Construction of tunnels
The most significant construction site for the METRONET Forrestfield-Airport Link tunnels was near the tunnel portal at High Wycombe. This area used to house the key infrastructure, including bentonite and slurry treatment plants, which supported the tunnel boring machines. With tunnelling completed, the infrastructure has been demobilised, making room for further transit-orientated development.
The tunnel boring machines were launched from this site in 2017 and arrived at Bayswater Junction in 2020, constructing the tunnels along the way. The tunnels vary in depth along the route, with the deepest point being beneath the Swan River, where the centre of the tunnel is approximately 26m below the riverbed. Overall, the depth of the tunnels averages 20m below the surface.
Cross passages (allowing access between the tunnels) and emergency exits have been constructed at various points along the rail route.