The Challenge of Long-Term Cover Performance
Designing engineered covers for waste rock dumps and tailings storage facilities is essential for achieving regulatory compliance and meeting sustainable closure objectives. These engineered covers aim to ensure long-term environmental protection by controlling water infiltration, preventing erosion, and supporting vegetation establishment. A significant challenge lies in reliably predicting how materials will perform at field scale over many decades. Understanding effects of scale during investigation and design is therefore critical. Research findings from various scales of investigation, including detailed studies from sites in Queensland by SGME, provide valuable insights into managing these complexities.
Scale Effects in Characterising Material Properties
Accurate characterisation of hydraulic properties forms basis of cover design, particularly for store-and-release systems in semi-arid climates. Soil water characteristic curves (SWCCs), relating water content to soil suction, are fundamental. However, research comparing laboratory measurements with field behaviour reveals significant potential discrepancies. Standard laboratory SWCC tests on reconstituted samples often fail to capture influence of in-situ structure, stress history, or weak cementation present in field materials. This typically results in laboratory tests underestimating water retention capacity or suction achievable under field conditions. Further, laboratory tests often focus on the drying curve, potentially overlooking effects of hysteresis —the difference between wetting and drying SWCCs — which influences how much water is stored during rainfall versus how readily it is released during drying cycles, impacting store-and-release cover efficiency. Relying solely on potentially non-conservative laboratory data introduces uncertainty into design predictions.
Insights From Intermediate Scale Column Trials
Intermediate-scale testing using large columns offers a method for comparative assessment under controlled conditions. Studies at a mine site in northwestern Queensland employed column trials to compare performance of different store-and-release cover options for site-specific materials including non-acid forming waste rock. These trials provided valuable comparative water balance data, useful for calibrating numerical models. While beneficial for comparing design options side-by-side, these studies also highlight limitations of this method. Column trials inherently exclude natural atmospheric conditions governing evaporation and cannot replicate intensity and variability of natural rainfall. Consequently, results primarily inform relative performance rather than providing absolute predictions of long-term field behaviour.
Learnings From Large-Scale Field Trials
Large-scale field trials provide most direct assessment of cover performance under actual site conditions. An extensive trial program at a semi-arid mine in north Queensland involved constructing three large 75 meter by 75-meter cover plots directly on a tailings storage facility. These trials tested different engineered reduced permeability layers, including mixtures conditioned with bentonite, overlain by thick infiltration and storage layers (ISL) of natural soil and rock. Instrumentation allowed detailed monitoring over ten years.
A key aspect investigated was performance of capillary breaks, where coarse materials limit upward capillary rise from underlying finer tailings and downward percolation. The substantial thickness of loose rock ISL contributed to this effect. Long-term monitoring demonstrated sustained performance, achieving very low percolation rates consistent with design objectives even through periods of extreme climate variability, including intense wet seasons and prolonged droughts. This decade-long dataset provided high confidence in performance capabilities of tested designs under real-world conditions. Practical insights were also gained regarding material sourcing challenges and importance of construction quality control in achieving intended hydraulic separation between layers. While yielding invaluable data, the study also reinforced understanding that such trials require significant investment in cost, land area, and time.
Integrating Multi-Scale Investigations
Developing robust cover systems requires careful integration of findings from investigations across multiple scales. Laboratory tests offer fundamental data but must be interpreted knowing potential differences from field behaviour related to sample structure and hysteresis. Column trials provide useful comparative data but lack full environmental realism. Large-scale, long-term field trials, as demonstrated at the north Queensland site, provide most reliable validation of performance under site conditions but demand significant resources. Incorporating information appropriately from all scales, acknowledging limitations of each, and validating designs through long-term field monitoring where feasible, leads to more confident, effective cover designs capable of meeting, long-term mine rehabilitation goals.