The San Marco Bell Tower, located in Venice’s Piazza Public Square, is a towering structure that has stood for over 800 years and is one of the city’s most iconic landmarks. Known to the Venetians as “il paron di casa” or “the landlord, the master of the house,” the tower was originally built to guard the dock of the Grand Canal.
However, in 1902, the bell tower experienced a catastrophic failure of its brick masonry, indicating possible issues with its foundation. As a result, the Italian government undertook a reconstruction effort in 1908 to restore the tower to its former glory. The collapse of the masonry also caused damage to the nearby Marciana library building in San Marco Square, resulting in a massive cleanup effort that took over a month to clear the 14,000 tons of debris from the area.
San Marco Bell Tower
In 1903, the Italian government funded a project to reconstruct the San Marco Bell Tower, which was completed in 1908. Geotechnical engineers were brought in to strengthen the foundation of the structure, increasing the area of the foundation from 222 m2 to 407 m2 and locking the new masonry foundation with the old one.
In 1950, cracks were observed at the plinth level of the foundation, raising concerns about the structural integrity of the tower. A committee of engineers was appointed to monitor the cracks continuously. It was determined that the foundation masonry block was detaching from the old foundation masonry block. As a result, the committee recommended reinforcing the foundation to prevent further collapse.
In 2007, the reconstruction of the foundation of the Bell Tower began, using prestressed titanium rebars laid at two levels along the perimeter of the foundation stone block. These rebars were installed to enhance the overall flexural stiffness of the foundation and prevent the propagation of cracks. The installation required excavation below the groundwater table in a significant area of San Marco Square, while taking care to avoid disturbance or differential settlement of the foundation, which could have caused a disaster for the Canal City.
To perform the excavation, numerous floating columns were constructed to create a watertight retaining wall around the foundation influence zone of the Bell Tower. This work was carried out with great care due to the sensitive environment and the presence of buried archaeological remains. The article discusses the process followed for strengthening the foundation of the San Marco Bell Tower and provides 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 situated in the San Marco Square, Venice, Italy. The site’s geology is incredibly delicate, consisting of soft and fine sediments deposited by the shallow lagoons on which the city was built. The soil profile at the site is typically sandy-clayey silt, ranging from the pavement level of San Marco Square to a depth of 5 meters. It is locally characterized as medium to fine sand with silt, and in addition to the sandy-clayey silt, it contains masonry and wooden piles debris, trachyte, and concrete blocks of varying thickness.
At a depth of 5 to 7 meters, a layer of silty clay, soft sandy, and clayey silt with organic debris and peat is present. The unconfined compressive strength values of this layer range from 1 to 2 MPa. From 7 to 10 meters deep, a layer of medium to fine sand is present, with medium to high unconfined compressive strength values of 7 to 15 MPa. Beyond a depth of 10 meters, alternate layers of clayey silts, silty clay, and silty sands are found.
The groundwater levels in the San Marco Square area typically fluctuate between 1 to 2 meters below the ground level. However, for safety reasons, the foundation of the San Marco Bell Tower was designed for a groundwater level of 0.90 meters.
2. Foundation Reconstruction
The collapse of the San Marco Bell Tower was attributed to the poor condition of the brick masonry, which was constructed in the 11th and 12th centuries, and had suffered damages from earthquakes and lightning between 1489 and 1745. Despite these damages, the tower had not been adequately repaired. Initially, it was believed that the failure was due to the construction of the spire and upper marble cell on top of the tower. However, detailed investigations revealed that the collapse was actually caused by the differential settlement of the foundation.
In response to the collapse, a multidisciplinary advisory committee was formed to oversee the reconstruction of the Bell Tower. The committee was tasked with considering various aspects, including design-related, historical, artistic, and technical factors. Based on their recommendations, the construction of a new tower began on the existing foundation, as the differential settlement was determined to be relatively insignificant at 10 cm. However, the new designs did not match the previous ones, and the absolute weight of the tower was reduced.
The advisory committee expressed concerns about the passive resistance of the old foundation and advised increasing the plan area of the foundation from 222 m2 to 407 m2. This nearly doubled the foundation area, indicating that the committee used different criteria for foundation design than what was originally employed. The reconstruction process took into account these changes and adjustments to ensure the stability and safety of the new Bell Tower.
Collapse of San Marco Bell Tower
The new construction technique employed was similar to the old technique, with the addition of driving 3086 piles inside a wide wooden pile fence, in addition to the existing piles. These newly driven piles were intended to consolidate the soil to a depth of 4 to 8 meters. Once the soil was consolidated, three thick wood planks were placed above the piles to create a horizontal base level for constructing the foundation block.
The foundation block was carefully designed, using stones that were cut in a way to form a solid block. Unlike the stones used in the older foundation, these stones were relatively lighter in weight. As a result, the vertical stresses on the soil were reduced from 900 KPa to 400 KPa, and the factor of safety was significantly improved.
However, after a couple of years, cracks developed in the foundation, indicating potential issues with the construction.
3. Cracks Propagation in the Bell Tower
The Piazza’s trachyte staircase steps were found to have cracks exceeding the allowable limit, particularly indicating shear stresses due to the stone’s fragile nature. The situation became more concerning when new cracks emerged at Procuratoria, leading engineers to excavate six trenches around the Bell Tower. This revealed sub-vertical cracks on the foundation masonry block’s outer surface.
In 1955, researchers from the University of Padova installed 22 mechanical Whittemore extensometers at the foundation level to measure elongation. The extensometers indicated that the linear movement of the crack was increasing over time, and it was predicted that the width of the cracks would reach 1 mm in 1975. Despite concerns raised by the committee, monitoring was halted after 1960, assuming the cracks would stabilize with time. As a result, foundation strengthening measures were delayed.
However, the committee identified the insufficient thickness of the stone block construction above the masonry foundation as the main reason behind the propagation of shear cracks. They recommended installing an external reinforced concrete chain and steel connections with the stone block.
In 1989, the Civic Tower of Pavia collapsed unexpectedly, prompting the Italian government to order a detailed survey of the Bell Tower.
4. Structural Review of the Bell Tower
The Istituto Sperimentale Modelli e Strutture (ISMES) was granted permission to conduct a comprehensive structural survey of the Bell Tower. To monitor the growth and movement of cracks in the foundation block and critical points of the Bell Tower, an automatic monitoring system was installed. At a height of 25 m from the foundation level, ISMES observed a group of vertical cracks in the shaft. The organization attributed the cracks to temperature variations in the external wall, but they posited that they were limited in depth and did not pose a significant threat to the brickwork of the Bell Tower.
However, the flat-jack measurements of vertical stresses in 50 critical points of the Bell Tower yielded alarming results. The magnitude of the vertical stresses was considerably higher than those found in the lower zone of the Bell Tower. The flat-jacks at the four corners of the lower part of the shaft recorded a critical concentration of vertical stress. This concentration was due to the deformability of the stone base of the foundation compared to the stiffness of the shaft section.
Researchers correlated the magnitude of the vertical stress with the shape of the foundation of the Bell Tower. They found a systematic correlation between the vertical cracks observed on the staircase of the foundation block and the magnitude of vertical stress. Monitoring of the cracks by the University of Padova in 1955 showed a crack width of 1 mm. However, when monitoring resumed in 1975, the results were surprising. Instead of stabilizing, 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 this information indicated that the masonry foundation block was undergoing differential settlement. This was due to 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 could be dangerous and could 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.
Therefore, the researchers proposed an effective strengthening program for the foundation to halt new developments and avoid any unexpected outcomes in the future.
5. Strengthening of the Foundation
Additional examinations were conducted on the stone block foundation to verify the effectiveness and uniformity of the connection between the old and new stone blocks constructed in 1903, regardless of any potential cracks. Six samples with a diameter of 50 mm were taken at a 45° angle between the connection of the old and new stone blocks. The analysis was performed on both surface and core samples of the foundation blocks. The results revealed that the connection between the foundation masonry blocks was deteriorating, indicating a loss of connection between the old and new foundation masonry blocks.
Samples taken for the assessment of foundation of the San Marco Bell Tower
A novel solution was proposed to address the structural issues of the Bell Tower, which was reversible, sturdy, and did not require any intrusive prostheses like the reinforced concrete ring recommended in 1955. The proposed solution aimed to maintain the original superstructure and foundation of the Bell Tower in their current plan and elevation. The plan involved installing prestressed titanium rebars at two levels along the perimeter of the foundation stone block area. The titanium rebars would be adjusted using jacks to apply sufficient force to prevent further opening of the cracks, and this force would be maintained indefinitely. The key principle behind this approach was to utilize frictional forces to apply a minimal amount of force through the titanium bars, as the friction between the stone blocks was already neutralizing the relative displacement. This meant that any increase in crack size would be counteracted by the titanium bars with minimal force required to resist crack development. As a result, no movement or disturbance would occur to the monument due to the application of small forces.
FAQs
When was the reconstruction of the San Marco Bell Tower undertaken?
The San Marco Bell Tower underwent reconstruction from 1903 to 1908. The reason for the renovation was not mentioned in the given context.
However, in 1950, some cracks were detected in the foundation block of the same bell tower. A committee of engineers was formed by the authorities to oversee the growth of cracks in the foundation. Upon observation, the committee found that the foundation masonry block was gradually separating from the old foundation masonry block.
As a preventive measure, the committee recommended strengthening the foundation to reduce the likelihood of another collapse. The reason behind the previous reconstruction of the San Marco Bell Tower in 1903-1908 is still unknown.
What was the reason behind strengthening the foundation of the San Marco Bell Tower?
In 1950, authorities discovered cracks in the foundation block of the San Marco Bell Tower. To address the issue, a committee of engineers was formed to monitor the growth of the cracks in the foundation continuously. Through their observations, the committee determined that the foundation masonry block was gradually detaching from the old foundation masonry block, posing a risk of collapse. As a result, the committee recommended strengthening the foundation to reduce the possibility of further damage or collapse.