The conventional deposition of mine tailings as a slurry will likely result in seepage, with the potential to contaminate surface and groundwater, particularly during deposition, and possibly post-closure. In a dry climate, tailings deposition can be cycled, excess supernatant water can be collected and stored elsewhere, and excess water can largely be evaporated. On closure, tailings stored in a dry climate may remain desiccated to the extent that incidental rainfall will not result in continued seepage to the environment.
It was postulated that if the tailings were to be allowed to desiccate for a sufficiently long time period, rainfall events will not infiltrate the full depth of the desiccated tailings and reform a hydraulic connection with the groundwater table, resulting in negligible risk of seepage post-closure. Any rainfall that re-wets the upper tailings layers would re-evaporate. The purpose of any cover would then be to limit dust and promote some vegetation, rather than to shed rainfall runoff.
To provide supporting information for the selection of appropriate closure design criteria, and to provide information relating to the unsaturated behaviour of tailings during deposition and desiccation cycles, a trial tailings cell at a mine in arid Western Australia has been instrumented with moisture and suction sensors located on towers placed prior to tailings deposition. The monitoring data collected over a period of 2 years suggest the following:
• Rapid desiccation of the tailings and the foundation occurs following the cessation of tailings deposition.
• Rapid wetting-up of the desiccated tailings occurs during the deposition of fresh tailings, with surficial wetting-up of the tailings during rainfall events.
• The majority of the considerable surplus water evaporates.
• The saturated and unsaturated states can be directly linked to deposition and desiccation cycles.
• An unsaturated state is maintained during rainfall events that occur during desiccation periods.
Following the collection of field data, a column experiment was carried out at The University of Queensland (UQ) under controlled conditions to provide information relating to the saturation and desiccation of tailings placed in layers, and allow a numerical model to be calibrated. The data collected during the column experiment indicated similar responses to the field data, with rapid desiccation of the tailings and the foundation following the cessation of tailings deposition, and then rapid wetting-up of the desiccated tailings occurs during the deposition of fresh tailings, with surficial wetting-up of the tailings during rainfall events.
Numerical modelling has been carried out to identify the ponding depth required to cause infiltration through the full depth of the desiccated tailings column. The numerical model was developed using field and laboratory data, including site and laboratory climate data. The numerical results indicate that, provided a surficial suction of 100 kPa or more is achieved, the tailings are likely to mimic a store and release cover system for surface water ponding depths up to about 500 mm (modelled to result from rainfall in one day and representing about twice the average annual rainfall for the site; hence very much a worst case). Rainfall events causing greater depths of surface ponding than 500 mm may need to be considered in the design of closure measures, if the agreed closure criteria include consideration of the Probable Maximum Precipitation (PMP).
Based on the work presented in this Thesis, the author recommends that the upper surface of the tailings studied in their dry climatic setting be regraded to spread the temporary retention of rainwater on the upper surfaces of the Tailings Storage Facilities (TSFs) to facilitate loss to evaporation. Terracing the upper surface to optimise evaporation would be advantageous, with the aim of limiting the depth of surface ponding to no more than 500 mm. To accommodate large, yet infrequent, rainfall events, specific locations should be identified to allow for additional rainfall collected on the surface to overflow into purpose-built drop structures, releasing the excess rain water to the natural environment.