Emergency drainage system at the dam 3 – Outardes 4 development

Abstract

A sudden increase (outburst) in the seepage discharge was recorded at the downstream toe of dam 3 of the Outardes 4 project. The normal rate of seepage of about 22.5 L/min increased suddenly to 18 000–22 500 L/min. The flow seems to have increased to its maximum rate in a period of not more than a few hours, followed subsequently by a gradual decrease to about 45 L/min in about 20 days.The rockfill dam, with a slightly inclined till core, has a maximum height of about 30 m and rests directly on a very competent bedrock. The quality of the grouted bedrock and the properties of the dam's construction material make it extremely unlikely that seepage through the rock or piping through the core and the transition zones could have caused this event.The presence of a roadfill or "berm" on the downstream rockfill zone, used as a temporary access road during construction and composed of a variety of unselected overburden and rock materials, could have acted as a barrier at the downstream toe of the dam. Thus, the recorded water outburst most likely has been caused by storage of the normal seepage water in the voids of the downstream rock shell and the sudden opening of a drainage channel through this roadfill barrier.Considering the risk of a such ponding in the downstream shell, an emergency drainage system was installed to avoid the repetition of a similar event in the future.The present paper deals with the design and construction of the drainage system whose major component was a tunnel excavated in the bedrock under the dam and illustrates the results of the tests carried out to monitor the ponding tendency in the downstream shell. Keywords: rockfill dam, drainage, tunnel (excavation), blasting (controlled).