An Analysis of the San Macro Bell Tower's Foundation Failure and Solutions

Venetians know it as the San Marco Bell Tower, which is the tallest human made structure in the city which was made around 800 years ago. The tower is located in Piazza Public Square, one of the city's most important symbols.

Later, it was discovered that the foundation masonry block was separating from its older counterpart. Accordingly, the committee recommended reinforcing the foundation a second time in order to reduce the chance that another failure would occur.

San Macro Tower Geographic Location

Due to its geographical location, the Bell Tower site contains soft and fine sediments collected by the shallow lagoons within which Venice was built. Due to the sand clay silt soil in San Marco Square, the pavement level is 5 meters deep. This type of soil is characterized by fine to medium sand with silt.

Trachyte and concrete blocks are collected with varying thicknesses alongside sand clay silt, masonry, and wood pile debris. There are layers of silt clay, soft sand, and clayey silt between 5 and 7 m in depth with organic debris and peat. There are about 1 to 2 MPa of unconfined compressive strength in this soil layer.

There are fine to medium sand layers between 7 m and 10 m in depth. This soil layer has medium to high unconfined compressive strength values. As soil depth increases, alternate soil layers may include clay silts, and silt sands. Under the ground level, the groundwater level changes by 1 m to 2 m in San Marco Square. As a precaution, a .90 m groundwater level was built into the foundation.

San Macro Bell Tower Foundation Reconstruction

Brick masonry used between the 11th and 12th centuries was in a bad shape when the San Marco Bell Tower tumbled. A number of earthquakes and lightning strikes caused damage to the tower between 1489 and 1745. However, the tower was still not sufficiently repaired after these damages.

The Bell Tower is believed to have fallen because of the upper marble cell and spire on the tower's top. The tower fell because of differential settlement of the tower's foundation.

The Bell Tower was rebuilt by a multidisciplinary committee following the failure. With regard to the reconstruction of the tower, all design, historical, artistic, and technical factors had to be considered.

As the committee recommended that the differential settlement portion was not important, construction of a unique tower began on the earlier foundation. It was discovered that the new designs did not match the earlier designs, resulting in a reduction of weight.

Crack Propagation

Cracks were visible on the trachyte staircase steps at the plinth level of the Piazza. Considering that the ruptures were outside the acceptable range, engineers were tasked with assessing the situation. Due to the weakness of the trachyte stone, the infrequent ruptures may be associated with shear stresses.

Following the discovery of fresh ruptures at Procuratoria, the engineers excavated six ditches around the Bell Tower in order to find out the real cause of the problem. In the outer wall of the foundation masonry block, several sub-vertical ruptures were observed.

It was determined that the cracks were small, so further monitoring was discontinued after 1960, with the hope that the ruptures would balance with time. In spite of some problems presented by the committee, the foundation-strengthening efforts slowed.

A detailed survey of Pavia's Bell Tower was ordered by the Italian government following the sudden collapse of the Civic Tower in 1989.

The Bell Tower Structural Review

A detailed structural survey of the Bell Tower was conducted by the Istituto Sperimentale Modelli e Strutture or ISMES. During the process of constructing the foundation block, a computerized monitoring system was set up to observe fissures as they grew and observed their movement as they developed. A number of critical points in the Bell Tower were also being monitored in real time by the system.

In the shaft, a set of vertical cracks was observed at a height of 25 m from the Bell Tower's foundation. In ISMES's recommendations, temperature variation in the external wall should be directed towards vertical cracks. In spite of the fact that ISMES advised limiting these ruptures to a certain depth, the brickwork of the Bell Tower should not be exposed to an extreme danger.

A vertical stress estimate of 50 critical issues of the Bell Tower, however, led to threatening results. For the calculation of vertical stresses, flat jacks were used. Compared to the lower zone of the Bell Tower, the vertical stresses were deemed to be fundamentally greater.

Furthermore, it is surprising that at the four intersections in the lower part of the shaft. The flat jacks consistently applied the appropriate vertical stresses. Due to the deformability of the stone foundation and the stiffness of the shaft section, the vertical stress at the shaft corners is concentrated.

Foundation Strengthening

It was also determined that the association between the old and new stone blocks, regardless of potential ruptures, was adequate and uniform across the stone block of the foundation.

A 45 degree angle was struck between the old and new stone blocks to pierce six samplings of 50 mm diameter. Both surface and core samples were taken from the foundation blocks for the study. A consensus was reached that the new masonry block was losing its connection to the old masonry block. In 1955, a reinforced concrete ring was suggested, which was intrusive and reversible.

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Plan and elevation of the Bell Tower were spared by presenting the key work in such a way that it was easy to maintain without damaging the superstructure or foundation. Throughout the foundation stone block area, pre-stressed titanium rebar was to be delivered on two levels.

Titanium bars should be subjected to constant pressure by jacks to prevent further cracks from developing. Through frictional forces, titanium bars were attached to the foundation block using a small amount of force.

Despite the titanium bars adding a considerable force, friction between stone blocks already balanced out the displacement. Thus, if the cracks expand, the titanium bars will counteract it with minimal forces necessary to withstand rupture development. Because of this, tiny forces would not move or disturb the monument.