Tunnels are dug in materials ranging from soft clay to hard rock. The method of construction depends on ground conditions, groundwater, length and diameter of the tunnel, the depth of the tunnel etc.
There are three basic types of tunnel construction:
1. Cut & Cover Tunnels
A simple method for shallow tunnels where a trench is excavated and roofed over with an overhead support system strong enough to carry the load of what is to be built above the tunnel. The ‘Poly Pipe’ was built this way.
2. Immersed Tube Tunnels
Involves immersing tubes into a trench, sealing them and then backfilling. It’s a common method for under-river tunnels. The ‘Chunnel’ was built using this method.
3. Bored Tunnels
A reliable tool or technique is required to dig into and through the earth – be it soft ground or solid rock. Through mountains, a tunnel boring machine is used to chew through the rock. In small tunnels, the entire tunnel diameter is excavated (full-face method), whereas in large tunnels the top-heading-and-bench method is used. Whilst rock tunnels require little or no support to ensure they are structurally sound, unstable ground requires support during excavation and following that too. Tunnel shields are used – a steel cylinder is literally pushed into the soft soil, then debris is removed and a permanent lining of steel or pre-cast concrete is installed.
A World View
Switzerland has 468 tunnels totalling 403 km.
The world’s longest and deepest rail tunnel is in Switzerland – it’s the 57.5 km long twin-bored Gothard base, surpassing Japan’s 53.9 km Seikan rail tunnel and the Channel Tunnel (50.5 km). It cost US$12.5b and opened in 2016 after 17 years of construction.
Italy has 900 km of tunnels. The longest is Gran Sasso d’Italia, a little over 10 kilometres long.
Norway is considered the world’s leader in tunnelling. It has 1400 tunnels, including the world’s longest tunnel – Laerdal Tunnel – a single tube tunnel with a length of 24 kilometres. It cost US$114m and opened in 2000.
China has the world’s longest twin-tube tunnel measuring 18.02 km – it opened in 2007 after almost 5 years of construction. Collectively, China has 16,229 tunnels!
Zhongnanshan Twin-Tube Tunnel
Our (Perth) Tunnels
Graham Farmer Freeway
Type: Cut & Cover
Length: 6400 m
Construction duration: Between 1996 and 2000.
Cost: Unknown. $313m was mentioned but may include more than just the tunnel.
City Railway Tunnels
Type: Cut & Cover; Bored
Length 1: 600 m (Cut & Cover)
Length 2: 700 m (Twin-bored)
Construction duration: Commenced in February 2004. Tunnels completed by October 2006
Cost: Unknown. Included in $324.5m ‘Package F’
Airport Link Railway Tunnels
Length: 8000 m
Construction duration: Started in late 2017. Let’s say 3 years as promised!
Cost: Unknown. The entire METRONET contracts amount to $1.86b plus over-runs, kick-backs and legal fees 😉
I’m not saying Perth tunnels are pathetic – we don’t have the mountains, and we don’t have the driver volume needed to make flat, fast, superhighways. But what is interesting is that we built 7.7 km of tunnels barely under the surface in a total of 6.7 years. And when METRONET is finished we’ll have built 15.7 km of tunnels in around 10 years.
If China built tunnels at the rate we do (and one at a time), their tunnels would have taken 10,000 years to construct!
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