Trenchless Australasia

There’s nothing boring about underground tunnels

Underneath every major city across the globe are networks upon networks of underground tunnels – many are used daily by commuters on trains or light-rail or even travelling by road.

The tunnels help to keep vehicles off the condensed city streets and for traffic to flow more freely across urban areas.

In 2022, the NSW state government committed investing $120 billion in infrastructure projects over the next decade with almost $77 millions set aside for transport projects. Furthermore WestConnex, which is Australia’s largest infrastructure road project to date, is estimated to cost $16 billion to complete. The goal of the project is to not only ease congestion but connect communities in Sydney and create thousands of job opportunities.

But Australia isn’t slowing down either.

UNSW civil construction expert, Johnson Shen from UNSW Civil and Environmental Engineering, says underground tunnels provide sustainable transport solutions as Australia’s population density continues to increase a cross the nation, but particularly in capital cities.

“We need to think differently about how we move people around whilst saving them time on their commute,” says Shen.

“If we look at other hugely populated cities such as Singapore or Tokyo, they have extensive networks of underground tunnels to support their above ground transport system. Why? Because when you have limited space but still need to make the city liveable for its residents, you either must start building bridges or build tunnels below ground.

“No infrastructure project is ever cheap, but the benefit to the community is priceless,” says Shen.

Not all underground tunnels are made the same

The construction process of underground tunnels varies depending on what the tunnel will be used for once completed.

Usually, underground tunnels that are used for transit tend to be circular in shame which can be achieved using a tunnel boring machine (TBM). Occasionally referred to as a ‘mole’, TBM’s are very large machines used to excavate tunnels and are capable of boring through a variety of ground conditions including soil, hard rock and sandstone.

“What makes them great for tunnels which will be used for transit is the shape,” says Shen.

“If you think about the size of tunnel needed for a train to pass through, it’s consistent all the way through between station platforms and they’re much smaller than the size of the trucks that use traffic tunnels.

“While TBMs are expensive to make, they can produce a smooth tunnel wall all the way through whilst reducing the disturbance to the surrounding ground as it bores.

“For example, a specialised TBM was used to excavate twin metro rail tunnels from Barangaroo to Blues Point under Sydney Harbour.”

Conversely, unlike tunnels used for transit, tunnels that are used for traffic tend to be constructed using Australian roadheaders.

A roadheader is a piece of excavation equipment designed to mine hard rock formation. With bulldozer-style tracks on the arms, the pineapple-like head fitted with metal picks break and excavate the rock material,” Shen says.

“Tunnels used by buses and cars are wider and taller than transit tunnels, so there is more rock to bore. Engineers could use wider TBMs but these are much more expensive to manufacture and operate.”

Taking a dive under water

Oftentimes engineers face the challenge of attempting to connect two areas that are separated by a large body of water – sometimes making going underwater the only solution.

Unlike the tunnel boring method of constructing underground tunnels, some underwater tunnels are built using what is known as the submerged tube method.

Shen explains that the construction design process must take into consideration the depth of the tunnel to ensure it’s not too close to the bottom of the water. Likewise, engineers will also undertake thorough geotechnical investigation works prior to construction beginning.

“Using the submerged tube method, each section of the tunnel is cast off-site before being towed by sea to the desired location,” says Shen.

“Section by section, each concrete tube is lowered into the water. During this stage, it’s important that each element is aligned correctly. Once submerged, the joint between each tube is emptied of water and then sealed tight – this process continues until each end is connected to the opening portals constructed on land.

“The trench is then backfilled with material to provide protection over the top of the tunnel, and the ground on either side is also reinforced.
“An example of this type of tunnel is Sydney Harbour Tunnel which connects Warringah Freeway at North Sydney, and the Eastern Distributor at South Sydney.”

Safety comes first

One of the most notable and dangerous concerns with underwater tunnels is keeping water from leaking into the tunnel.

However, while it may seem likely that water may burst through the walls of the tunnels at any given moment, Shen assures that they are very safe.

“There are protocols in place so that specialists are conducting routine checks on the tunnel so make sure they’re watertight,” he says.

“Building underground tunnels is one of the most complex engineering systems in the world. There are so many moving parts and things to consider but ultimately, the end goal is to connect communities and make it easier for people to move around quickly, but safely too.”

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