Instrumented column testing on long-term consolidation and desiccation behaviour of coal tailings under natural weather conditions

The sedimentation, consolidation and desiccation behaviour of sub-aerially deposited tailings in tailings storage facilities, despite occurring simultaneously and being essential to maximise tailings dry density and dewatering efficiency, are usually analysed in laboratories using distinctive testing equipment, which is unable to reveal their interactions. A large instrumented column was constructed to test the three processes of tailings in a single apparatus, capable of monitoring the changes in the hydrological parameters of coal tailings with depth and surface settlement under natural weather conditions. The column was initially filled with coal tailings slurry and exposed to natural weather for two years, during which five wetting–drying periods were identified. The monitored results indicate that exposing the slurry tailings for one month would lead to the formation of an unsaturated tailings layer with a thickness of 150 mm, likely representing the optimised tailings deposition cycle and thickness, respectively, under the given tailings characteristics and semi-arid climates. Settlement mainly occurred after major rainfall events due to the collapse of cracks and cavities formed during the preceding desiccation. The maximum thickness of the unsaturated tailings increased over the first four periods as the predominant desiccation and settlement strengthened the evaporative capillary forces, despite it decreasing greatly in the last period due to insufficient desiccation. An empirical model was proposed to consecutively estimate the surface moisture content of deformable tailings and validated by achieving a good agreement in the water mass estimated by two methods during wetting–drying cycles.