Three-dimensional quantification of macropore networks of different compacted soils from opencast coal mine area using X-ray computed tomography

Abstract

Heavy machinery compaction in mining area is widespread, and seriously jeopardizes soil restoration. Characterization of soil macropores with three-dimensional methods is essential for understanding the reclaimed minesoil (RMS) development. The aim of this study was to demonstrate the application of X-ray computed tomographic (CT) scanning for analyzing soil macropore structure and to explore how soil compaction affects soil macropore properties. To achieve this, soil samples collected from Pingshuo opencast coalmine were tested through a laboratory scheme, which was conducted to simulate the real soil pore situation under increasing soil bulk density treatments of 1.3, 1.4, 1.5, 1.6, 1.7, and 1.8 g cm−3 (T1.3-T1.8). Three-dimensional (3D) macropore networks were observed via CT, and macroporosity, macropore number, area, volume and its connectivity were characterized. Soil compaction had a major impact on soil macropore characteristics. The macropores of the 1.3 g cm-3 treatment were well connected, whereas those of the 1.8 g cm-3 treatment were poorly connected and isolated from each other. Macropore number and macroporosity decreased significantly as the soil compaction increased. Moreover, a 3D visualization of soil pore connectivity results suggested that the macropore connectivity markedly decreased with increasing compaction level, with average Euler number increasing from 0.677 in the 1.3 g cm-3 treatment to 0.999 in the 1.8 g cm-3 treatment. Soil bulk density was negatively correlated with macropore throat number and thickness, indicating that the soil compaction causes a loss of pore connectivity. Despite using a coarse resolution, the combination of laboratory simulation tests and 3D visualization of soil macropores provides valuable data for identifying the differences in soil pore structure among compacted RMSs and monitoring the persistent effects of soil compaction caused by heavy machinery in the mining area. Furthermore, we recommend a new method combining 3D visualization techniques and hydraulic analysis in the study of compacted soils, and the establishment of soil restoration measurements to avoid further compaction in mining areas.