Long-term creep and permeability evolution of tailings sand at elevated stresses

Tailings dams are typically constructed utilising the coarser fraction of tailings and are engineered to contain the finer fraction of the waste. For extremely tall tailings dams, the high internal stress near the bottom of the dam may induce particle breakage in the embankment material (coarser fraction), resulting in an increase in fines content. This can further lead to reduced hydraulic conductivity, raised water level, and endanger the stability of the dam. Another often neglected aspect is that grain crushing may not happen instantaneously at the moment of the application of the load but develop gradually over time due to delayed crack growth (or static fatigue) within the grains, even at relatively intermediate or low stress levels. Considering the expected long service life (50–100 years) of typical tailings storage facilities (TSF), it is critical to quantify the degree of grain breakage and the induced creep and permeability reduction under sustained stresses over extended period of times. We conducted a 450-day long creep test on tailings sand retrieved from a currently active TSF at in situ vertical stress (3 MPa) using a custom-built high-stress-permeability oedometer cell. The experiment shows a clear change in vertical strain and hydraulic conductivity, with hydraulic conductivity steadily declining during the creep stage before reaching a stable value. The results are subsequently interpreted by combining a rate-dependent breakage mechanics model which tracks the evolution of grain size distribution over time and several hydraulic conductivity models that account for both porosity and grain size effects. The model predictions generally agree well with the experimental data. Finally, possible sources of discrepancies and recommendations for future long-term creep tests on tailings materials are discussed.