A tale of two towers: Big Ben and Pisa - Royal Academy of ...

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A Tale of Two Towers Ground movements and possible building damage were of major concern for this project situated as it is close to priceless historic buildings. The Department of the Environment called in the Building Research Establishment to advise on the project. Predictions of the ground movements were made using computer modelling. This is one of the earliest examples of the application of the finite element method in geotechnical design. For the analysis the London clay was assumed to behave in a linearly elastic way and laboratory testing at that time supported the use of such simple behaviour. The assumed values of Young’s modulus increased with depth and were based on the back-analysis of measurements of retaining wall movements of other excavations in London Clay (Cole and Burland, 1972 2 ). The predictions from the computer model were published prior to commencement of the work (Ward and Burland, 1972 3 ). Such a prediction published prior to construction has come to be known as a Class A prediction. The graph adjacent shows the observed inward displacements of the southerly retaining wall on completion of excavation (full line) which can be compared with the Class A prediction. It can be seen that the agreement, though not perfect, is very reasonable. The situation proved to be far less satisfactory for the ground surface movements around the excavation.The graph below shows the horizontal and vertical surface movements with distance from the edge of the retaining walls. The points show measurements made on various buildings and the full line shows the class A prediction. It can be seen that, although the predicted horizontal movements are once again in reasonable agreement with the observations, the shape of the predicted settlement profile differs significantly from the observations. The observed settlements were concentrated much closer to the edge of the excavation than the predicted values and were larger than them. A consequence of this was that, whereas the Big Ben Clock Tower was predicted to tilt away from the excavation by about 1/6000 it actually tilted towards the excavation by about 1/7000. We had got the magnitude about right but the direction wrong! Observed and predicted horizontal displacements of the Observed and predicted ground surface displacements outside the car park The Royal Academy of Engineering 5
A Tale of Two Towers We found this result very puzzling. However, shortly after we published our measurements (Burland and Hancock, 1977 1 ), Dr Brian Simpson FREng of Ove Arup showed that, by using a bilinear stress-strain law with a high initial stiffness, the agreement between observations and predictions could be greatly improved - particularly with respect to the vertical movements as shown by the broken lines in the two graphs on page 5 (Simpson et al, 1979 4 ). Simultaneously with this theoretical work, Professor Vaughan FREng began laboratory studies at Imperial College in which axial strains were measured locally on soil samples instead of between the end plattens as had traditionally been done. These measurements gave highly non-linear stressstrain behaviour with stiffnesses at small strains which were much larger than those inferred from traditional measurements. It now became clear that the pattern of ground movements observed at New Palace Yard, in which the vertical movements are concentrated close to the edge of the excavation, is due to the non-linear nature of the stress-strain behaviour of the soil. This process of prior publication of predictions, though uncomfortable at the time, has proved highly beneficial as it forced us all to ponder long and hard as to the explanation for the discrepancies. Without such public disclosure it would have been all too tempting to quietly ignore the discrepancies and move on to other things. The work at New Palace Yard and the measured response of the Clock Tower has spawned a whole new important area of study of the behaviour of the ground at small strains - indeed whole international conferences are now devoted to the subject. These studies are proving vitally important for modelling interaction effects between ground and structure, particularly in the urban environment where underground construction is a vital part of infrastructure development. We now travel to Italy to consider the challenges faced by the Pisa Commission. 3. MOVEMENTS OF THE PISA TOWER The ground underlying the Pisa Tower consists of three distinct layers. Layer A is about 10m thick and primarily consists of soft estuarine deposits of sandy and clayey silts laid down under tidal conditions. Layer B consists of soft sensitive normally consolidated marine clay which extends to a depth of about 40m. This material is very sensitive and loses much of its strength if disturbed. Layer C is a dense sand which extends to considerable depth. The water table in Horizon A is between 1m and 2m below ground surface. The surface of the marine clay is dished beneath the Tower showing that the average settlement is between 2.5m and 3.0m - a good indication of how very soft the ground is. The axis of the tower is not straight - it bends to the north. In an attempt to correct the lean during construction the masons placed tapered blocks of masonry at the level of each floor to bend the axis of the tower away from the lean. 6 The Royal Academy of Engineering Soil profile beneath the Leaning Tower of Pisa
Page 1 and 2: 2002 Joint Lecture The Royal Academ
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Page 5: A Tale of Two Towers Vertical cross
Page 9 and 10: A Tale of Two Towers The discovery
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