Feature article

Propping up the wall: how to rescue a leaning tower Image courtesy of John Burland Civil engineer John Burland talks about the perils John Burland in front of the tower and practicalities he helped to save, in Pisa of supporting some of the world’s most iconic buildings. Physics Physics Earth science By Susan Watt History General science “Mexico City Engineering was founded on an ne of the joys of being a civil old volcanic lakeAges of 10+ very, very soft engineer, believes John Bur- volcanic sediments, which are incred- This article offers an insight into the life and work of an engineer: his land, is that “every single ibly compressible,” he says. “Pump- decision to study engineering, how science and engineering methods projectO is unique; no buildings are ing water from beneath that area for are combined, and his small and large projects. The Leaning Tower of alike”. There is, however, one feature the city’s water supply has caused the Pisa and its problems are well known, but it is especially exciting to shared by most of the buildings on ground to settle, literally by metres. be told the story of how it was saved by the man who did it. which John has worked: they seem to That has caused huge problems, have been in danger of falling down. because theThe settlement topic is interdisciplinary, isn’t uniform. can be easily understood without much Most famously, John was one of the Half of theknowledge Metropolitan of physics, Cathedral and, is because the article is written like an ad- engineering experts called on in 1990 built on anventure, area where can anbe oldused Aztec in many ways and to enhance the teaching of to help rescue the Leaning Tower of temple onceseveral stood, subjects. which compressedTeachers can find their own ways of using the article, Pisa, Italy, but the Big Ben clock tower for example in geography, general science, history, physics or engi- in London, UK, and even the Metro- neering lessons, but also in language lessons, to read and translate the politan Cathedral in Mexico City have article. For students aged 10-15, I would recommend the article for also benefitted from his expertise. general reading, while older students can benefit from it in the context Rescuing the Big Ben clock tower of lessons on engineering. The article is also suitable for comprehension questions such as: The Big Ben project was not so much a rescue as damage limita- · What problems may occur if massive buildings are built on unsta- tion. As a soil mechanics engineer, ble ground? Describe these problems in detail. John was already involved in deep · Which methods can be used to stabilise unsafe ground and how excavation projects in London when do they work? the decision was taken in the early Gerd Vogt, Higher Secondary School for Environment 1970s to build a multi-storey car park and Economics, Yspertal REVIEW underneath the Houses of Parliament www.scienceinschool.org Science in School I Issue 26 : Spring 2013 I 9 and the Big Ben tower. John was ap- Mistake or not, John’s expertise you’re trying to meet them. I don’t proached to help with the design and was again called on years later when, believe you can do anything else,” he construction, to ensure that the state in the early 1990s, an even deeper says. buildings would not be damaged by excavation was needed for an under- the work on the 20 m-deep car park ground train line being run beneath How it all began beneath. The fear was that another fa- the Houses of Parliament. Again As an engineering student, John mous leaning tower might be created, John’s team managed both to measure initially disliked the discipline of this time in London. and to control the movement – by soil mechanics, in which he is now Constant monitoring was crucial pumping cement into the ground regarded as such a prominent expert, to the project, as John recollects: “My before the excavation started – but but warmed to it while studying for team was involved in taking measure- not without a lot of attention from the his PhD at the University of Cam- ments as the car park was constructed, media. “The press got very excited bridge. After Cambridge, he joined and we discovered all sorts of unex- about it, and we still get questions the Building Research Establishment pected things – as you do when you occasionally, and see headlines such (BRE) in London, a UK government take measurements. For example, we as ‘The Palace of Westminster is about research organisation that had pio- predicted that the Big Ben clock tower to subside into the River Thames’,” neered the study of soil mechanics. “I would move towards the east, and it John says. wanted to go there and work on real actually moved westwards. We got John has had a lot of practice at buildings, and find out how they re- the amount right, but we got the di- dealing with the international press, ally behaved,” he says. rection wrong, and we had to explain as well as in tackling technical issues. At BRE John worked on his first to Black Rod [the House of Lords of- So how does he approach this? “My high-profile project – the Sydney ficial] why, even though we were out line is to tell it as it is – say what the Opera House – and honed his skills in by 180°, it wasn’t a serious mistake!” technical challenges are and how foundations and excavations. “I had a wonderful 16 years at BRE work-

Image courtesy of John Burland ing on all sorts of projects, but one in particular required digging deep excavations in urban areas,” he says. “So I began to get involved with some of the deep excavations that were taking place in London – the Barbi- The new can Arts Centre, for example, which underground train was a huge excavation in the City of station under London. And because of this work, I construction at was asked to become involved with Westminster the Houses of Parliament car park. That became a seminal project that was published in the engineering journals. And I guess it was because of my involvement with that that I got involved with Pisa.” The Leaning Tower of Pisa in danger This was in 1990, when the famous Leaning Tower of Pisa was leaning rather too far for comfort. Not only that, but a medieval tower in the northern Italian town of Pavia had collapsed the previous year, killing four people. So the Italian government set up a multidisciplinary commission of international experts to prevent the Pisa tower meeting the same fate, and John was asked to join it.

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of ground it was built on, as John explains: “The ground there is so soft that it’s like foam rubber, and in this situation there’s a critical height for a building of a given weight and di- A cross-section through Burland John of courtesy Image the Leaning Tower of ameter before it starts to lean and fall Pisa, before John and his over. And the Pisa tower is exactly at colleagues began their that critical height: amazing.” work to rescue it So the tower was hovering between

stability and collapse. “It was going to fall over, but it was impossible to say when – a storm or an earthquake could have finished it off. So we had to do some very urgent work if we were to stabilise it,” John says. The problem is that once a tower starts leaning, it becomes increasingly unstable. “As the inclination increases, the centre of gravity moves, produc- ing an additional overturning force,” he explains. “So it gets worse and worse, and then it collapses.” At the time, the tower was moving at about

1.5 mm a year, but the movement was Physics accelerating – which was terrifying, John says. Of course, working on such iconic buildings involves more than just engineering. “The problem was In the beginning, not everyone so it’ll never fall down’. So our job incredibly complex, because we had agreed that the tower needed sav- was to bring some science to bear.” to obey the laws of conservation of ing. “There was this huge mythology The first step was to investigate how about the tower. People were saying the tower had been built originally, to ‘Don’t touch it, it’s quite safe as it is; if understand why it was leaning. The you do anything to it, it’ll fall over’,” team discovered that the tower had says John. “There was a quote from probably begun to lean from day one. the mayor saying ‘It’s in the Piazza “You can see tapered layers of ma-

dei Miracoli [the Square of Miracles], sonry, showing that the builders were Image courtesy of John Burland trying to correct the slant even as they Image courtesy of tiseb / Flickr were building it,” says John. The Leaning Tower He and other technical experts spent of Pisa, pictured two years trying to understand why before John’s the tower was leaning and what was work on it began, controlling its movements, which was is surrounded by absolutely fundamental to solving mythology. the problem, John believes. “Up until then, people had just proposed solu- tions at random, but until you really understand things, you’re likely to get it wrong.” They discovered that the leaning The Leaning Tower of Pisa, after John’s was caused by a combination of the work, is still leaning, but more stable. tower’s dimensions and the type

www.scienceinschool.org Science in School I Issue 26 : Spring 2013 I 11 Image courtesy of John Burland ancient monuments, which forbid you to change the character of the monu- ment. So you couldn’t go and stick a great big prop on one side,” John says. “And it had to continue leaning: the Italians didn’t want a vertical Leaning Tower.” Stabilising the Leaning Tower The team’s solution was inspired by the tunnelling work that John had already carried out in London. “When you drill a tunnel in the ground, you always cause the ground to settle a little bit. And I thought that if we could magically induce the ground to settle a little bit on the high side of the foundations, we could perhaps reduce the inclination of the tower enough to The Metropolitan Cathedral of Mexico City, the oldest and largest cathedral in Latin make it stable,” he says. “So we came America, photographed during John’s work on it up with this idea of extracting small amounts of soil, rather like mini- But there was a risk that putting any stabilise the water table,” says John. tunnels.” idea into practice could itself make the The solution to this problem was tower topple. “There were all sorts of engineering of a much more standard “There were all sorts of ideas about what you could do to sta- type, as John explains: “We put in bilise the tower, and all of them would some drains on the north side, which ideas about what you work if you could implement them, ran the water into wells that are kept could do to stabilise the but implementing them would cause at a constant level by discharging into the tower to fall over,” says John. So a conduit. That’s all been in operation tower, and all of them the team did a huge amount of com- since about 2006, and it really has sta- would work if you could puter analysis and precise monitoring bilised the water table. And that’s one implement them, but im- to test the tunnelling approach before of the major reasons why the move- going ahead. “In the end we extracted ment has stopped.” plementing them would about 70 cubic metres [of soil], a cou- So is the tower now permanently cause the tower to fall ple of lorries full, so it was really quite secure and stable? “We’ve continued over.” small,” he says. our measurements for the past three But the monitoring and historical or four years and for all practical pur- investigations had also revealed an- poses it’s not moving,” he says. other factor: the tower leaned further Saving Latin America’s oldest Image courtesy of John Burland to the south in winter than in sum- mer. “Originally I thought this was cathedral something to do with the sun going Remarkably, at the same time as he round the tower, but then we began was in the international spotlight for to measure the way the water table the Pisa project, John was also work- changes seasonally. Whenever there ing on another endangered historical is a rainstorm – and in Tuscany they building, this time in Mexico City. As get incredibly heavy rain storms in the in Pisa, the problem was that one side winter – the water table comes up a of the Metropolitan Cathedral − the little bit more on the north [high] side oldest and largest cathedral in Latin than on the south [low] side, so it lifts America − was subsiding more than the north side and pushes the tower the other. Once again, changes in the Drainage was part of the solution to further to the south. So in addition to water table were found to be the un- stabilising the Leaning Tower of Pisa reducing the inclination, we had to derlying cause, as John explains.

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- . See: page 57. Issue 26 : Spring 2013 I Feature articleFeature To learn how to learn how To use this code, see This article is based on an interview This article is based emy of Engineering, Ingenia maga- articles about engineer zine offers ing written for the non-specialist. See: www.ingenia.org.uk In 2005, Ingenia published an inter view with John Burland. See the magazine’s website (www.ingenia. link: or use the direct org.uk) http://tinyurl.com/bou227h Burland, see: Smith D (2009) Engineering beats Physics Education Pisa’s problem. doi: 10.1088/ 44(3): 311-313. 0031-9120/44/3/M02 the full you might like to browse articles pub- collection of feature lished in Science in School www.scienceinschool.org/features science is a freelance Susan Watt Science in School Science in School Dr Eleanor Hayes. given by Professor John Burland to given by Professor Science in School of the editor-in-chief former students all over the place who all over the former students Burland, I’ll never ‘Oh, Professor say, us of the model you showed forget - they remem such and such’. Because remember ber models; they don’t equations.” Acknowledgement Resources Acad- Published by the UK’s Royal For a further interview with John this article, If you enjoyed reading She studied natural writer and editor. sciences at the University of Cam bridge, UK, and has worked for the Science Museum, London, and the British Council. Her special interests the history and philosophy of sci- are ence and science education. - John is understandably passionate “My experience with children is “My experience with children This philosophy seems to have about this unique building, regarding about this unique building, regarding his most as perhaps the Pisa project satisfying achievement in a lifetime of But he is also passion daring projects. ate about teaching, and believes that is the right hands-on approach a truly way to engage the next generation of scientists and engineers. that they long to get their hands on things,” he says. “And in some senses I’m quite dismayed at how the the computer, virtual world, through can computerise You that. is replacing models, but it’s not the same as giving to handle and to something children because that’s the way push and prod, cultivate curiosity and the you really intrinsic understanding of how things work.” become a hallmark of the Burland to teaching, as John says: approach “I’m well known for bringing physi- cal models into class, and I bump into John’s first high-profile project was on first high-profile project was John’s Australia. House, the Sydney Opera When a multi-storey car park was built car park was When a multi-storey in of Parliament underneath the Houses called in to make sure was 1979, John (on the right) tower that the Big Ben clock did not become another leaning tower. Engineering in the classroom Image courtesy of Moyan Brenn / Flickr Brenn of Moyan Image courtesy - The result is that, over the 300 or is that, The result So is a building ever in such a bad And buildings don’t get any more the ground and made it much firmer. it much firmer. and made the ground built on un But the other half was the cathedral so disturbed ground, set- differential underwent these vast and more.” tlements of 1.5 metres existence, so years of the cathedral’s and in the 1990s it has cracked badly, still continuing. the movement was the experts in Mexico Coincidentally, to sta- considering soil extraction were bilise the cathedral at the same time as for this approach John was proposing in doing it “They were the Pisa tower. the principle but way, a very different of us was the same,” he says. “Three on the Pisa commission got involved with the Mexican cathedral because of - that link.” The soil extraction success fully evened out the rate of sinking the cathedral’s foundations, across the risk of further reducing greatly cracking. state that John would advise just giving up? “The decision is probably economic in the end, but it’s also a question of how important the build- he says. ing is historically,” important historically than the Lean- enhance the of Pisa. To ing Tower success of the engineering work, the tower has now been cleaned – as John of seeing on a recent had the pleasure visit. “It’s now absolutely mind-blow- ing, this beautiful white marble tower, dramatically leaning, with the most beautiful inlays and patterns in the marble. It absolutely takes your breath he says. away,” www.scienceinschool.org

Image courtesy of jimmyharris / Flickr