BC Hydro’s Site C Clean Energy Project will be the third dam and hydroelectric generating station on the Peace River in northeast British Columbia, Canada. The Main Civil Works Contract is valued at approximately $1.75 billion and includes two large diversion tunnels to temporarily reroute a short section of the Peace River (3,000 m3/s), to allow construction of the dam across the main channel of the river. This paper will describe the complexity of the execution of twin 750 m long and 11 meters in diameter tunnels that have being carried out using 4 roadheaders (150 Tons each) at the same time, under extreme weather conditions (temperatures below -40ºC) and integrating very demanding silica exposure control methods. The lining of the tunnels includes pouring up to 2 m concrete using a 12 meters long custombuilt slip form, full-round, creating a perfect circle.