2002 Joint Lecture The Royal Academy of Engineering and The Royal Society of Edinburgh A tale of two towers: Big Ben and Pisa Speaker: Professor John Burland FREng FRS A Tale of Two Towers: Big Ben and Pisa Professor John Burland DSc(Eng) FREng FRS FICE FIStructE Imperial College of Science, Technology and Medicine Born in the UK, Professor Burland was educated in South Africa and studied Civil Engineering at the University of the Witwatersrand. He returned to England in 1961 and worked with Ove Arup and Partners for a few years. After studying for his PhD at Cambridge University, Professor Burland joined the Building Research Station in 1966, became Head of the Geotechnics Division in 1972 and Assistant Director in 1979. In 1980 he was appointed to the Chair of Soil Mechanics at the Imperial College of Science, Technology and Medicine. He is now Emeritus Professor and Senior Research Investigator at Imperial College. In addition to being very active in teaching and research, John Burland has been responsible for the design of many large ground engineering projects such as the underground car park at the Palace of Westminster and the foundations of the Queen Elizabeth II Conference Centre. He was London Underground’s expert witness for the Parliamentary Select Committees on the Jubilee Line Extension and has advised on many geotechnical aspects of that project, including ensuring the stability of the Big Ben Clock Tower. He was a member of the Italian Prime Minister’s Commission for stabilising the Leaning Tower of Pisa. He has received many awards and medals including the Kelvin Gold Medal for outstanding contributions to Engineering and the Gold Medals of the Institution of Structural Engineers and the Institution of Civil Engineers. He has been awarded three Honorary Doctorates including one from Glasgow University. Photograph courtesy of James Hunkin 2002 Royal Academy of Engineering/ Royal Society of Edinburgh Lecture A Tale of Two Towers: Big Ben and Pisa © Burland, John ISBN 1-903496-04-7 February 2002 Published by THE ROYAL ACADEMY OF ENGINEERING 29 Great Peter Street, Westminster, London SW1P 3LW Telephone 020 7222 2688 Facsimile 020 7233 0054 www.raeng.org.uk The Royal Academy of Engineering is a Registered Charity (No. 293074) A Tale of Two Towers A Tale of Two Towers: Big Ben and Pisa 1. INTRODUCTION This lecture tells the story of the movements of two world famous towers resulting from nearby construction activities and the application of novel geotechnical protective measures. The Big Ben Clock Tower was constructed in 1858, soon after the old Houses of Parliament were destroyed by fire. The clock tower consists of load-bearing brickwork with stone cladding rising to a height of 61m; this supports a cast-iron framed spire, giving a total height of 92m. The tower is supported on a mass concrete raft 15m square and 3m thick which is founded within the Terrace Gravels of the River Thames, at a depth of about 7m below Aerial view of the Big Ben Clock Tower and the Palace of Westminster ground level. The tower is estimated to have a weight of 85MN, giving an average bearing pressure of about 400kPa. The clock face is 55m above ground level and is out of plumb towards the northwest by 220mm. Thus the inclination is about 1/250 - an amount which is often quoted as being just discernable to the casual onlooker. This explains why tourists are often seen debating the verticality of the clock-tower! The Leaning Tower of Pisa stands within the Piazza dei Miracole and is the bell tower of the magnificent Romanesque Cathedral. The tower is an architectural gem and would be one of the most important monuments of medieval Europe even if it were not leaning. The Pisa tower is very nearly 60m high, has a 20m diameter masonry foundation and weighs 145MN. The foundation rests on a deep deposit of very soft estuarine and marine sediments. The tower leans due south and in 1990 the seventh level, which forms the base of the bell chamber, overhung the ground by 4.5m. It is estimated that The Leaning Tower of Pisa The Royal Academy of Engineering 3 A Tale of Two Towers the bearing pressure beneath the south edge of the foundation is about 1000kPa. Construction of the tower began in 1173 and took place in three stages. By 1178 the fourth level had been reached when work ceased for nearly 100 years. Between 1272 and 1278 construction took place up to the seventh level when work again ceased. Finally in 1360 work on the bell chamber commenced and was completed in about 1370. Our calculations show that if the long pauses between the three phases of construction had not taken place the tower would have fallen over. The pauses allowed consolidation of the soft sediments to take place thereby increasing the strength of the ground. There is one further important historical feature relating to the construction of the tower. In 1838 the architect Alessandro della Gherardesca Vertical cross-section through the Leaning excavated a walk-way (catino) around the base of the Tower of Pisa tower. It is known that the tower lurched to the south by nearly half a metre which brought it very close to collapse. Because of fears for its stability brought about by the collapse of a bell tower in Pavia in 1989, the Pisa Tower was closed to the public in January 1990 and the Italian Prime Minister set up a Commission in March 1990, under the chairmanship of Professor Michele Jamiolkowski, to implement stabilisation measures. 2. CONSTRUCTION OF THE UNDERGROUND CAR PARK AT THE PALACE OF WESTMINSTER In the early 1970’s an 18.5m deep underground car park was constructed in New Palace Yard and the project is described by Burland and Hancock (1977)1. The excavation comes to within 16m of the Big Ben Clock Tower and 3m of Westminster Hall. It was constructed using what is termed ‘top-down’ construction. The pile foundations and reinforced concrete diaphragm retaining walls were constructed first. Then the ground floor was cast and thereafter excavation took place downwards with successive floors being constructed from the top downwards. This method provides very effective support to the retaining walls thereby minimising surrounding surface ground movements. It is also environmentally friendly since it reduces noise and dust during construction. Model of the underground car park at the Palace of Westminster 4 The Royal Academy of Engineering 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, 19722). The predictions from the computer model were published prior to commencement of the work (Ward and Burland, 19723). 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, 19771), 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, 19794). 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. Soil profile beneath the Leaning Tower of Pisa 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 A Tale of Two Towers Careful analysis of the relative inclinations of the masonry layers has revealed the history of the tilting of the tower. At the end of the first construction phase it was actually leaning northwards by about one quarter of a degree. Then, as construction advanced above the fourth storey, it began to move towards the south and accelerate so that by 1278, when the seventh level had been reached, it was inclining southwards by about 0.6 of a degree. This had increased to about 1.6 degrees by 1360 when work on the bell chamber commenced. In 1817 two British architects used a plumb line to measure the inclination which by then was 5 degrees. Thus the construction of the bell chamber caused a very significant increase in inclination. Advanced computer modelling has revealed that the rapid increase in inclination as the seventh level was reached and the bell chamber was added is directly analogous to constructing a tower from model bricks on a soft carpet (Burland and Potts, 19945). It is possible to build to a certain critical height, but no higher, however careful one is - a phenomenon known as leaning instability. The tower was just at its critical height and was very close to falling over! The excavation of the catino brought the tower even closer to collapse. Precise measurements begun in 1911 show that during the twentieth century the inclination of the tower has been increasing inexorably each year and the rate of tilt has doubled since the 1930’s. In 1990 the rate of tilt was equivalent to a horizontal movement at the top of about 1.5mm per year. Moreover any interference with the tower resulted in significant increases in tilt. For example, in 1934 consolidation of the foundation masonry by means of grout injection resulted in a sudden movement south of about 10mm and ground water abstraction from the lower sands in the 1970’s resulted in an increase in movement of about 12mm. These responses confirm how very sensitively poised the tower was and how delicate any method of stabilisation would have to be. There has been much debate about the cause of this progressive increase in inclination. It has usually been attributed to creep in the underlying soft marine clay, the assumption being made that the south side was settling more than the north side. A careful study of the geodetic survey measurements going back to 1911 revealed a most surprising form of motion of the foundations which was radically different to previously held ideas. The theodolite measurements onto the first cornice (V1 in the diagram on page 4) showed that it had not moved horizontally - apart from two occasions when man had intervened. Also precision level measurements which commenced in 1928 showed that the centre of the foundations had not displaced vertically relative to the surrounding ground. Therefore the rigid body motion of the Tower could only be as shown here, with an instantaneous centre of rotation at the level of the first cornice vertically above the centre of the foundations. Motion of Tower foundations during The direction of motion of points FN and FS are shown by progressive increase in inclination vectors and it is clear that the foundations have been moving northwards with FN rising and FS sinking. The Royal Academy of Engineering 7 A Tale of Two Towers The discovery that the motion of the Tower was as shown has turned out to be crucial in three respects: 1. The observation that the north side had been steadily rising led directly to the suggestion that the application of a lead counterweight to the foundation masonry on the north side could be beneficial as a temporary stabilising measure by reducing the overturning moment. 2. The pattern of ground movements depicted led to the very important conclusion that the seat of the continuing long-term rotation of the Tower lies in horizon A and not within the underlying marine clay as had been widely assumed in the past. It can therefore be concluded that the latter stratum must have undergone a considerable period of ageing since last experiencing significant deformation (which was probably in 1838 when Gherardesca excavated the catino). This ageing resulted in an increased resistance to yield - a conclusion that proved to be of great importance in the successful modelling of the application of the temporary counterweights. 3. In the light of the measured motion of the Tower foundations, and consistent with the seat of the movement lying within Horizon A, it was concluded that the most likely cause of the progressive seasonal rotation was a seasonally fluctuating ground-water level in Horizon A due to seasonal heavy rainstorms that always occur in the period September to December each year. Accordingly a number of stand-pipes were installed in this Horizon around the Tower. Measurement made over a four year period have confirmed this hypothesis - commencement of rotation each year coincides with very sharp rises in the ground water level in the Horizon following each heavy rainstorm. Measures have been proposed to stabilise the ground water levels beneath the Tower. It is true to say that the identification of the form of motion of the foundations of the Leaning Tower of Pisa is the single most important finding in the development of the strategy for temporary stabilisation in which 600t of lead weights were placed on a concrete ring clamped to the base of the Tower by circumferential post-tensioning. This measure was implemented between July 1993 and January 1994 and proved to be very effective. Immediately following the application of the lead weights, activities commenced in London alongside the Big Ben Clock Tower that required urgent attention. We therefore have to return to London to attend to these and leave the Tower for a while to ponder on its permanent stabilisation. 4. THE INFLUENCE OF THE JUBILEE LINE EXTENSION ON THE BIG BEN CLOCK TOWER The construction of Westminster Station on London Underground Limited’s new Jubilee Line Extension (JLE) was predicted to produce significant movements of the Big Ben Clock Tower 6 (Harris et al 2000 ). A north-south cross-section through the new Westminster Station and the Clock Tower is shown in opposite. The Station consists of two 7.4m diameter platform tunnels in a vertically stacked arrangement below Bridge Street at depths of 21m and 30m below ground level. Alongside is a 39m deep excavation which forms an underground ‘box’ to house the access escalators and is the deepest basement ever to have been constructed in London. 8 The Royal Academy of Engineering A Tale of Two Towers Prior to any substantial excavation within the station escalator box, the 4.85m diameter running tunnels were driven as pilot tunnels. The lowest west-bound tunnel was constructed in March 1995 and the upper east-bound tunnel in October 1995. The running tunnels were then enlarged to 7.4m diameter to form the platform tunnels, the westbound and east-bound enlargements being carried out in February 1996 and November 1996 respectively. The retaining walls for the station box consist of reinforced concrete diaphragm walls. Like the adjacent Palace of Westminster car park, excavation was carried out using the top-down method with the struts and floors being installed progressively from the top down as excavation progressed. In order to minimise surrounding ground movements, low-level struts were installed in tunnels close to the base of the diaphragm walls prior to excavation below the main roof slab. Excavation within the diaphragm walls was undertaken between September 1995 and September 1997. Careful computer modelling of the tunnelling and excavation was carried out and was greatly aided by the measurements made during the construction of the underground car park in the 1970’s. Despite the provision of very stiff diaphragm walls and low level tunnelling struts, it was recognised that the combination of the two platform tunnels and the station box could lead to unacceptable tilting of the Clock Tower. The concern was that excessive tilting would lead to cracking where the Tower and the Palace of Westminster were Cross-section showing proximity of the new Westminster station connected. A contingency protective measure to Big Ben was called for and the relatively new technique of compensation grouting was adopted. The principle of compensation grouting is to inject grout (a mixture of cement, sand and water) under pressure into the ground at chosen locations so as to counter any subsidence that an overlying building might be experiencing. This is done by installing into the ground a number of steel tubes (known as TAMs, the abbreviation for ‘tubes à manchettes’) with holes machined into them at regular intervals, typically about 0.3m. Covering each hole is a short rubber sleeve which acts as a one-way valve allowing grout to be pumped out under pressure without flowing back in. Any hole can be selected for grout injection and the system allows repeated grouting through the same hole if required. The provision of grouting tubes below one of London’s busiest areas was not a simple matter and the horizontal array of grouting tubes were installed by drilling radially outwards from a vertical shaft which was located in the middle of Bridge Street. The tubes were about 50m long and were drilled beneath the foundation of the Clock Tower and immediately to the north. The Royal Academy of Engineering 9 A Tale of Two Towers The elevation of the tubes was chosen so that they were just within the London Clay to avoid encountering the ground water in the overlying gravel. Sixteen tubes were installed beneath the foundation having a maximum spacing of about 2.5m. The graph below shows the measured tilt of the Clock Tower throughout the construction period and for three years afterwards. The tilt is expressed as horizontal movements northwards in mm at a height of 55m. The dates of the various construction activities are indicated on the figure: the passage of the four tunnel drives are shown across the top of the figure and the dates of installation of the props at various depths within the escalator box are shown along the bottom. The maximum permissible limit on the change in tilt had been set at 1/2000 which is equivalent to 27.5mm at a height of 55m. A trigger level for initiating grouting was set 1/2500 (22mm). A construction control 40 range of between 15mm and 25mm tilt was adopted. 30 Tunnel Progress: Pilots Tilt of Clock Tower (mm/55m) WB Enlargements EB WB EB As anticipated, northward tilt of the Clock Tower commenced as the west-bound running tunnel 10 passed by and an immediate tilt of 4mm was recorded. Time 0 dependent movements then took place and it became clear that it -10 would be necessary to implement Nov-94 Nov-95 Nov-96 Nov-97 Nov-98 Nov-99 Nov-2000 compensation grouting in order Optical Plumb to keep the tilt of the Clock Measured horizontal movements of Big Ben at clock face level between 1994 Tower within permissible limits. and 2000 Between February 1996 and September 1997, when the deepest level of the escalator box was reached, grouting was undertaken to keep the tilt within the construction control range and this was generally achieved. Altogether 24 grouting episodes were undertaken in which a total volume of 122m3 of grout was injected. Without any compensation grouting the cumulative increase in tilt of the Clock Tower would have been at least 120mm which would certainly have resulted in significant cracking of the Palace of Westminster. 20 Construction Control Range Grouting Episodes Start of Grouting Box Excavation Progress [m]: 9 1 3 16 22 25 31 35 39 Since the end of construction, no further grouting has been undertaken. It can be seen from the graph above that time-dependent tilt has continued at a decreasing rate. This is consistent with computer predictions and is still being monitored very closely. The measurements indicate that the long-term tilt has almost stabilised at around 35mm. The damage to the Palace of Westminster has been very localised and very slight. The innovative technique of compensation grouting, which has never before been applied to a structure as fragile and of such historic importance as the Big Ben Clock Tower, has been extremely successful and is a great credit to the contractor, Balfour Beatty/AMEC. In the controversy that surrounds London Underground, the successful construction of the Jubilee Line Extension has not been given the recognition that it deserves. 10 The Royal Academy of Engineering A Tale of Two Towers The problems at Pisa are now pressing and we need to return there to decide on the permanent stabilisation measures. 5. STABILISATION OF THE PISA TOWER USING SOIL EXTRACTION The internationally accepted conventions for the conservation of valuable historic monuments requires that their essential character should be preserved, together with their history, craftsmanship and enigmas. Thus any invasive interventions on the Tower had to be kept to an absolute minimum and permanent stabilisation schemes involving propping or visible support were unacceptable and in any case could have triggered the collapse of the fragile masonry. As described on our previous visit to Pisa, temporary stabilisation of the foundations was achieved during the second half of 1993 by the application of 600t of lead weights to the north side of the foundations via a post-tensioned removable concrete ring cast around the base of the Tower at plinth level. This caused a reduction in inclination of about one minute of arc and, more importantly, reduced the overturning moment by about ten percent. In September 1995 the load was increased to 900t in order to control the movements of the Tower during an unsuccessful attempt to replace the unsightly lead weights with temporary ground anchors. A permanent solution was sought that would result in a small reduction in inclination by about half a degree which is not enough to be visible but which would reduce the stresses in the masonry and stabilise the foundations. Given that the foundation of the Tower was on the point of instability and that any slight disturbance to the ground on the south side would almost certainly trigger collapse, finding a method of reducing the inclination was far from straight forward and gave rise to many heated debates within the Commission. Many possible methods of inducing controlled subsidence of the north side were investigated. These included drainage by means of wells, consolidation beneath the north side by electro-osmosis and loading the ground around the north side of the Tower by means of a pressing slab pulled down by ground anchors. None of these methods proved satisfactory. A method known as soil extraction gradually evolved. This involves installing a number of soil extraction tubes adjacent to and just beneath the north side of the foundation as illustrated . The method had been successfully used previously, notably to reduce the damaging differential settlements within the Metropolitan Cathedral of Mexico City. But using it on a tower that was on the point of falling over was altogether another matter. How could we be sure that removal of soil Location of soil extraction tubes adjacent to and beneath the Tower The Royal Academy of Engineering 11 A Tale of Two Towers from beneath the high side would not create instability of the Tower? Over a number of years the method was studied first by means of physical models, then by numerical modelling and finally by means of a large-scale trial. A key finding from the model studies and numerical analysis was the existence of a critical line located about half a radius in from the northern edge of the foundation. Provided soil extraction from beneath the foundation took place north of this line the response of the Tower appeared always to be positive. However, if extraction took place south of this line the Tower would become unstable. Using a large-scale trial foundation in the Piazza, a drill was developed by the contractor Trevi which consisted of a hollow-stemmed continuous flight auger (otherwise known as an Archimedes screw) housed inside a contra-rotating 180mm diameter casing. This arrangement ensured that the drill could be advanced without any disturbance to the surrounding ground. The sequence of operations for carrying out an extraction is illustrated below. The trials showed that the cavities formed in the silty soil of Layer A closed gently and that repeated extractions could be made from the same location. The trial foundation was successfully rotated by about 0.25o and directional control was maintained even though the ground conditions were somewhat non-uniform. Very importantly, an effective system of communication, decision taking and implementation was developed. In August 1996 the Commission agreed to carry out limited soil extraction from beneath the Tower with a view to observing its response. A target of a minimum of 20 arc seconds reduction in inclination was set as being large enough to demonstrate unequivocally the effectiveness of the system. Due to bureaucratic and administrative delays it was not until the end of 1998 that preparatory work actually began. In December 1998 some temporary safeguard cables were attached to the third storey of the Tower. These stretched horizontally some 100m north of the Tower, passed over pulleys on the top of two massive A frames and Sequence of operations of the soil extraction drill were lightly tensioned by means of lead weights. In the event of adverse movements of the Tower these safeguard cables could be tensioned by adding additional lead weights to hold the Tower steady. It was never intended that they should be used to actually move the Tower northward. Preliminary soil extraction was carried out over a limited width of 6m using twelve bore holes lined with 220mm diameter casings. The auger and rotating casing had to be moved from hole to hole so that the operation was slow and cumbersome with a maximum of two extractions each day. The carefully developed system of communication and control involved a system of twice daily faxes from the site containing real-time information on the inclination and settlement of the Tower. A daily fax was issued by the responsible engineer (the author) summarising the 12 The Royal Academy of Engineering A Tale of Two Towers observed response, commenting on it and then giving a signed instruction for the next extraction operation with clearly stated objectives. It was rather like riding a bicycle by fax! Green, amber and red trigger levels were set for taking action in the event of adverse responses of the Tower. These included both rates and magnitudes of changes of inclination and settlement. The trigger levels were set after a careful study of about six years of records of movements of the Tower so as to avoid over stringent requirements and false alarms. On 9th February 1999, in an atmosphere of great tension, the first soil extraction took place. For the first few days, as the Results of soil extraction drills were advanced towards the edge of the foundation, the Tower showed no discernable response. Then slowly it began to rotate northwards. The results of preliminary soil extraction are shown at the left hand side of this figure. When the northward rotation had reached about 80 arc seconds by early June 1999 soil extraction was stopped. Northward rotation continued at a decreasing rate until July 1999 when three of the lead weights were removed whereupon all movement ceased. The success of preliminary soil extraction persuaded the Commission that it was safe to undertake full soil extraction over the full width of the foundations. Accordingly, between December 1999 and January 2000, 41 extraction holes were installed at 0.5m spacing with a dedicated auger and casing in each hole as shown below. Full soil extraction commenced on 21st February 2000 and the results of both preliminary and full soil extraction are shown above. It can be seen that a much higher rate of northward rotation was achieved than for preliminary soil extraction averaging about 6 arc seconds per day resulting from the removal of about 120 litres of soil. It was gratifying to note that significant uplift of the southern edge of the foundation took place indicating a reduction in bearing pressure at this highly stressed region. Towards the end of May 2000 progressive removal of the lead ingots commenced, Drilling rig and 41 extraction tubes The Royal Academy of Engineering 13 A Tale of Two Towers initially with two ingots per week (about 18t). In September 2000 this was increased to three per week and then to four per week in November 2000. Removal of the lead ingots resulted in a significant increase in overturning moment but the soil extraction continued to be effective. On 16th January 2001 the last lead ingot was removed from the post-tensioned concrete ring and thereafter only limited soil extraction was undertaken. In the middle of February the concrete ring itself was removed and at the beginning of March progressive removal of the augers and casings commenced with the holes being filled by a bentonitic grout. Finally in the middle of May the safeguard cables were removed from the Tower which resulted in a southward rotation of a few arc seconds. To counter this, a small amount of additional soil extraction was carried out with the final extraction and auger removal taking place on 6th June 2001 - the date when the Tower was released by the Commission from intensive care. Pageantry during hand-over ceremony on 16 June 2001 The target reduction of inclination had been half a degree, being an amount not visible to the casual observer but sufficient to stabilise the foundations and reduce the stresses in the masonry by a significant amount. In fact a total reduction of 1830 arc seconds was achieved which is equivalent to a northward movement of the seventh floor of 440mm. The Tower is now back at its inclination in 1838 at the time Gherardesca dug the catino and before its dramatic lurch south. On 16th June 2001 the Tower was formally handed back to the civic authorities at a colourful ceremony and celebrations continued throughout the next day to mark the feast of San Ranieri, the patron saint of Pisa. On the 15th December 2001 the Tower was re-opened to the public nearly twelve years after it had been closed. An obvious question is how will the Tower behave in the future? Two scenarios have been developed. A pessimistic one is that the Tower will remain stable for a while, followed by a resumption of rotation southwards at a much reduced rate. With this scenario it would take over 100 years before another intervention on the foundation is required. An optimistic scenario is that continuing rotation will cease apart from small cyclic movements caused by seasonal changes in the ground water table and also the influence of differential subsidence which is affecting the whole Piazza and which is reflected in the Tower. 14 The Royal Academy of Engineering A Tale of Two Towers 6. CONCLUSION The conservation of both the Big Ben Clock Tower and the Tower of Pisa has provided immense civil engineering challenges. Both compensation grouting and soil extraction are highly innovative methods of stabilisation that are completely consistent with the requirements of architectural conservation. Their implementation has required advanced computer modelling, large-scale development trials, an exceptional level of continuous high precision monitoring and carefully developed systems of day by day communication and control. REFERENCES 1. J.B. Burland and Hancock,R.J.R.(1977). Underground car park at the House of Commons, London: Geotechnical aspects. The Structural Engineer, 55;2;87-100. 2. K.W. Cole and Burland,J.B.(1972). Observations of retaining wall movements associated with a large excavation. Proc. 5th European Conf. on Soil Mechanics and Foundation Engineering, Madrid 1972, 1;445-453. 3. W.H. Ward and Burland,J.B.(1972). The use of ground strain measurements in civil engineering. Phil. Trans. Royal Soc, London, A, 274, pp 421-428. 4. B. Simpson, O’ Riordan, N.J. and Croft, D.D. (1979). A computer model for the analysis of ground movements in London Clay. Geotechnique 29, No 2, 149-175 5. J.B.Burland and Potts,D.M.(1994). Development and application of a numerical model for the Leaning Tower of Pisa. Proc. Int. Symp. on Pre-failure Deformation Characteristics of Geo-materials, Hokkaido, Japan, Vol 2; 715-738. 6. D.I.Harris, Mair, R.J., Burland, J.B.and Standing, J.R.(2000). Compensation grouting to control tilt of Big Ben Clock Tower. Geotechnical Aspects of Underground Construction in Soft Ground. Ed. by Kusakabe, Fujita & Miyazaki, Balkema, 2000, p.225-232. The Royal Academy of Engineering 15 A Tale of Two Towers 16 The Royal Academy of Engineering The Royal Academy of Engineering The objectives of The Royal Academy of Engineering are to pursue, encourage and maintain excellence in the whole field of engineering in order to promote the advancement of the science, art and practice of engineering for the benefit of the public. The Academy comprises the United Kingdom’s most eminent engineers of all disciplines. It is able to take advantage of their wealth of knowledge and experience which, with the interdisciplinary character of the membership, provides a unique resource with which to meet the objectives. Its activities include an extensive education programme, research chairs and fellowships, visiting professorships, industrial secondments and international travel grants. It provides expert advice on engineering matters to government and other bodies and administers the UK’s premier annual prize for innovation in engineering, The Royal Academy of Engineering MacRobert Award. Election to The Academy is by invitation only. Up to sixty Fellows may be elected annually, together with Honorary Fellows and Foreign Members who have made exceptional contributions to engineering. All are elected by their peers for personal achievement of exceptional merit and distinction. Fellows are distinguished by the title “Fellow of The Royal Academy of Engineering” and use the designatory letters “FREng”. The Academy was founded in 1976 as The Fellowship of Engineering on the initiative of HRH The Duke of Edinburgh and a group of distinguished engineers. It was granted its Royal Charter in 1983 and, with the consent of HM The Queen, adopted the present title in 1992. The Royal Academy of Engineering 29 Great Peter Street, Westminster, London SW1P 3LW Telephone: 020 7222 2688 Facsimile: 020 7233 0054 Website: www.raeng.org.uk Registered Charity No. 293074 View publication stats
