The Leaning Tower of Pisa in Italy is perhaps the most famous tower structure in the world, thanks to its lean (nearly 5.50 out of plumb – Fig-1) which makes it a great international tourist attraction. When the lean started becoming progressive and excessive, threatening its very survival, a geotechnical engineering solution, simple in principle, namely, controlled ground extraction, which was rarely attempted before, corrected the excessive tilt and stabilized it at the inclination that existed at the beginning of the 19th century, which continues to make it a tourist attraction as did the original Leaning Tower of Pisa. The purpose of the exercise, therefore, was not to correct the entire tilt and bring it to a perfectly vertical position, but to restore it to the original tilt, which alone would make it continue to be the object of attraction that it was. The success of the above exercise marked the culmination of the 11-year effort of the International Committee which was entrusted with the task of restoration of the tower to its original tilt.
History of the Tilt
The tower was constructed at a time when the discipline of geotechnical engineering, as it is known today, was non-existent. (Had it been constructed today, a detailed subsoil investigation would have been commissioned and carried out prior to the design and construction of the tower, that would have indicated the need for a deep foundation, instead of the shallow foundation on which it stands, but then we would not have had the Leaning Tower of Pisa!) This error has turned out to be a blessing in disguise, as it were, for the tourism industry.
The culprit in this case was the foundation soil on which was laid a shallow foundation. Hence a solution had to be sought from within the ambit of the geotechnical engineering discipline. It, however, turned out to be a common-sense solution, without much of geotechnical engineering input!
It is seen from Fig-1 that the tower leans southwards. The inclination, close to 5.50 , creates an overhang of nearly 4.5 m at the top with respect to the bottom edge of the plinth.
The subsoil condition is shown in Fig-2 in which three distinct Horizons – A, B and C can be identified of which Horizon B comprises of layers B1, B2, B3 and B4 which are mostly marine deposits. The tower was founded in the sandy silt layer of Horizon A.
The construction of the Tower started in 1173 but was suspended after five years. The construction was resumed in 1272 and in six years the tower was brought to its top seventh cornice. To this was added the bell chamber at the top which was completed in 1370.
It was during the second stage of construction, when its weight approached two-thirds of the full weight, that the inclination began to appear. This continued till 1993 when the authorities were seized of the problem which made them attempt temporary interventions to arrest the trend for averting the risk of toppling of the structure. Increasing tilt caused increasing eccentricity and consequently increasing moment due to self weight which triggers further tilt at an increasingly faster rate. This was referred to as leaning instability. In geotechnical engineering terms, the foundation, rather the structure itself, failed in both bearing capacity and settlement, the latter taking the form of acute differential settlement.
The modern monitoring of the Tower’s inclination started in 1911. It was found that the rate of inclination per annum increased from 76 mm in the 1940s to 152 mm in the 1980s.
When the continuing increase in inclination began to be noticed with alarm, the Government of Italy realized that unless the tilt is stopped, the consequences would be catastrophic, leading to the eventual toppling of tower itself in a few decades. Accordingly the Prime Minister appointed an ‘International Committee for the Safeguard and Stabilisation of the Leaning Tower of Pisa’. The committee which was the 17th in the long history of the monument, with Professor M. B. Jamiolkowski of the Technical University of Torino, Italy, in the chair, was a multi-disciplinary body with experts drawn from a wide spectrum including history of medieval arts, archaeology, construction stones, architecture, structural engineering and geotechnical engineering.
Final Measure of Stabilization
After a number of initial attempts, the final measure of correcting the Tower’s tilt chosen was the gradual extraction of the soil from the lower part of Horizon A on the northern part of the tower. This measure had been suggested as early as 1962. It was successfully applied in 1992 to mitigate the impact of very large differential settlements suffered by the Metropolitan Cathedral of Mexico City. Encouraged by the same, numerical analyses, physical model studies in the Centrifuge and large scale field trials were performed which proved the feasibility of the approach.
The final scheme of ‘under excavation’ is illustrated in Fig-3. Forty-one inclined boreholes were drilled on the northern side starting with guide tubes fixed on an R.C. beam. Extraction was carried out using helical augers.
The aim of this exercise was achieving a reduction in the Tower’s inclination by 1800 seconds of arc (1 degree of arc = 3600 seconds of arc). The work was started on 21-2-2000 and was stopped on 19-1-2001 when the total extracted volume of soil reached 37 m3 and the inclination was reduced to nearly 1620 seconds of arc. The tower continued its rotation northwards and in July 2001 the total reduction in inclination reached 1842 sec. which corresponded to a reduction of 446 mm of the overhang which was 1/10th of the value of h Fig.49.1) recorded in 1993. It is significant that, apart from ground extraction, no steps were undertaken to stabilize the foundation soil with a view to ensuring long term performance. In Dec. 2001, when all the related works were over, the Committee ‘returned’ the Tower to the Italian Government, with a request to carefully monitor the structure for the next ten years, ringing the curtain down on a gigantic task the like of which has not been witnessed before.