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Tuesday, September 06, 2016

More about Tunnelling

by Tim Inglehearn, lead project manager

In our last post I talked to you about Horizontal Directional Drilling (HDD) – one of the technology options we’re thinking about using to go underneath the Menai Strait. Today we’ll look at tunnelling, the other technology option we could use.

The tunnelling project most people will be familiar with is the construction of the Channel Tunnel. The method we’d use to build our tunnel is pretty similar, although our tunnel would be a lot smaller.  

The first thing we’d need to do is build a tunnel shaft – this lets us get our tunneling equipment down to the depth we need. Every tunneling job is unique so the machine we’d need would be designed and built specifically for this project. This is an impressive and complex piece of kit that drills, collects sediment and transfers it on a conveyor belt so it can be removed. We’d need to assemble all of the machines pieces and build it in sections at the bottom of the tunnels shaft as it could be quite long.

Tunnel shaft entrance

The geology in North Wales and the depth of the Menai Strait means our tunnel shaft would need to be approximately 100 metres deep. That’s about three and a half times the Marquess of Anglesey column, or roughly the length of a football pitch. To fit all of the equipment this would be dug out in sections and the walls lined with concrete.

We’d lower a tunnel boring machine (pictured below) to the bottom of the tunnel shaft and begin work digging a tunnel. We’d need a tunnel about 4-5 metres wide (roughly the length of a family saloon car), to keep the cables cool and allow access for maintenance.  

Tunnel boring machine (TBM) being lowered into tunnel shaft

The tunneling machine would continue digging until it reached its destination in Gwynedd.  This is known as a reception site – just like with HDD. At our reception site, another shaft – called a breakthrough shaft – would await our tunnelling machine’s arrival. This would be a similar depth of about 100 metres.

Once the tunnelling machine has broken through, we’d remove it and begin laying our high-voltage power cables. To do this, we’d mechanically pull them from large drums from the drive site, through the tunnel, until reaching the breakthrough shaft.

Both our drive site and reception site would need to be close to the shore and they’d be influenced by the geology on either side of the Menai Strait – we’ve currently identified four areas where they could be.

Once the shafts and tunnels are complete and the cables in place, we’d build tunnel head houses at the entrance to each shaft. These are roughly the size of a three storey building and give our construction team an access point for them to do their maintenance work. They also house the ventilation equipment that’d keep our cables cool.

Completed tunnel with cables

Example of a tunnel head house under contruction

At National Grid we’ve got a massive amount of experience of constructing tunnels. We’re in the middle of a seven-year project to create a 32km electrical network under the streets of London at the moment. Take a look – it’ll give you a better idea of what our tunnel under the Menai Strait could look like.   

We’ve now looked at both technologies we might use. We’re completing our planning work at the moment and will be back to provide more information later in the year.

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