Contractor Acciona is building both tunnels, which will be used to temporarily reroute a short section of the Peace River to allow for construction of the dam across the main channel of the river. River diversion is scheduled for September 2020.

Tunnel 1 is approximately 700m long and 12m in diameter. Excavation of the tunnel began in July 2018.

The excavation work is being carried out using roadheaders, and a tunnel lining is now underway on Tunnel 1 using a custom-built slip form.

Each pour takes two to three days to cure, and the liner thickness can range between 40 centimetres and 200 centimetres. In total, about 41,000 cubic metres of concrete will reinforce the walls of both tunnels.

Breakthrough on Tunnel 2 is expected to happen in July. Once operational, the capacity of the two tunnels will be 3,000 cubic metres of water per second. On average, the diversion tunnels will typically pass river flows between 500 and 1,800 cubic metres per second.