Risks associated with construction on submerged tailings beaches

A rockfill dyke will be built in a lake by end dumping method. Its crest will be at elevation 521.5 m compared to the nominal lake level at 520 m. Its height will vary from 1.5 m on 37.0 m of tailings to 22.0 m on about 0.5 m of tailings or native silty clay. Tailings consist in fine-grained soils showing segregation. Tailings are compressible and may develop excess pore water pressure on loading. Tailings overlie a thin layer of relatively soft native silty clay. The till beneath the silty clay is considered incompressible compared to the material above. After construction, tailings and silty clay will consolidate and dyke stability will improve. Since dyke stability will increase with time, the primary concern is its stability during construction. Three potential modes of failure were identified and recommendations were provided regarding measures that should be undertaken to increase its stability during construction.