Explainer: how to build a tunnel

Under the sea or through mountains, it’s all the same. Roger Wollstadt

Londoners will be aware that there is a lot of work going on under their feet at the moment. There is the new Crossrail railway with eight subterranean stations, expansion of Tottenham Court Road, Bond Street and Victoria London Underground stations, new cable tunnels and a huge new sewer tunnel. The digging is also set to continue well into the next decade, with many other projects underway.

And London is not alone. Cities need tunnels to function efficiently and sustainably, and to make them attractive places to live. Increasing urban populations in cities across the world are creating a huge demand for tunnels. As cities grow, the need to put trains, roads and utilities underground becomes unavoidable. Urban planners are beginning to recognise the benefits of using underground space and are coming up with novel ways of using it. In Norway there is an underground ice rink. In Hong Kong there are plans afoot to put sewage treatment facilities and waste transfer stations underground, freeing up the surface for development.

Tunnelling used to be one of those “black arts”, heavily based on experience and empirical data. Now, however, it is technologically advanced in both design and construction. Excavation is completely mechanised wherever possible, and miners are more likely to be seen behind a control panel than holding a shovel.

The daily running costs are high, and there isn’t space in the tunnel to throw more resources at the problem, so it is important to have the best people you can afford. It also makes tunnelling a 24/7 business, where working 12 hour shifts is the norm.

For long and reasonably straight tunnels of constant diameter, tunnel boring machines (TBMs) are used. They consist of a cylindrical steel can, called a shield, with a rotating cutterhead in front. The teeth or discs on the cutterhead break up the soil or rock, which passes through openings and onto a conveyor that carries the spoil out of the tunnel.

At the back of the TBM, precast concrete segments are brought in and erected to form a circular ring. The TBM pushes itself forward off the front face of the ring, excavating the ground again, until it has advanced far enough to insert another ring.

For shorter tunnels, tunnels that change size, turn sharp corners or have junctions, it is not possible to use a TBM. In the old days temporary timber support would have been used, followed by either a masonry or brick lining. This was quite slow, so in the late 19th century masonry and brick were largely superseded by cast iron rings. These rings came in segments that were bolted together, and these can be seen on many London Underground stations, including those on the Jubilee Line Extension (East of Green Park), built in the mid-1990s. Now sprayed concrete, also known as shotcrete, is more common.

Sprayed concrete is propelled using high pressure compressed air and the velocity of impact ensures the air is driven out of the concrete so it is dense, strong and durable. To help it stick, a chemical accelerator is added so that the concrete sets as soon as it hits the wall and quickly gains strength.

Tunnelling under a city is a delicate business. Creating a void causes the surrounding ground to relax even if a stiff lining support is installed very quickly. These ground movements can result in settlement of buildings, or distortion of other tunnels already in the ground. This risk is managed by predicting the ground movements as accurately as possible and using construction methods that minimise them. Monitoring and mitigation are carefully controlled.

For me, tunnels are like museums or hospitals – a symbol of human civilisation. To excavate ground that has not been touched for millions of years, to work all the hours of the day and night, to produce a structure that will last for hundreds of years, and to do it all in the service of and with the minimum disruption to the surface humans, is truly quite a feat.