The failure triggered liquefaction of the tailings, which accelerated at more than 0.1 g. The results show that although the timing of the failure could have been approximated using equivalent ductile properties, predicting the shallow, planar geometry of the failure surface observed would have required a precise representation of the brittle response. Special modelling difficulties were posed by the need to incorporate the strain softening behaviour of the clays. It was a progressive type of failure, allowed by the brittle response of the pre-consolidated and cemented Guadalquivir blue clays and the high pore pressures left from the incomplete consolidation of the dam’s foundation.
The dam underwent displacements of up to 55 m along a 700 m length, releasing large quantities of acidic waters and 5.5 million m3 of pyrite and pyroclastic tailings. The mechanisms and causes of the sudden failure of the Aznalcóllar tailings pond were investigated. The results are important for optimizing the slope geometry of open-pit mines and can be replicated in other regions. Slope geometry optimization obtained with this drawdown scenario led to adequate inter-ramp and overall safety factors for the final pit design, reducing the barren-to-ore ratio to 0.38, much less than the present ratio (≈ 3). The groundwater control system obtained with 11 horizontal drains and 1 pumping well was considered the most adequate from the geotechnical and economic perspectives. The factors of safety obtained, the drawdown costs and the water table elevation of each section were selected as indicators for obtaining the optimal drawdown scenario using a multi-objective tool. Stability analyzes using the limit equilibrium method were used to obtain the bench, inter-ramp, and overall factors of safety of different representative sections. The groundwater simulation scenarios included the combination of deep horizontal drains and pumping wells. The optimization approach analyzed different scenarios of groundwater drawdown for the final pit of a phosphate mine to improve the pit slopes stability. This study documents a mining plan using groundwater drawdown scenarios that illustrate how geotechnical, economic, and environmental indicators can be combined to obtain optimum slope geometry for open-pit mining. The use of numerical models to simulate groundwater drawdown and slope stability can be an alternative to assess cost-benefit trade-offs for decision-making. However, safe geotechnical conditions generally involve redesigning the geometry of slopes and groundwater drawdown, significantly increasing the costs of mining operations.
The design of open-pit mines should balance safety and economy.
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