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San Marco Bell Tower is the tallest man-made structure in Venice. It was first built around 800 years ago. It is located in Piazza Public Square, the tower is among the most typical symbols of the city. The Venetians call the tower “il paron di casa” (the landlord, the master of the house) as it was made to protect the dock of the Grand Canal.
The tower was rebuilt by the Italian government in 1908 due to the collapse of the brick masonry of the bell tower in 1902. The total failure of the masonry work was signifying of a maybe flawed foundation. This incident additionally caused the collapse of the Marciana library building situated in the famous San Marco Square. The failure was so bad that took over a month to groom the 14,000 tons of debris from the location.
In 1903, the Italian government financed the project, and the tower was rebuilt in 1908. To maintain the foundation of the structure, the geotechnical engineers expanded the area of the foundation from 222 m2 to 407 m2, and the new masonry foundation was sealed with the aged masonry foundation.
In 1950, a few shear ruptures were marked at the plinth level of the foundation, which again extended serious problems regarding the structural integrity of the tower. The leaders selected a committee of engineers to always monitor the appearance of cracks in the foundation.
It was later resolved that the foundation masonry block was reaching separated from the aged foundation masonry block. Therefore, the committee recommended reinforcing the foundation again to reduce the possibility of another failure.
1. Geology of the San Marco Bell Tower
The geology of the Bell Tower site is exceptionally sensitive and contains of soft and fine sediments gathered by the shallow lagoons in which the city of Venice was constructed.
The following topics express the average soil profile available at the San Marco Bell Tower site:
From the pavement level of San Marco Square to a 5 m depth, the soil is classified as sandy-clayey silt. It is locally represented as medium to fine sand with silt.
In complement to the sandy-clayey silt, masonry, and wooden pile debris, trachyte, and concrete blocks are gathered with varying thicknesses.
From 5 m to 7 m depth, a layer of silty clay, soft sandy, and clayey silt is found with organic debris and peat. The unconfined compressive strength values of this soil layer are about 1 to 2 MPa.
From 7 m to 10 m depth, a layer of soil is medium to fine sand. The unconfined compressive strength values of this soil layer are medium to high (7 to 15 MPa).
After 10.0 m depth, alternative layers of soil are clayey silts, silty clay, and silty sands.
The groundwater levels change in the San Marco Square area between 1 m to 2 m under the ground level. However, on the safer side, the foundation was created for a groundwater level of 0.90 m.
2. Foundation Reconstruction
The San Marco Bell Tower tumbled due to the low-grade situation of the brick masonry, which was made between the 11th and 12th centuries. Also, the tower mourned damage due to earthquakes and lightning between the period of 1489 and 1745. Even after these damages, the tower was not sufficiently fixed.
As the Bell Tower tumbled, it was assumed to have fallen due to the construction of the spire and upper marble cell on the top of the tower. But detailed analyses indicated that the tower actually fell due to the differential settlement of the foundation of the tower.
Following the failure, a multidisciplinary advisory committee was created to rebuild the Bell Tower. The committee had to examine all the design-related, historical, artistic, and technical factors associated with the reconstruction of the tower.
The construction of a unique tower started on the earlier foundation as the committee recommended that the portion of the differential settlement was not important (the differential settlement was 10 cm). But the new designs didn’t check with the earlier designs and the absolute weight was decreased.
The committee was also unsure about the passive resistance of the old foundation and recommended to have the intended area of the foundation be advanced from 222 m2 to 407 m2. The change in the foundation area was nearly doubled which indicated that the advisory committee used some different criteria for foundation design.
The new construction approach was similar to the old method. In addition to the existing piles, a sum of 3086 piles was pushed inside a wide wooden pile fence. The recently pushed piles reduced the soil to a depth of 4 to 8 m. Once the soil was consolidated, three thick wood planks were delivered above the pile to deliver a horizontal base level for creating the foundation block.
The foundation block was planned carefully and the stones employed in the block were carved in a way as to construct a solid block. The stones were moderately lighter in weight than the stones employed for the older foundation. Thus, the vertical stresses on the soil were decreased from 900 KPa to 400 KPa and the factor of safety was substantially enhanced. However, a couple of years later, the foundation originated a few ruptures.
3. Cracks Propagation in the Bell Tower
At the plinth level of the Piazza, numerous fissures were marked on the trachyte staircase steps. The ruptures were not within the acceptable limit, thus fetching the attention of the engineers. Infrequent ruptures showed a reasonable theory for being associated with shear stresses; not unexpected though, because of the weak nature of the trachyte stone.
However, the engineers examined the problem more extremely when fresh ruptures were found at Procuratoria (office of Procurators of Saint Mark) following which six ditches were excavated all around the Bell Tower to see the real reason behind the problem. It was found that several sub-vertical ruptures showed up on the outer surface of the foundation masonry block.
Investigators from the University of Padova installed 22 mechanical Whittemore extensometers (extensometers are employed for calculating elongation) at the foundation level in 1955. Measurements from the extensometer revealed that the linear movement of the ruptures was rising with time. They indicated that the width of the cracks would reach 1 mm in 1975.
The cracks occurred to be small and further monitoring was ended after 1960 hoping that the ruptures would balance with time. Thus, the foundation-strengthening efforts were slowed despite some problems presented by the committee.
However, the committee recommended that the major reason behind the propagation of shear ruptures was the inadequate thickness of the stone block construction exceeding the masonry foundation. They recommended establishing an external reinforced concrete chain and steel links with the stone block.
In 1989, the Civic Tower of Pavia tumbled suddenly and in view of this incident, the Italian government called a detailed survey of the Bell Tower.
4. Structural Review of the Bell Tower
The Istituto Sperimentale Modelli e Strutture (ISMES) was assigned to perform a detailed structural survey of the Bell Tower. A computerized monitoring system was established to monitor the growth of fissures and recognize the ongoing movement of fissures in the foundation block. Also, in real-time, the system was observing the movement of many critical points of the Bell Tower.
At a height of 25 m from the foundation level of the Bell Tower, a set of vertical cracks was marked in the shaft. ISMES recommended that the temperature variation in the external wall be directed to the formation of vertical cracks. However, ISMES recommended that these ruptures were limited to a certain depth and not convey an extreme danger to the brickwork of the Bell Tower.
However, threatening results appeared from the vertical stresses estimated at 50 critical issues of the Bell Tower. Flat jacks were employed to calculate the vertical stresses. The magnitude of vertical stresses was regarded as fundamentally more increased than those specified in the lower zone of the Bell Tower.
On the other hand, it was surprising that the necessary magnitude of vertical stresses was followed by the flat jacks at the four intersections in the lower part of the shaft. The critical concentration of the vertical stress at shaft corners was because of the deformability of the stone base of the foundation in contrast with the stiffness of the shaft section.
The magnitude of vertical stress was associated with the shape of the foundation of the Bell Tower. Therefore, the correlation of systematic vertical cracks marked on the staircase of the foundation block was completed. The monitoring of the cracks executed by the University of Padova in 1955 discovered a crack width of 1 mm. However, in 1975, when, the monitoring was continued, the results were stunning. The ruptures didn’t stabilize with time; instead, the cracks were still delivering a linear growth in their width. The fissure width advanced about 2 mm in 1975, twice the size indicated in 1955.
All the reports confirmed that the differential settlement of the masonry foundation block was a result of improvement. The reason behind the differential settlement was the inadequate thickness of the stone block added above the masonry foundation during the rebuild in 1903. The researchers indicated that the progressive growth in the width of the fissure and the rise in vertical stresses in the masonry shaft can be risky and this splendor can produce a local failure of the Bell Tower.
The consequence was that the vertical stresses estimated by flat jacks near the corners of the base were between 2 and 4 MPa, against the mean value of 0.8 MPa. An effective strengthening program of the foundation was proposed by the researchers to control the new developments and to bypass any unexpected outcomes with time.
5. Strengthening of the Foundation
Further studies were performed across the stone block of the foundation to ensure that the association between the old stone block and the new stone block built in 1903 was adequate and uniform regardless of any potential ruptures.
Six samplings of 50 mm diameter were pierced at a 45° angle between the connection of the old and new stone blocks. The study was taken out on both the samples obtained from the surface and the core of the foundation blocks. It was acknowledged that the foundation masonry block was yielding its connection with the old foundation masonry block.
A remarkable solution was offered, which was reversible, sturdy, and didn’t apply any intrusive prostheses such as the reinforced concrete ring that was suggested in 1955.
The key work was presented in such a way that the superstructure and the foundation of the Bell Tower would stay unharmed both in plan and elevation. The plan was to deliver prestressed titanium rebars at two levels along the perimeter of the foundation stone block area.
The pressures involved in the titanium bars by jacks would be good to prevent any further opening of the cracks and should be endless. The principle behind establishing the titanium bars to the foundation block was to involve a small portion of force by handling the benefit of frictional forces.
The titanium bars were only adding a determinate force because the friction forces acting between the stone blocks were already balancing the comparable displacement. Thus, if there’s an expansion in the size of the cracks, it will be counteracted by the titanium bars with minimal forces needed to withstand the development of rupture. Hence, no movement or disturbance would happen to the monument due to the application of tiny forces.
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