Reutilization of gangue wastes in underground backfilling mining: Overburden aquifer protection

Gangue backfilling mining (GBM) can effectively alleviate the gangue accumulation pollution and the overburden aquifer destruction. To efficiently evaluate the reutilization of gangue wastes by GBM and its advantage in overburden aquifer protection, non-Darcy hydraulic properties and deformation behaviors of granular gangues were studied through laboratorial, theoretical, and in-situ aspects. A series of compression and seepage tests on granular gangues under the variable original grain size grade (GSG) and stress rate were conducted. Laboratorial testing results convince that, hydraulic properties (porosity and permeability) of the granular gangue decline with the increasing original GSG and decreasing stress rate. The crushing ratio of the sample increases with the increase of original GSG and the decrease of stress rate. The fractal dimension reveals more obvious increases in the samples with the higher original GSGs and lower stress rates. The Kruger model (a classical theoretical model) was employed to predict the permeability evolution based on the porosity. However, the invalid pores in rocks would result in the model's underestimation. To this end, an improved model was established to predict the permeability evolution by the fractal dimension, and the improved Kruger model has better accuracy than the original one. Finally, according to the laboratorial testing and theoretical predicted results, friendly-environmental strategies for overburden aquifer protection were proposed. The effectiveness of these strategies was successfully verified by an in-situ application. It is concluded that the high filling stress, low gangue original GSG, and low filling stress rate in GBM can effectively reduce the risk of overburden aquifer destruction.