Effect of different fly ash additions on the properties of unsaturated soil in open-pit mine dumps

Internal dumps in mines are the main source of risks that threaten the safe production of mines, and improving their impermeability and stability is one of the core issues in this study. In this study, fly ash was used as the surface soil improver for the internal dump, and different fly ash additions (0%, 5%, 10%, 15%, 20%) were designed to improve the properties of unsaturated soil materials. The unsaturated properties, permeability, and strength properties of soil materials were studied through centrifuge tests, permeability tests, and direct shear tests. The results show that the matrix suction of the five kinds of improved soil materials with different addition amounts of fly ash is improved, but when the addition amount is greater than 15%, the improvement effect is weak. A secondary response surface model of fly ash addition, volumetric water content, and matrix suction was established to predict matrix suction. With the increase in the amount of fly ash added, the permeability coefficient of the soil shows a downward trend of the power function, which can have a better impermeability and release erosion effect. After fly ash is added, all soil materials show typical stress hardening behavior, and there is an optimal value (15%) for the improvement of the soil antideformation ability. Affected by the pore size, particle gradation, and fly ash hydration products, the shear strength and cohesion of the sample first increased and then decreased, while the internal friction angle first decreased, then increased and then decreased. The above research results are of great significance for improving the stability and impermeability of the internal dump in mines. © 2021, Saudi Society for Geosciences.