Why tunnels work

Tunnelling is becoming a popular solution as cities around the world look to expand their transport networks. They are very useful in locations where above-ground options are exhausted and communities place a high value on land at ground level.

Tunnels allow for effective, secure train operations and offer a viable alternative to an above-ground rail system. They save space and reduce cost. They also minimise potential environmental impacts on existing residential and business areas, compared to above-ground options.

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.

Experience in building tunnels 

Salini Impregilo (now Webuild), part of the joint venture with NRW to build the Forrestfield-Airport Link, is a world leader in complex construction with more than 110 years’ experience building tunnels and infrastructure around the globe. Their work to date includes the delivery of more than 1500km of tunnels and more than 7500km of metros and railways. The company is experienced working in challenging environmental conditions in major cities such as Doha, Qatar and Riyadh, Saudi Arabia, where they have been involved in two of the biggest metro systems in the world.

Read more information about some of the company’s current or completed projects. 


The diameter of each tunnel


The number of pre-cast concrete segments


The depth of the tunnels as they run under the Swan River