Geotechnical precautions: the lead bars
The risky situation that had been noticed in the Tower and that had led to closing it down drove the experts to immediately take temporary, reversible measures that would increase safety and give them time to complete their surveys and studies before planning the final measures. From a geotechnical point of view, due to the fact that the Tower continued to move by approximately 1.2 millimetres a year, this meant fitting the monument with a counterweight consisting of heavy lead bars (May 1993 – July 1994). The bars (weighing a total of about 700 tons) were placed over a concrete ring and installed on the northern side, between the pillars of the first row, slightly more on the west side; they were laid down one by one, at appropriate time intervals. During and after the application, the inclination of the Tower actually went back north, for a total of 48 seconds of an arc.
Structural precautions: the steel hoops
As a structural precaution, in 1992 the second order (just below the first moulding and on the outer facing of the first open gallery) was fitted with hoops made of steel cables to prevent the existing cracks expanding and the instability of the marble facing increasing. At the end of the stabilisation work, the hoops installed in 1992 were replaced with similar but less visible hoops.
Geotechnical work
The results of the preliminary surveys
From a geotechnical point of view these surveys and studies suggested that a slight reduction in the inclination of the Tower (a ‘return north’ by about half a degree, i.e. about one tenth of the inclination measured in 1990) would have been sufficient to halt the inclination and improve stability in the long term. This would have also reduced the stress on the walls and would have minimised the strengthening work.
The under-excavation
The system selected to ‘straighten’ the Tower was the under-excavation, i.e. the controlled removal of specific amounts of soil. A number of pipes were driven slantwise into the ground and pushed until they reached the foundations; acting as drills, they dug out the soil proportionally to their size. After removing the pipes, the small cavities left in the ground were slowly plugged up by the pressure of the soil, causing a minimal subsidence on the surface. This technique was used in the area north of the Tower and it caused the Belfry to ‘fall back’ northwards, thus decreasing its incline. This extremely delicate operation was calibrated and adapted to the local circumstances and carried out in stages, to give the soil time to strengthen between extractions. Of the many proposed techniques, this had the advantage of being ‘invisible’ and not altering the appearance of the monument.
The ‘critical line’
After the first laboratory surveys and tests that began in 1994, a large-scale experimental under-excavation was conducted in 1999 in a designated area of the square, between the Cemetery and the walls. The experiment was successful even if it confirmed, as already observed in laboratory conditions, that there was a ‘critical line’, i.e. a point beyond which the effects of the under-excavation became negative: in other words, the Tower would lean southward again.
The guys
In 1997 the experts decided that, before conducting any test on the Tower, a safety construction should be set up for fear that something unexpected might occur. So the Belfry was «guyed», i.e. anchored to the ground by two big steel cables wrapped around the third order of columns and secured to two anchoring devices. Installed loose, the guys could be tightened up at any time, if needed.
The first results
After installing the guys, February 1999 saw the beginning of a preliminary under-excavation, slower and more distant from the foundations than the final one. The result was remarkable: even after finishing the soil-strengthening operation, the Tower kept moving northwards, to such an extent that in September 1999 it had ‘moved back’ by 130 seconds of an arc, going back to its 1968 inclination. Then some of the lead bars began to be removed and from then on the Tower stopped moving (except of course for its ‘natural’ and seasonal cyclic movements).
The final under-excavation
The final under-excavation took place between February 2000 and February 2001. 41 inclined drillings dug out 38 cubic metres of soil, 70 per cent north of the foundations and the remaining 30 per cent underneath them. During a break in the operation, in August 2000, some lead bars were removed and the ‘postponed’ effect of the under-excavation actually managed to prevent the Tower from moving back southward. The remaining bars were removed in January 2001 and the guys were removed in June.
The Tower ‘straightens up’ by half a degree
So the Tower ‘straightened up’ by half a degree, as planned. The current situation is the same as it was about two hundred years ago, before the excavation of the conch in 1838-1839, and it will presumably stay like this: even if the Tower started to lean again by as much as 6 seconds of an arc a year (a ‘worst-case’ scenario that the geotechnical engineers consider unlikely), it would take 300 years before it went back to the way it was in 1990. All this has been done without visibly altering the appearance and nature of the monument.
The structural work
The purpose of the final structural work was to increase the resistance of the critical areas. This was preceded by a vast battery of structural studies and surveys that provided an extremely detailed map of the ‘health’ of the Tower prior to the strengthening work; this may be useful as a baseline to assess any future change.
The Deformometers
These studies were based on a full architectural survey, which located the ‘critical’ cracks for the installation of the monitoring detectors («deformometers») which ‘spy’ on any enlargement of these cracks and any deterioration of the state of the structure. A comparison between the new survey with the one of 1965-1971 showed that new cracks have appeared since then in the south-western section and on the stair steps, in the marble facing, in the vaults of the open galleries and in rifts in the lintels of the open galleries.
Other tests on the Tower
Between 1990 and 1994, this stone-by-stone mapping was completed by a number of non-destructive tests and other experiments in the body of the monument to find any cavity or void that was not visible to the naked eye. To do this, the results of different tests had to be combined: infrared thermal imaging, which shows up any cavity in the wall by measuring thermal differences on the surface; radars, which reveal any cavity or fracture based on the reflection of electromagnetic waves from the walls; sonic tomography, i.e. the measurement of the time the sound takes to travel through the wall; colour endoscopy through small surface holes, called «cores», made in the wall from the outside; and the sampling of materials.
Dynamic identification
The movements of the Tower were also reconstructed and measured in ‘normal’ and exceptional conditions: for instance, movements caused by changes in temperature during the day or during the year (due to the thermal dilation and the cooling of the stones at night, the top of the Tower moves in an ellipsis over 24 hours) or by the effect of an earthquake, which was simulated by installing some vibrating masses on the Tower. The result of these and other measurements is the so-called «dynamic identification» of the Belfry, showing how it responds to winds or earthquakes. The one specifically conducted on the colonnade showed the response of each column to such stress.
The final structural work
The final structural work took place in 1998-1999. In the ‘critical areas’ of the first and second levels under the pressure of the inclination, the wall was strengthened by injections of mortar and the installation of stainless steel bars; identical bars were also used to strengthen the lintels of the open gallery and their connections to the outer facing of the Tower. The temporary hoops were replaced with less visible ones, made of stainless steel wire, 4 millimetres in diameter. To retain and increase the stability of the conch, the outer wall was strengthened; in addition, a number of steel cables were buried all around the Belfry, under the floor of the conch, and pre-tensioned to strengthen the connections between the conch and the Tower.
Monitoring
In 1991, with a view to working on the Tower for the first time, the experts realised that there were no appropriate instruments to measure the short-term response of the monument to such work. The Girometti-Bonechi inclinometer and the Civil Engineers’ level that had been working since 1934, was not able to provide reliable information except in the long term. For this reason, a new monitoring system was designed and installed and was left in situ even after the end of the work; it will continue to provide information to measure the health of the Belfry under any condition. The new system provides static and dynamic monitoring. Static monitoring is currently provided by four groups of instruments.
1) Three wire pendulums and some telecoordinometers on four levels, which, by detecting the horizontal shifting of any points on the outer surface of the Belfry, show any shifting of the vertical axis of the monument.
2) Ten electric deformometers measure any change in the cracks of the structure under normal conditions.
3) A “weather hut” records weather information (solar radiation, temperature, wind speed and direction)
4) Five accelerometers record any vibration of the Tower under the effect, for example, of seismic winds.
This complex system provides the exact picture of the state of the Tower at any time. In addition, over the last few years, it has provided information on the movements and alterations that the Tower undergoes in normal circumstances, depending on the cycle of the day and year, or any change in temperature, solar radiation, and wind.
