Evaluation of Cover System Field Trials with Compacted Till Layers for Waste Rock Dumps at the Boliden Aitik Copper Mine, Northern Sweden

摘要

The Boliden Aitik copper mine is located outside Gallivare, northern Sweden. Since mining started in 1968, more than 500 Mt of waste rock have been deposited in waste rock dumps (WRDs). In the 1990s, a dry cover system was designed to reduce oxygen diffusion to the underlying waste rock and limit the subsequent formation of metal leaching and acid rock drainage (ML/ARD). The original cover system (applied to WRD5 and parts of WRD2) consists of 1.0 m compacted till placed in two lifts covered by 0.3 m organic material. Recent studies concluded that predicted annual diffusion flux estimates based on calibrated soil-plant-atmosphere modelling were similar in magnitude, but greater than, rates predicted in the 1996 design. Therefore, to increase the level of water retention within the compacted till and reduce oxygen ingress to the underlying waste rock, increased compaction from the current practice is required.Multiple field and laboratory studies were completed to evaluate the cover system design and performance. Field compaction trials conducted in 2012 confirmed that increased compaction would enhance water retention characteristics within the cover system. A field study of freeze / thaw effects on the compacted till highlighted frost penetration to the compacted layer as an area of concern for long term performance of the cover system. Cover system trials implemented in 2013 were instrumented with monitoring instrumentation that will allow performance to be evaluated over time under site-specific conditions, providing essential insight into cover system response to climatic variations in terms of temperature and water storage dynamics.