On 24 November Professor Lord Mair CBE FREng FRS, Sir Kirby Laing Professor of Civil Engineering and Head of Civil and Environmental Engineering at the University of Cambridge, addressed an audience of over 150 people at the Academy’s annual Hinton Lecture. This year’s lecture focused on the role of geotechnical engineering in creating underground infrastructure, with examples drawn from projects including Crossrail and London’s Northern Line extension. The lecture is now available to watch on RAEng.tv.
Professor Lord Mair outlined the huge challenge for engineers building underground, beginning with the need to understand the sometimes complex geology of an area before tunnelling begins. To construct the Stepney Green crossover during Crossrail, for example, a 15 metre wide cavern had to cut across different layers of sand and clay, requiring a range of excavation techniques. By understanding the types of rock they would encounter at each end of the site, engineers were able to use specialist equipment to relieve water pressure before digging through and creating a single, structurally sound cavern which was hailed as a huge success.
Crossrail also used precision compensation grouting methods, where an extensive network of tubes was installed beneath key structures before tunnelling began. By carefully monitoring the ground and movement of any buildings on the surface, engineers knew which areas to target by injecting supporting grout along the tubes above the deep tunnels. By adding grout at the precise locations requiring reinforcement, the technique compensated for any settlement of the ground caused by the tunnelling. Previously used to support Big Ben during the Jubilee Line extension, Professor Lord Mair revealed how precision compensation grouting is being used to protect listed buildings at Finsbury Circus today.
Another significant challenge for new underground infrastructure is navigating through the work of previous engineers by avoiding existing railway tunnels, utilities and building piles. The forthcoming Northern Line extension will present one particular challenge where sophisticated monitoring will be critical, as it will involve tunnelling directly through the piles of Battersea Dogs and Cats Home without disturbing its residents, who must remain in situ throughout construction. During Crossrail excavation, the equipment used was so precise that the Crossrail tunnel could be dug to touch the Royal Mail tunnel running past it at Bond Street without causing any damage.
Such close encounters provide unique opportunities to assess what happens in previously untested situations, through careful monitoring with the latest sensor technology. Professor Lord Mair revealed how his team were involved in the installation of optical fibres and sensors to monitor and analyse the impact of creating openings and passageways in existing tunnels. The findings of the analysis will allow subsequent projects to save time and materials on cross tunnel construction. Fibre optic load sensing technology is also being used in National Grid power tunnels, and Professor Lord Mair also discussed wireless sensor networks which can be installed to monitor existing structures as well as new constructions.
When asked about the future of tunnelling technology, Lord Mair predicted that sensors will become increasingly important, particularly in the achieving the ‘holy grail’ of tunnelling without volume loss. Throughout the lecture, Professor Lord Mair paid tribute to colleagues, particular PhD students, whose state-of-the-art work on these sensors and other engineering technology has helped to make projects like Crossrail a success.