Seismic Design and Safety Criteria for Tailings Dams: A comparison with Water Storage Dams

There exist up-to-date seismic design and safety criteria for water storage dams and guidelines for the conceptual design of large dams in seismic regions, published by the International Commission on large Dams (ICOlD). Tailings dams store waste from mining activities, which is often classified as hazardous. They are mainly embankment dams, and the way of construction differs from that of water dams. Water dams are usually completed before the reservoir is filled, whereas in tailings dams the construction of the dam and the filling with the waste material proceed stepwise at the same time, leaving an adequate freeboard to protect the tailings dam from overtopping. Thus, because of the incremental construction and reservoir filling, embankment dams are built in which consolidated tailings form also part of the embankment. In general modern dams that can resist strong ground shaking are earth core rockfill dams, where seepage through the dam body is controlled by an impervious core, which is protected from internal erosion by filters, Moreover, seepage through the foundation is controlled by a grout curtain or cut-off walls, depending on the type of foundation. large water dams are usually founded on rock but this may not be the case for smaller embankment dams. If no sediment flushing is provided, the reservoirs formed by water dams will eventually fill up with sediments and their final state may not be too different from that of tailings dams. The difference being in the properties of tailings and in the way these tailings are placed. In water dams there is no control on how the sedimentation process occurs. Most sediment deposition will be during large floods. Both the sediments and tailings materials are assumed to liquefy under strong ground shaking. Therefore, the stored materials are basically liquids during strong earthquakes. If we take this into account, then certain types of tailings dams cannot be safe. Some recent failures of tailings dams in Brazil and Australia have shown that they failed due to static liquefaction. Therefore, it is obvious that they would have also failed under seismic action. Based on this comparison, the seismic safety requirements for tailings and water dams should be the same for the same risk classes of projects. In the case of tailings dams, the hazardous materials remain in the reservoir for hundreds of years and in the case of safety concerns the reservoir cannot be lowered as in the case of water dams, which would allow a fast increase in dam safety. Thus, the seismic safety of tailings dams should be even larger than that of water storage dams. These seismic aspects of water and tailings dams are discussed in the paper.