San Marco Bell Tower: Foundation Reconstruction of the Tallest Structure in Venice

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San Marco Bell Tower, the tallest structure in Venice, was first constructed around 800 years ago. Located in Piazza Public Square, the tower is among the most distinct symbols of the city. Venetians call the tower “il paron di casa” (the landlord, the master of the house) as it was constructed to guard the dock of the Grand Canal.

The tower was reconstructed by the Italian government in 1908 due to the failure of the brick masonry of the bell tower in 1902. The total collapse of the masonry work was indicative of a possibly flawed foundation. This incident further caused the failure of the Marciana library building located in the famous San Marco Square. The collapse was so bad that took over a month to clean the 14,000 tons of debris from the location.

In 1903, the Italian government funded the project and the tower was reconstructed in 1908. To strengthen the foundation of the structure, the geotechnical engineers increased the area of the foundation from 222 m² to 407 m² and the new masonry foundation was locked with the old masonry foundation.

In 1950, a few shear cracks were observed at the plinth level of the foundation, which again raised serious concerns regarding the structural integrity of the tower. The authorities appointed a committee of engineers to continuously monitor the formation of cracks in the foundation.

It was later determined that the foundation masonry block was getting detached from the old foundation masonry block. Therefore, the committee suggested reinforcing the foundation again to decrease the possibility of another collapse.

Recently in 2007, the reconstruction of the foundation of Bell tower started, and prestressed titanium rebars were laid at two levels along the perimeter of the foundation stone block. The bars were installed to enhance the overall flexural stiffness of the foundation and stop the further propagation of cracks. The installation required the excavation below the groundwater table within a significant area of San Marco Square and in the presence of buried archaeological remains.

In such a sensitive environment, the foundation work was carried out with utmost care as the slightest disturbance or differential settlement of the foundation could have caused a disaster for the Canal City. To perform the excavation of the foundation, numerous floating columns were constructed to make a watertight retaining wall around the foundation influence zone of the Bell Tower.

In this article, we discuss the process followed for strengthening the foundation of the San Marco Bell Tower and a detailed description of the structural and geotechnical works that made the project feasible.

1. Geology of the San Marco Bell Tower

The San Marco Bell Tower is located in the San Marco Square in Venice, Italy. The geology of the Bell Tower site is extremely sensitive and consists of soft and fine sediments deposited by the shallow lagoons in which the city of Venice was built.   

The following points describe the average soil profile available at the San Marco Bell Tower site:

1. From the pavement level of San Marco Square to a 5 m depth, the soil is sandy-clayey silt. It is locally characterized as medium to fine sand with silt. 
2. In addition to the sandy-clayey silt, masonry, and wooden piles debris, trachyte, and concrete blocks are deposited with variable thickness.
3. 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 layer are around 1 to 2 MPa. 
4. From 7 m to 10 m depth, a layer of medium to fine sand is found. The unconfined compressive strength values of this layer are medium to high (7 to 15 MPa).
5. After 10.0 m depth, alternative layers of clayey silts, silty clay, and silty sands are found.
6. The groundwater levels fluctuate in the San Marco Square area between 1 m to 2 m below the ground level. However, on the safer side, the foundation was designed for a groundwater level of 0.90 m.

2. Foundation Reconstruction

The San Marco Bell Tower collapsed due to the poor condition of brick masonry, which was constructed between the 11th and 12th centuries. Also, the tower had suffered damages due to earthquakes and lightning between the period of 1489 and 1745. Even after these damages, the tower was not adequately repaired.

As the Bell Tower collapsed, it was thought to have failed due to the construction of the spire and upper marble cell on the top of the tower. But detailed investigations revealed that the tower actually failed due to the differential settlement of the foundation of the tower.

Following the collapse, a multidisciplinary advisory committee was formed to reconstruct the Bell Tower. The committee had to look after all the design-related, historical, artistic, and technical aspects associated with the reconstruction of the tower. 

The construction of a new tower began on the previous foundation as the committee suggested that the amount of differential settlement was not significant (the differential settlement was 10 cm). But the new designs didn’t match with the previous designs and the absolute weight was reduced.

The committee was also doubtful about the passive resistance of the old foundation and advised to have the plan area of the foundation increased 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 technique was analogous to the old technique. In addition to the existing piles, a total of 3086 piles were driven inside a wide wooden pile fence. The newly driven piles consolidated the soil to a depth of 4 to 8 m. Once the soil was consolidated, three thick wood planks were provided above the pile to provide a horizontal base level for constructing the foundation block. 

The foundation block was designed carefully and the stones used in the block were cut in a way as to form a solid block. The stones were relatively lighter in weight than the stones used for the older foundation. Thus, the vertical stresses on the soil were reduced from 900 KPa to 400 KPa and the factor of safety was substantially improved. However, a couple of years later, the foundation developed some cracks. 

3. Cracks Propagation in the Bell Tower 

At the plinth level of the Piazza, many cracks were observed on the trachyte staircase steps. The cracks were not within the allowable limit, thus got the attention of the engineers. A few cracks indicated a reasonable theory for being associated with shear stresses; not surprising though, because of the fragile nature of the trachyte stone. 

However, the engineers looked into the problem more seriously when new cracks were discovered at Procuratoria (office of Procurators of Saint Mark) following which six trenches were excavated all around the Bell Tower to find the real reason behind the problem. It was discovered that several sub-vertical cracks showed up on the outer surface of the foundation masonry block.     

Researchers from the University of Padova installed 22 mechanical Whittemore extensometers (extensometers are used for measuring elongation) at the foundation level in 1955. Measurements from the extensometer showed that the linear movement of the crack was increasing with time. They predicted that the width of the cracks would reach 1 mm in 1975.   

The cracks appeared to be small and further monitoring was halted after 1960 hoping that the cracks would stabilize with time. Thus, the foundation strengthening measures were delayed despite some concerns raised by the committee.

However, the committee suggested that the main reason behind the propagation of shear cracks was insufficient thickness of the stone block construction above the masonry foundation. They suggested installing an external reinforced concrete chain and steel connections with the stone block. 

In 1989, the Civic Tower of Pavia collapsed unexpectedly and in view of this incident, the Italian government ordered  a detailed survey of the Bell Tower.  

4. Structural Review of the Bell Tower

The Istituto Sperimentale Modelli e Strutture (ISMES) was authorized to conduct a detailed structural survey of the Bell Tower. An automatic monitoring system was installed to monitor the growth of cracks and identify the continuous movement of cracks in the foundation block. Also, in real-time, the system was monitoring the movement of several critical points of the Bell Tower.    

At a height of 25 m from the foundation level of the Bell Tower, a group of vertical cracks was observed in the shaft. ISMES suggested that the temperature variation in the external wall led to the formation of the vertical cracks. However, ISMES suggested that these cracks were limited to a certain depth and did not represent a severe threat to the brickwork of the Bell Tower.

However, alarming outcomes emerged from the vertical stresses estimated in 50 critical points of the Bell Tower. Flat-jacks were used to measure the vertical stresses. The magnitude of vertical stresses was considered fundamentally higher than those determined in the lower zone of the Bell Tower. 

On the other hand, it was shocking that the critical magnitude of vertical stresses was observed by the flat-jacks at the four corners 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 correlated with the shape of the foundation of the Bell Tower. Therefore, the correlation of systematic vertical cracks observed on the staircase of the foundation block was made. The monitoring of the cracks conducted by the University of Padova in 1955 found a crack width of 1 mm. However, in 1975, when, the monitoring was resumed, the results were surprising. The cracks didn’t stabilize with time; instead, the cracks were still showing a linear increase in their width. The crack width reached about 2 mm in 1975, twice the size predicted in 1955.

All the information affirmed that the differential settlement of the masonry foundation block was a development in progress. The reason behind the differential settlement was the insufficient thickness of the stone block added above the masonry foundation during the reconstruction in 1903. The researchers suggested that the progressive increase in the width of the crack and the increase in vertical stresses in the masonry shaft can be dangerous and this phenomenon can produce a local collapse 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 suggested by the researchers to stop the new developments and to avoid any unexpected outcomes with time.

5. Strengthening of the Foundation

Further examinations were conducted across the stone block of the foundation to confirm that the connection between the old stone block and the new stone block constructed in 1903 was effective and uniform regardless of any potential cracks. 

Six samplings of 50 mm diameter were penetrated at a 45° angle between the connection of the old and new stone blocks. The analysis was carried out on both the samples received from the surface and the core of the foundation blocks. It was recognized that the foundation masonry block was losing its connection with the old foundation masonry block.

A unique solution was proposed, which was reversible, sturdy, and didn’t involve any obtrusive prostheses such as the reinforced concrete ring that was recommended in 1955.

The solution was proposed in such a way that the superstructure and the foundation of the Bell Tower would remain unchanged both in plan and elevation. The plan was to provide prestressed titanium rebars at two levels along the perimeter of the foundation stone block area.  

The forces applied to the titanium bars by jacks would be adequate to stop any further opening of the cracks and should be perpetual.  The principle behind installing the titanium bars to the foundation block was to apply a small amount of force by taking advantage of frictional forces.

The titanium bars were only adding a limited force because the friction forces acting between the stone blocks were already neutralizing the relative displacement. Thus, if there’s an increase in size of the cracks, it will be counteracted by the titanium bars with minimal forces required to resist the development of crack. Hence, no movement or disturbance would occur to the monument due to the application of small forces.      

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