Risk of collapse of the Garisenda Tower

The Torre della Garisenda is one of the most iconic symbols of the city of Bologna. Located in the heart of the historic center, it has witnessed centuries of history and has fascinated visitors and scholars from all over the world. This medieval tower, 48 meters high, is known for its inclination, which has made it a unique landmark in the Italian architectural panorama. Built in the XNUMXth century, the Tower today is a cause for concern for the conditions of potential collapse in which it finds itself.

Below, we will analyze the structure of the Garisenda Tower, the activities of the Technical-Scientific Committees (CTS) for Conservation and the monitoring system adopted. Furthermore, we will report the suitable solutions and the guidelines for the technical design of the necessary interventions, identified by the CTS.

Garisenda Tower of Bologna

The Torre Garisenda is a medieval tower located in Bologna. It is one of the two symbolic towers of the city, together with the Torre degli Asinelli (the taller of the two).

The Tower was built in the 60th century by the Garisendi, a wealthy family of bankers. In the past it reached 48 meters, but currently its height is 3,22 meters and it is tilted by 3,97 degrees, which makes it one of the most tilted towers in the world, after the Tower of Pisa (XNUMX degrees).

Garisenda was mentioned by Dante Alighieri in the Divine Comedy, in Canto XXXI of the Inferno. Dante compares the tower to the fallen angel Lucifer, who is imprisoned at the bottom of Hell.

Due to its tilt and potential collapse, the tower has undergone numerous restoration works over the years. The most recent works were carried out in 2019 and have stabilized the tower and prevented further collapse. However, further consolidation work has recently become necessary to prevent the structure from collapsing.

Architecture and construction of the Garisenda Tower

The Garisenda Tower has a square section, built using bricks and local limestone. The tower originally reached a height of about 60 meters, but over the centuries it was reduced due to the subsidence of the foundations and partial collapses that occurred.

Despite its inclination, the Garisenda Tower was built in a robust and resistant way, using innovative architectural techniques for the time. The inclination is due to the subsidence of the ground on which the tower rests.

Towards the end of the 19th century, the tower underwent a restoration which included the addition of a base cladding in selenite ashlar.

Risk of collapse of the Garisenda Tower

In recent decades, concerns have emerged regarding the structural stability of the Garisenda Tower. Investigations and analyses conducted by experts have revealed that the tower is subject to a number of factors that could increase the risk of collapse of the structure. The inclination and erosion of the surrounding soil have been identified as the main risk factors. The presence of cracks and the degradation of building materials due to water infiltration are additional elements that contribute to the instability of the tower.

Structural integrity analysis of the Garisenda Tower

To assess the structural integrity of the Garisenda Tower, in-depth analyses were conducted using modern techniques such as a monitoring system and analysis of static and dynamic loads. These analyses confirmed concerns about the stability of the tower and revealed the need for urgent interventions to avoid its collapse. Experts highlighted the need to strengthen the structure, repair cracks and constantly monitor the evolution of the surrounding terrain.

Monitoring, protection actions and subjects involved

The Garisenda Tower has been equipped with a constant monitoring system since the 90s, thanks to the installation of various detection instruments that have improved over time following technological evolution.

2018

In 2018, the Municipality of Bologna conducted further investigations on the Torre Garisenda to better understand its structural conditions. The investigations revealed the presence of water infiltrations, which caused dampness in some points of the masonry.

To solve the problem, the Administration drew up a project that included:

• Waterproofing of the summit terrace
• Partial closure of the pontoon holes
• Sealing the mortar joints of the four facades

The project received the opinion of the Superintendency in December 2018 and the works were carried out in 2019.

2019

In 2019, the Municipality of Bologna appointed a Scientific Committee to support the consolidation interventions of the Torre Garisenda. The Committee proposed to create an active belting of the tower's base to block the tilting phenomena.

In February 2019, the Committee approved the project to consolidate the base in three phases:

• First phase: interventions on the base, divided into five sub-phases.
• Second phase: production of metal supports.
• Third stage: definitive consolidation of the base trunk, with reinforcement and replacement of the stone elements, stitching and rejoining of the walls and injections of mixtures.

The project was approved by the Superintendency in 2019 and concerns temporary interventions, i.e. some interventions of the first phase.

In December 2019, the municipal administration approved the project drawn up by engineer Dallavalle and entrusted the works to the CORMA consortium.

2020

In 2020, despite the pandemic, the consolidation work on the Garisenda Tower continues. Investigations and laboratory tests are carried out to evaluate the type of mortar to be injected and the impact of the foundations for the two metal guard elements.

In December 2020, the Administration approved the continuation of the intervention, which includes:

• Monitoring and checking the condition of the tower base bag
• Microinjections
• Passive confinement orthogonal to the walls
• Active confinement orthogonal to the walls
• Preliminary works for definitive interventions

The works are started and the necessary investigations are carried out to understand the degradation of the materials that make up the so-called “bag”. The passive confinement orthogonal to the walls is created by means of a wooden structure around the tower, which is held in tension by metal belts.

2022

In October 2022, the mortar suitable for the microinjection works between the cracks of the tower is identified. The Technical-Scientific Committee, which follows the works, addresses the issue of how to control the spread of the injections and the effectiveness of this intervention.

Although the municipal administration approves the project for the microinjections and re-stirring of the tower base and entrusts the work to a company, the construction site is suspended to allow a new and larger CTS to learn about the contents of the project and to request the improvement of some aspects.

2023

In September 2023, the new committee approved an experimental intervention on one side of the tower, to then extend it to the entire base in case of a positive outcome. In the same session, the structural engineers agreed on the need to continuously monitor the effects of the injections over time, establishing the tolerability thresholds of the intervention.

Engineering red alert

In July 2023, the CTS delivers a reading of the monitoring data indicating an advancement of the crushing at the base of the Garisenda Tower, not only on the south-east side but also on the south-west side.

At the end of September, the CTS approved the microinjection and pilot pile projects, with a monitoring plan to report any alarms. From the data collected by the monitoring system, it emerged that the Tower was developing a new trend of movement in a southerly direction, or “an anomalous trend compared to the trend observed in past years, which was essentially towards the East”.

The CTS declares the situation in “engineering red alert” and blocks the works, pending further analyses.

To be precise, the engineering red alert is different from the civil protection red alert. The first indicates structural criticalities that must be monitored, the second indicates an imminent risk of collapse.

The CTS decides to proceed by implementing the pilot project and at the same time the system for processing the alarm thresholds.

CTS meeting on 4 October 2023

On October 4, 2023, a restricted meeting of the CTS was held, in which the data collected on the tower were presented and which confirm what was previously stated. Namely, "the development of worrying residues in the deformations of the South West (SW) corner of the base and the modification in the historical movement trend of the pendulum."

We report the conclusions of the meeting:

“At the end of an in-depth discussion, the following conclusions were reached.

1. The available factual elements, resulting from investigations and monitoring, continue to report a static situation of the tower of high criticality, with negative evolution, and which in engineering terms can now be defined as a "red code".
2. Consequently, the actions to be undertaken from now on can only be configured in a civil protection regime.
3. In this perspective, a meeting will be held with the Fagioli company, to explore the possibility of creating a support structure for the tower in a very short time, with the aim of preventing a possible collapse or, in the extreme case, controlling the collapse mechanism to minimize all possible risks arising from it. Only in the presence of an effective support will it be possible to proceed with other improvement interventions, but, at the moment, the activity that involved injections into the base sack is suspended.
4. If this road is not passable, it will be inevitable to activate the procedures foreseen by the civil protection plan, with the closure of Piazza Ravegnana and the evacuation of the buildings at risk.”

Site inspections October 2023

In October 2023, 3 inspections were carried out by some members of the CTS. The inspections were aimed at also photographically representing the reasons for alarm provided by the processing of monitoring data.

From the inspections carried out, the following emerged from the technical report:

1. “How the previously detected phenomenon of selenite disintegration at the base of the tower is still in development and widely spread on the internal walls of the tower. The photographic image shown in Figure 9 as an example illustrates the condition of chemical/physical disintegration of the selenite that constitutes the facing containing the so-called “sack” in conglomerate.
2. Like the vertical micro cracks in the bricks placed immediately above the selenite base already detected in 2020 in correspondence only with the South side of the South West corner of the tower (see Figure 4), they have now spread widely also on the East and West sides (Figures 6, 7 and 8). It should be noted that these vertical micro cracks in the bricks (which often do not involve the mortar) are of particular importance and attention as they were identified by Prof. Luigia Binda (Technical Consultant of the Pavia Public Prosecutor's Office regarding the identification of the causes of the collapse of the Civic Tower which occurred in 1989) as an indicative sign of the creep phenomena under static load which caused the collapse of the Civic Tower of Pavia. The photographic images reported illustrate only some of the numerous micro cracks detected.”

CTS Technical Report of 15 November 2023

On October 20, 2023, at the request of Mayor Matteo Lepore, the Prefect convenes a meeting of the Committee for Public Order and Safety, to discuss the safety of the Garisenda tower.

During the meeting, Lepore communicates that he would have asked the CTS with the utmost urgency, a definitive and clarifying dossier on the data that emerged from the surveys on the Tower. In particular, the mayor of Bologna asks that the technical report include:

1. data analysis highlighting the coherence between the different measuring instruments, the presence of any anomalies, in particular of the strain gauges;
2. all the elements "to determine suitable solutions for the consolidation of the Garisenda tower" and "provide guidelines, through written summaries for the technical design of the consolidation interventions" as foreseen in the committee's tasks, approved with Council resolution PG.395798/2023 dated 6/06/2023.
3. provide useful elements for an update of the simulation of the collapse of the Garisenda tower and of the civil protection plan.

On November 15, 2023, the Technical Scientific Committee for the consolidation of the Garisenda Tower presents the technical report, with the following answers to the questions posed.

1. Data analysis highlighting the coherence between the different measuring instruments, the presence of any anomalies, in particular of the strain gauges.

Based on the data emerging in recent years, the CTS presents the following conclusions:

“In the last period, the monitoring system has highlighted how the overall situation has unfortunately significantly worsened, with worrying implications for the general stability of the tower, for the following main reasons:

• more than a year after the completion of the ringing works on the base, which should have created an effective confinement and therefore an improvement in the mechanical performance of the materials that compose it, the medium-term data do not allow us to detect any significant improvement in the trends preceding these interventions; on the contrary, the speed of the deformations in the base (crushing under constant load) appears to have significantly increased;
• as clearly demonstrated by the pendulum measurements of the last period, the tower seems to have modified its millenary movement of the top towards East / South-East, undertaking a new movement towards South.

This unexpected and accelerated trend suggested to the CTS to immediately suspend all other ongoing activities (particularly consolidation) and to bring attention to maximum alert conditions, believing that the safety conditions no longer exist for operating on and near the tower, except within the framework of a civil protection plan.”

2. All the elements "to determine suitable solutions for the consolidation of the Garisenda Tower" and "provide guidelines, through written summaries for the technical design of the consolidation interventions" as foreseen in the committee's tasks, approved with council resolution pg. 395798/2023 dated 6/06/2023

“In the context outlined above, the Municipality of Bologna has adopted a civil protection plan, which contemplates among the various scenarios also the possibility of sudden and unpredictable collapse. The definition and management of this plan is outside the tasks of the Technical Scientific Committee.

Once the civil protection plan has been defined, it is appropriate that the Municipality of Bologna urgently entrust a specific specialized company with the safety of the tower and the surrounding areas.”

“Considering the critical nature of the situation and the objective difficulty in operating, in order to safeguard the safety of people and prevent the loss of the property, as well as the surrounding buildings, the Committee suggests operating according to two levels of intervention, distinct in timing and purpose:

• installation of protective barriers, an intervention that must be carried out in a very short time: these barriers must be positioned in such a way as to maximize the area in which the debris can accumulate in the event of a collapse, so as to contain them even in the event that the collapse should occur with an overturning component; these barriers must be of a height sufficient to contain the volume of the debris, which is expected to be greater than the volume of the tower's masonry, due to the voids that will remain in the debris;
• implementation of provisional interventions, to be carried out as quickly as possible, with the function not only of confining any possible collapse but also, in a subsequent phase, of allowing for consolidation and restoration interventions, also contemplating the possibility of partially relieving the load on the base.”

Further clarifications from the CTS:

• All of the above must obviously be achieved as quickly as possible.
• The provisional interventions to be carried out must be reversible in nature and must be conceived in such a way as to allow, once completed, the execution of all possible and appropriate future interventions on the tower.
• The CTS also recommends that such structures allow, or rather encourage, a comprehensive examination of the building.
• In the event that part of the surfaces need to be covered, it is requested to acquire high definition and georeferenced images in advance, capable of providing a detailed survey.
• As regards future consolidation interventions, it is represented that these may also include those aimed at reducing the effects of the weight of the tower on the base and lower part of the same.

3. Provide useful elements for an update of the collapse simulation and the civil protection plan

“With regard to the subject of question 3, it should be noted that the identification of the areas at risk for the possible collapse mechanisms of the tower is beyond the tasks of the Technical Scientific Committee.

Here we can only limit ourselves to recalling the work carried out in 2020 (with subsequent updates in 2021) by Professors Tomaso Trombetti and Angelo di Tommaso, who developed simulations to define the areas affected by potential collapse (implosion and/or overturning). These simulations require appropriate updates, also in light of the changed trends in the displacement of the top of the tower.“

Securing the Garisenda Tower

At the end of November, work began on the containment structure of the Garisenda Tower, which represents the first phase of the safety process. After inspections to verify the underground utilities and the removal of the electrical cables that interfered with the affected area, carried out by the Municipality, preliminary work began to position the containers and the metal fence.

First phase: Garisenda Tower containment belt

In the first days of December, the positioning of containers and the first elements that will constitute the containment structure for the first safety phase began in Piazza di Porta Ravegnana.

The contractor Fagioli will have to build a protection belt to contain debris in the event of a collapse, in order to reduce the vulnerability of surrounding buildings and protect the population. This belt will prevent access to the restricted area and will be composed of metal modules that will be anchored and ballasted to the ground. In addition, specially designed metal rockfall protection nets with pre-tensioning and ground anchoring systems will be used.

The construction of the protection belt will require an adequate logistics plan for the transportation of materials and the pre-assembly of the modules at an Operations Hub located in via dell'Industria in Bologna, outside the historic center. This will minimize construction activities in Piazza Porta Ravegnana and minimize the impact on mobility. Thanks to this logistics solution, the transportation of the structure can be done mainly with ordinary means, avoiding the use of exceptional means except in sporadic cases.

It is estimated that the completion of the Garisenda protection belt construction site will be by January-February 2024.

The economic framework indicated in the Council resolution amounts to just over 4,2 million euros.

Second phase: Cylinder around the Garisenda

The next phase to ensure the safety of the Garisenda Tower will last at least six months. The mayor of Bologna, Matteo Lepore, explained this during a press conference. During this phase, a cylinder will be built around the Garisenda Tower, which will take significantly longer.

Third phase: restoration of the Garisenda Tower

The third phase, the restoration phase, will take even longer, Lepore says. “We will discuss this later,” the mayor cuts short and adds, “The time required for the first phase was very short,” implying that the restoration phase will also be defined soon.

Our photo gallery of the works, click on the image to view the photos in detail ▼

March 28, 2024 – The mayor announces the intervention to secure the Garisenda Tower

The mayor of Bologna, Matteo Lepore, announced the details of the intervention to secure the Torre Garisenda in a press conference at Palazzo d'Accursio.

The methods of intervention

Lepore explained that the steel trusses already successfully used for the tower will be used to stabilize the tower. Tower of Pisa. These pylons, appropriately modified, will speed up the work and contain costs.

The phases of the project

The intervention will take place in three phases:

1. Installing the trusses: inside the perimeter of the red containers the two steel structures will pull the Garisenda to reduce the inclination and prevent a possible collapse. Reducing the inclination angle will ensure the stability of the tower.
2. Monitoring: the tower will be constantly monitored to evaluate the effectiveness of the intervention.
3. Restoration: once the safety measures have been completed, we will proceed with the structural consolidation and subsequent conservative restoration of the tower.

The timings

The goal is to complete the intervention by 2024. Once the pylons are installed, the tower “will exit the yellow phase” and will be “secured, entering the green phase“, the mayor specified.

The words of Mayor Lepore:

“The Torre Garisenda is a symbol of Bologna and an important piece of our history,” said Mayor Lepore. “With this intervention, which uses innovative technology already successfully tested in Pisa, we want to secure and save the Tower of Bologna.”

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Cover photo: Trifonov_Evgeniy from Getty Images