Have you ever wondered why the route alignment for metro tunnels are always bendy and curvy? See the planning route for Crossrail2 below for instance. Even in between two stations, the tunnel is never ever a straight line. Doesn’t a straight-line tunnel make everyone’s life easier – the train runs fastest on a straight line, making the journey time shortest, passenger comfort the best, and wear to the tracks minimal. Just why not ever a straight line?
Figure above: planning route for Crossrail2, one of my key projects
Well, the rationale is absolutely correct, and it is not because tunnel engineers are incompetent of drawing a straight line. In fact, tunnel engineers try to keep a straight line where possible when planning the tunnel route. But reality sucks – we can never get what we want in a straightforward manner. The reality is full of constraints and our new tunnels have got to get around them.
• Existing underground assets, such as buried pipes, tunnels, deep basements and piles. Cities like London has over 100 years history of tunneling. There are a maze of various tunnels and deep foundations under the city. At certain locations the space underneath the city is virtually hollow after all the digging over the years. A new tunnel has got to find new space that bypass all existing ones, and believe me all the best spots have been taken. Some early-built tunnels such as the Circle line are ‘cut-and-cover’ tunnels, which means you dig up the streets, put a tunnel box in and put the streets back on top. You won’t feel or see anything from the streets, but there is a live railway line running inches under your feet. New-build tunnel nowadays in London and other developed cities rarely have the luxury to do ‘cut-and-cover’ and mostly resort to deep-mined tunnels, which are comparatively costly, risky, slow-progress, but causes minimal amount of disruption to the city. Deep-mined tunnels are key-hole surgeries to cities to provide the transport or services it needs.
• Ground conditions. Difficult ground increases risk for tunneling. It may increase not only the likelihood for catastrophic collapse of tunnels, slow down the tunneling progress, but also the impact on existing third party assets (by causing settlements which make them crack).
• Surface settlement requirement. This ties into the first two points. The surface settlement must be controlled down to an acceptable level, so as not to wreak havoc on buildings at the surface, otherwise the client will face claims after claims for the damages they have done. This requires the tunnel to have sufficient cover (thickness of soil that is ‘covering’ the tunnel before reaching the surface), and specific tunneling techniques such as earth pressure balancing. Nevertheless, sometimes the building is simply too important to tunnel under – think the gold bars in the vault of Bank of England or the queen’s face when you tell her you have to tunnel under her bedroom in Buckingham Palace. In these situations, it is simply better to steer away from them.
In addition, there are requirements from the railway operational perspective:
• Available surface space. For a metro line, although the majority of it is underground, some things have to stick out of the ground, such as stations or shafts. People need to get down to the tunnel through stations. They also need to breath. So intermittently there might be a need for a ventilation shaft to pump in fresh air for people. These shafts can also be used for escape and fire intervention in case of serious incidents. They can also serve as ‘draught reliefs’ which reduces the ‘piston effect’, meaning your ears won’t feel too sore when the train runs quickly inside the tunnel. If you ever took the Jubilee Line to go from Canada Water to Canary Wharf, you will know what I am talking about. We have to find free space at the surface to plop these stations and shafts and you know how hard it is to find space in London. Plus metro stations need locations with good access and ideally close to important/popular places. They also typically need at least two access/egress points, and be of rectangular box in shape.
• Minimum bending radius for the tracks – trains can not make too much of a sudden turn at high speed. An alignment route too bendy means speed restrictions on the trains. Then the client’s business (commercial) case takes a massive hit as it relies on how quickly the trains can take people from one station to another. Sometimes because of the orientations of the stations (see previous bullet point), you have to swing around your tunnels at its allowed bending radius to connect the stations together.
• Maximum gradient for the tracks – the tracks can not rise up or dive down too quickly. This again has to do with speed restrictions, as well as passenger comfort and safety. And believe it or not, there is actually a minimum gradient as well. Because there might be water coming into the tunnel, either by leakage of ground water, or water that is used to clean the tunnel, there is a minimum gradient of the tunnel required, to prevent water from lodging and keep water flowing towards a low point where a sump with a pump is located.
There are a much wider spectrum of other considerations, such as:
• Worksite selection – noise, pollution, public relations, regeneration
• Construction logistics
• Stakeholder management
• Collaborative development
A tunnel engineer has to take all these into consideration and every single one of the factors is likely to deviate the tunnel out of its straight position. No wonder it can never be a straight line.
There is a myriad of topics on the planning of tunnels and I will keep making further blog posts on this subject.
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