Effect of Grain Crushing on the Hydraulic Conductivity of Tailings Sand

Tailing dams are constructed to impound the byproducts of mining operations and are often built using the coarser fraction of the tailings resulting from gravity separation processes. The high stresses anticipated at the bottom of tall tailings dams can lead to grain crushing, resulting in a reduction in the hydraulic conductivity and, ultimately, long-term performance issues if the water drains at a slower rate than required. Therefore, in order to accurately evaluate the margin of safety in designing tall tailings dams, understanding the evolution of the hydromechanical behavior of the tailings is important. This paper presents the results from a series of laboratory-scale high-stress compression tests combined with hydraulic conductivity measurements using a specially designed oedometer cell. The experimental results were subsequently interpreted by an enhanced continuum breakage mechanics model. The grain-size distributions predicted by the continuum breakage model were further combined with six commonly used hydraulic models to predict the reduction of hydraulic conductivity of tailings sand subjected to grain crushing. The results suggest that a stress increase from 5 to 40 MPa can cause an increase in fines content from 11% to 17%, respectively, and a corresponding reduction in hydraulic conductivity of up to one order of magnitude for the tested materials. The paper highlights that the breakage mechanics theory combined with proper hydraulic conductivity models can offer reliable predictions of the evolution of the hydromechanical behavior of tailings sands subjected to high stresses.