to differences in stacking and abrupt changes in texture. Specifically, a weakly permeable layer was formed in the transition zone between the topsoil and sandy-soil layers, while cavities with thicknesses ranging from 0.5 mm to 1.0 mm occurred in the transition zone between the sandy-soil and mudstone layers due to differential swelling and shrinkage. Therefore, water stagnation zones were formed in the transition zones, thus enhancing the water storage capacity. The hydraulic conductivity of various reconstructed soil layers and their transition zones decreased in the order of the sandy-soil layer, the transition zone between the topsoil and sandy-soil layers, the topsoil layer, and the transition zone between the sandy-soil and mudstone layers. The water migration in the reconstructed soil profile was jointly influenced by the microscopic morphology, interconnectivity, and orientations of pores. The results of this study reveal the differentiation patterns of pore structures and water migration characteristics in the reconstructed soils at the microscopic scale, providing a theoretical basis for the ecological restoration of open-pit coal mines in arid regions.