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

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: 


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 

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The Royal Academy of Engineering is a Registered Charity (No. 293074)

A Tale of Two Towers: 


1. INTRODUCTION 

A Tale of Two Towers 

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 

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 

Aerial view of the Big Ben Clock Tower and the Palace of Westminster 

The Leaning Tower of Pisa 

The Royal Academy of Engineering 

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Vertical cross-section through the Leaning 

Tower of Pisa 

4 


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 

excavated a walk-way (catino) around the base of the 

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 

Model 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.


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 


5


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 


Soil profile beneath the Leaning Tower of Pisa


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, 1994 5 ). 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. 

The direction of motion of points F N and F S are shown by 

vectors and it is clear that the foundations have been 

moving northwards with F N rising and F S sinking. 

Motion of Tower foundations during 

progressive increase in inclination 


7


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 

(Harris et al 2000 6 

). 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


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 

connected. A contingency protective measure 

was called for and the relatively new 

technique of compensation grouting was adopted. 

Cross-section showing proximity of the new Westminster station 

to Big Ben 

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. 


9


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 

Tilt of Clock Tower (mm/55m) 

10 

40 

30 

20 

10 

0 

Tunnel Progress: 

Pilots Enlargements 

WB EB WB EB 

Construction 

Control 

Range 

Grouting Episodes 

Start of 

Grouting 

Box Excavation 

Progress [m]: 

9 1 3 16 22 25 31 35 39 

-10 

Nov-94 Nov-95 Nov-96 Nov-97 Nov-98 Nov-99 Nov-2000 

Optical Plumb 

Measured horizontal movements of Big Ben at clock face level between 1994 

and 2000 


(22mm). A construction control 

range of between 15mm and 

25mm tilt was adopted. 

As anticipated, northward tilt of 

the Clock Tower commenced as 

the west-bound running tunnel 

passed by and an immediate tilt 

of 4mm was recorded. Time 

dependent movements then took 

place and it became clear that it 

would be necessary to implement 

compensation grouting in order 

to keep the tilt of the Clock 

Tower within permissible limits. 

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 122m 3 

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. 

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.


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 


11


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 0.25 o 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 



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 

21 st 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 


13


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 16 th 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 6 th June 2001 - the date when 

the Tower was released by the Commission from intensive care. 

Pageantry during hand-over ceremony on 16 June 2001 

14 


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 16 th 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 15 th 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.

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. 


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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. 


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

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