Professor Robert Mair recently gave a Public Lecture at the Royal Society in London. The Royal Society hosts regular Public Lectures, designed to give a perspective of science in society from leading scientists, writers and broadcasters. The title of the lecture was "What's going on Underground? Tunnelling into the Future".
In Shanghai 140 km of tunnel were built in 2008 alone and there will be 400km of new tunnels completed by 2010. In Rome a new metro line beneath the Colosseum is about to be constructed. In London the exciting Crossrail project has got the green light. Urban congestion is a serious problem in many cities, so the creation of underground space and in particular the development of underground transport is environmentally essential. How can tunnels be built in ground sometimes as soft as toothpaste? What can go wrong? Will buildings above be affected by subsidence? What else is underground already that might get in the way? Geotechnical engineering, the application of the science of soil mechanics and engineering geology, plays a key role in answering these questions.
Professor Mair presented the latest methods of tunnelling that will ensure that London's Crossrail scheme can be safely constructed without damage to the buildings above ground. Work on the Crossrail scheme which will connect Maidenhead in Berkshire to Shenfield in Essex via Heathrow airport and London began in mid-January and the new line is due to open in 2018. The project will involve major new tunnels and stations constructed beneath many buildings in central London. The tunnels and stations will be considerably larger than a typical London Underground station and so extra care will need to be taken to protect existing buildings.
According to Professor Mair, engineers will use an innovative technique called compensation grouting to protect certain key buildings during construction of the tunnels. Before tunnelling, steel tubes known as 'tube a manchettes' (sleeved tubes) will be installed in the ground between the tunnel and the buildings above. During tunnelling, grout (liquid cement) will be injected from any one of a large number of holes in the steel tubes into the ground to compensate for the ground movements being caused by the tunnel excavation. The result is that the building experiences only a slight settlement, compared with what might have been a severe settlement if no compensation grouting had been undertaken.
Speaking about the technique, Professor Mair says: "The most famous example of the successful application of this technique was the protection of Big Ben from tilting during construction of Westminster Station for the Jubilee Line Extension. It has also recently been used to protect King's Cross Station during tunnelling works beneath it and is now being used to safeguard buildings from tunnelling settlements in many international projects, including metros in Amsterdam, Barcelona and Rome."
Other advances in tunnelling include the development of sophisticated closed face tunnelling machines which provide continuous support to the tunnel face while the ground is being excavated. An 8m diameter tunnelling machine was used for the recent Channel Tunnel Rail Link project that built the tunnels connecting Folkestone with St Pancras Station for the Eurostar. "Tunnels as large as 15m in diameter - three times the size of a London double-decker bus - have recently been completed in the cities of Madrid and Shanghai. In Shanghai the ground was as soft as toothpaste."
Professor Mair describes the critical importance of geology and the development and application of the latest tunnelling techniques. Examples of current and future projects from all over the world demonstrate the size, technical challenges and complexity of modern underground construction. Protection from subsidence is critical and new ways to ensure buildings are unaffected during tunnelling are explained.