The Coniston concrete dam upgrade and rehabilitation project is located on the Wanapitei River 10 km east of Sudbury, Ontario. The Wanapitei River is part of the Lake Superior and Lake Huron drainage basin. It drains south into the northern portion of Georgian Bay. The mean average flow is 39m3/s at the Coniston Dam generating station. The bedrock geology consists of Middle Precambrian metasediments of the Mississagi Formation and gneissic rocks of the Grenville Province. Bedrock knobs are the dominant landform having a very thin discontinuous ground moraine consisting of a bouldery sandy till. There are glaciofluvial, glaciolacustine and organic deposits. Lakes and rivers reflect local faulting and long linear lakes and straight rivers are common. The river bottom sediments consist of nonplastic silts sensitive to erosion and piping.

The main concrete dam is founded on bedrock. Concrete restoration work including concrete grouting of the dam structure was performed. No adverse bedrock faulting or jointing were reported. In order to carry out the concrete dam rehabilitation a temporary upstream cofferdam was designed with well graded rockfill with 500mm maximum size. The crest width was 6m with a fine aggregate leveling course for construction traffic. The crest level was at Elev. 238.0m and the base overlying bedrock was at approximately Elev. 231.0m The upstream and downstream slopes were 2H:1V. The upstream slope was designed with geotextile for filtration and a geomembrane barrier for seepage control. The geomembrane design was continued into an upstream cutoff trench to be excavated underwater and keyed into bedrock. The 1m wide cutoff trench was to extend from shore to shore along the entire length of the cofferdam.

Significant underseepage and erosion of the temporary rockfill cofferdam and leakage of the bypass flumes were investigated to assess the probable causes and responses to flooding of the upstream settling basin area of the main concrete dam.