With international policies pushing for the reduction of carbon emissions around the world, large-scale exploitation of surface lignite (brown coal) mines is gradually becoming uneconomical. As a result, many countries around the world are requiring coal mining companies to plan for the entirety of a mining operation’s life cycle, which includes developing a plan to decommission and close mines when they are no longer profitable.
Surface coal mining poses a particular set of geotechnical challenges to decommissioning engineers. These include the long-term stability of the slopes around the edges of the pit, particularly when the pit is likely to gradually fill up with water in the years after the mine has stopped functioning.
To further investigate slope stability issues, researchers from the National Technical University of Athens and the Public Power Corporation of Greece recently examined the condition of the Amyntaion surface lignite mine. Their goal was to determine under what parameters the safety factor of a decommissioned coal mine could change over time, particularly while it is filling up with rainwater.
The research reveals that that until the water level in the mining pit reaches a critical depth of approximately 15–35% of the final equilibrium, the safety factor of the slope instability decreases slightly (by about 3% in the case study, and up to 5–10% in other conditions) compared to its value at the end of exploitation. At higher water levels, the safety factor increases significantly, as the beneficial effect of the lake water pressure acting on the slope overcomes the adverse effect of pore water pressure rise inside the slope.
In typical mines, the critical water depth is achieved within a few years, since the surface area of the pit is smaller at deeper levels; thus, more favourable slope stability conditions are usually reinstated a few years after a mines closure, while the small reduction in safety during the initial stages after closure is inconsequential.
The paper also investigates the factors that influence the degree of the small reduction in the short-term safety, and produces normalised graphs that show the evolution of the safety factor as the lake water level rises. This work could form the basis of preliminary closure studies for other kinds of surface coal mines. However, the authors note that more of these kinds of investigations are required to provide more data to accurately plan mining operation closures and decommissioning while ensuring long-term public safety and environmental remediation.