How Engineers Straightened the Leaning Tower of Pisa

Summary

The Leaning Tower of Pisa's stabilization project is a remarkable tale of engineering and historical preservation. Built in 1173 on uneven soil, the tower's tilt increased over centuries, leading to its closure in 1990 due to near-collapse. A committee of experts employed innovative methods, including counterweights and underexcavation, to reduce the tilt by half a degree, ensuring the tower's safety for future generations while maintaining its iconic lean.

Takeaways

• 🏗️ The Leaning Tower of Pisa was constructed in 1173 and began tilting due to uneven soil deposition and sedimentation from a nearby river.
• 📐 By 1990, the tower's tilt had reached 5.5 degrees, leading to its closure over fears of imminent collapse.
• 🔍 A committee of experts was appointed to investigate and stabilize the tower, using methods like drilling boreholes, soil testing, and building scale models.
• 📈 The tower's tilt wasn't uniform, allowing historians to track its lean's history and estimate foundation settling since 1817.
• 🏢 The committee initially used counterweights (lead ingots) to temporarily stabilize the tower, which successfully reduced the tilt slightly.
• 🔨 The anchoring solution involving deep anchors faced challenges, causing the tower to tilt in the wrong direction and necessitating additional counterweights.
• 💡 The committee explored several solutions, including groundwater pumping, electroosmosis, and underexcavation, to stabilize the tower.
• 🚧 Underexcavation, a method of carefully removing soil beneath the tower, proved to be the most effective solution after successful tests.
• 🛠️ The stabilization project reduced the tower's tilt by half a degree, bringing it back to the stability of the early 1800s, but not completely straightening it to preserve its historical character.
• 📚 The stabilization efforts represent an 850-year-old process of managing the tower's unique lean, which is central to its appeal and historical significance.
• 🎥 The story of the Leaning Tower of Pisa's stabilization is a fascinating example of the ingenuity and logistics behind large-scale engineering projects.

Q & A

How old is the Leaning Tower of Pisa?

The Leaning Tower of Pisa was started in 1173, making it over 850 years old as of the time of the transcript.

What caused the Leaning Tower of Pisa to lean?

The tower leans due to the uneven distribution of sand and clay deposited by a river and the sea in the area where Pisa is located, combined with the movement of sediment from tidal changes.

When was the Leaning Tower of Pisa closed to the public?

The tower was closed to the public in 1990 due to concerns that it was near collapse.

What was the average tilt of the Leaning Tower of Pisa in 1990?

The average tilt of the Leaning Tower of Pisa in 1990 was five-and-a-half degrees.

How did the construction of the Leaning Tower of Pisa progress?

Construction began in 1173, was interrupted by battles in 1178, resumed nearly a century later, and the belfry was completed one century after that. The tower was already tilting when work resumed, and builders compensated by making one side taller than the other.

What were the initial measures taken to stabilize the Leaning Tower of Pisa?

Initial measures included installing a modern monitoring system, building a concrete ring around the base of the tower, and placing lead ingots on the north side as counterweights.

What was the factor of safety estimated for the Leaning Tower of Pisa in 1993?

In 1993, the factor of safety for the Leaning Tower of Pisa was estimated to be 1.07, indicating that the soil could only withstand a 7% increase in weight from the tower.

What was the temporary solution that initially worked but was later replaced?

The temporary solution that initially worked was the use of lead counterweights. This was later replaced with the idea of deep anchors, which unfortunately did not work as planned and caused the tower to tilt in the wrong direction.

What technique was used to permanently stabilize the Leaning Tower of Pisa?

The technique used to permanently stabilize the Leaning Tower of Pisa was underexcavation, which involved carefully removing soil from below the tower to gradually tilt it upright.

How much did the tilt of the Leaning Tower of Pisa decrease after the stabilization project?

The tilt of the Leaning Tower of Pisa was reduced by about half a degree after the stabilization project, effectively reversing time to the early 1800s when its likelihood of toppling was much lower.

Why was the Leaning Tower of Pisa not completely straightened?

The Leaning Tower of Pisa was not completely straightened because the tilt is integral to its historical character and a big part of why it is of interest and importance to people.

Outlines

00:00

🏗️ The Origins and Construction of the Leaning Tower of Pisa

This paragraph delves into the geological and historical context of the Leaning Tower of Pisa. It explains how sediment deposition from a river in Italy led to the uneven foundation on which the tower was built in 1173. The narrative highlights the tower's tilt, which increased over six centuries, prompting its closure in 1990 due to the risk of collapse. The Italian Government assembled a committee to address the structural issue, employing methods from recreational geology and heavy construction to stabilize the iconic structure.

05:04

🔩 Challenges and Temporary Solutions in Stabilizing the Tower

The second paragraph discusses the challenges faced by the committee tasked with stabilizing the Leaning Tower of Pisa. It outlines the initial investigations, including soil testing and construction of scale models, which confirmed the tower's precarious state. The committee's first solution involved installing a modern monitoring system and using lead counterweights to temporarily stabilize the tower. However, subsequent attempts to fix the tilt with deep anchors led to new complications, requiring additional measures to prevent further tilting and eventual collapse.

10:09

🛠️ The Successful Underexcavation Technique to Save the Tower

This paragraph describes the final and successful method used to stabilize the Leaning Tower of Pisa. After testing various solutions, including electroosmosis, the committee settled on underexcavation, a technique that involved removing soil from beneath the tower to correct its tilt. The process was meticulously executed with a safeguard system in place, and it successfully reduced the tower's tilt by half a degree. The tower's stabilization was a continuation of its 850-year-old history, preserving its distinctive lean while ensuring its safety for future generations.

Keywords

💡Leaning Tower of Pisa

The Leaning Tower of Pisa is a world-renowned architectural structure known for its unintended tilt. Located in Pisa, Italy, it was originally intended to be a bell tower for the cathedral but began to lean due to uneven foundation soil. The tower's tilt became a significant tourist attraction and an engineering challenge, leading to various stabilization efforts. In the context of the video, the Leaning Tower of Pisa serves as the central focus, detailing its history, construction, and the engineering feats that prevented its collapse.

💡Tilt

Tilt refers to the degree to which the Leaning Tower of Pisa deviates from the vertical. It is a critical measure of the tower's stability and safety, with the average tilt reaching five-and-a-half degrees in 1990. The tilt is not uniform throughout the structure, which allows historians to trace the tower's construction history and the foundation's settling over time. The video explains how the tower's tilt was monitored and reduced through various engineering methods.

💡Underexcavation

Underexcavation is the process of carefully removing soil from beneath a structure to alter its position or to reduce its tilt. In the case of the Leaning Tower of Pisa, this technique was used to extract soil from beneath the tower's north side, gradually straightening it and reducing the tilt. This method was developed after other techniques, such as electroosmosis, proved ineffective. Underexcavation was a key component of the stabilization project and was executed with precision to avoid further destabilizing the tower.

💡Counterweights

Counterweights are heavy objects used to balance a structure and prevent it from tipping or falling. In the Leaning Tower of Pisa's case, lead ingots were used as counterweights on the north side of the tower to counteract the overhang caused by the tilt. This temporary measure was part of the initial efforts to stabilize the tower before more permanent solutions were implemented.

💡Electroosmosis

Electroosmosis is a soil stabilization technique that involves the use of electric current to facilitate the movement of water through soil. The process takes advantage of the negative surface charge of clay particles, causing water to migrate toward the cathode where it can be removed. This technique was considered for the Leaning Tower of Pisa to selectively consolidate the clay below the tower's north side and reduce its tilt. However, the soil's high conductivity rendered electroosmosis ineffective in this case.

💡Stabilization

Stabilization in the context of the Leaning Tower of Pisa refers to the series of engineering interventions aimed at preventing the tower from collapsing due to its severe tilt. These interventions included the use of counterweights, underexcavation, and the installation of a drainage system. The goal of stabilization was to ensure the tower's safety without completely removing its iconic lean, which is part of its historical character.

💡Foundation

The foundation of a structure is the base upon which it rests and is designed to support the structure's weight. For the Leaning Tower of Pisa, the foundation's uneven settlement on layers of sand and clay caused the tower to lean. Understanding the foundation's behavior and how it settled over time was crucial for the tower's stabilization efforts. The foundation's condition directly influenced the tower's tilt and the engineering solutions employed to correct it.

💡Incline

Incline, in the context of the Leaning Tower of Pisa, refers to the angle or gradient at which the tower leans. The incline is a critical measurement for assessing the tower's stability and the effectiveness of stabilization efforts. The tower's incline was monitored and adjusted through various engineering techniques to prevent collapse.

💡Groundwater

Groundwater is the water found beneath the earth's surface, which can affect the stability of structures built on it. In the case of the Leaning Tower of Pisa, fluctuating groundwater levels exacerbated the tower's tilt by altering the soil's stability. Controlling groundwater levels was one of the measures taken to stabilize the tower and prevent further leaning.

💡Catino

The Catino is a part of the Leaning Tower of Pisa's structural complex, which was initially thought to be separate from the tower itself. However, during the stabilization efforts, it was discovered that parts of the tower were resting on the Catino. This realization led to the tower being structurally attached to the Catino as part of the stabilization process, increasing the effective area of the foundation and contributing to the tower's stability.

💡Engineering

Engineering in the context of the Leaning Tower of Pisa refers to the scientific and technical disciplines applied to understand, predict, and solve the problems associated with the tower's tilt and stability. The engineering efforts included the analysis of the tower's construction history, soil testing, and the development of innovative stabilization techniques such as underexcavation.

Highlights

The Leaning Tower of Pisa was constructed in 1173 on a site with a unique soil composition of more sand to the north and more clay to the south.

The tower's tilt was a result of the unstable soil and construction was interrupted multiple times due to battles, leading to adjustments in the structure to correct the lean.

By 1990, the tower's tilt had increased to 5.5 degrees, leading to its closure over fears of collapse.

A committee was appointed to find a solution to stabilize the tower, using methods from recreational geology and heavy construction.

The tower's tilt was not uniform, allowing historians to track its leaning history and estimate foundation settling.

The committee's initial investigations included drilling boreholes, soil testing, and building scale models to test the tower's stability.

The factor of safety for the tower was estimated to be 1.07 in 1993, indicating it was very near collapse.

Temporary measures included installing a modern monitoring system and placing lead ingots as counterweights on the north side of the tower.

The tower was temporarily stabilized by adding counterweights, but a more permanent solution was needed.

Three main ideas were proposed for a permanent fix: groundwater pumping, electroosmosis, and underexcavation.

Electroosmosis was attempted but failed due to the soil's high conductivity, rendering the process ineffective.

Underexcavation was the final solution, involving the careful removal of soil beneath the tower to correct its tilt.

The underexcavation process was tested and refined, ultimately successfully reducing the tower's tilt by about half a degree.

The stabilization project respected the tower's historical character, ensuring the iconic lean was preserved.

The Leaning Tower of Pisa's stabilization is an example of ingenious engineering and a testament to the value of independent educational content.

The logistics and precision involved in the tower's stabilization make it a fascinating case study in engineering.

The stabilization project was a continuation of the tower's 850-year-old process of leaning, now secured for future centuries.

The story of the Leaning Tower of Pisa's stabilization highlights the importance of independent creators in providing deep, educational content.

Transcripts

Long ago, maybe upwards of 1-2 million years ago,  a river in the central part of what’s now Italy,  

emptied into what’s now the Ligurian Sea. It  still does, by the way, but it did back then  

too. As the sea rose and fell from the tides  and the river moved sediment downstream, silt  

and soil were deposited across the landscape. In  one little spot, in what is now the city of Pisa,  

that sea and that river deposited a little bit  more sand to the north and a little bit more clay  

to the south. And no one knew or cared until  around the year 1173, when construction of a  

bell tower, or campanile (camp-uh-NEE-lee) for  the nearby cathedral began. You know the rest  

of this story. For whatever reason, we humans  love stuff like the Leaning Tower. There’s just  

something special about a massive structure that  looks like it’s about to fall over. But you might  

not know that it almost did. Over the roughly six  centuries from when it was built to modern times,  

that iconic tilt continued to increase to a point  in 1990 when the tower was closed to the public  

for fear that it was near collapse. The Italian  Government appointed a committee of engineers,  

architects, and experts in historical restoration  to decide how to fix the structure once and for  

all (or at least for the next several centuries,  we hope). And the way they did it is really pretty  

cool, if you’re into recreational geology and  heavy construction. And, who isn’t!? I’m Grady,  

and this is Practical Engineering. Today  we’re talking about the Leaning Tower of Pisa.

Five-and-a-half degrees. That was the average tilt  of the tower in 1990 when all this got started.  

I have to say average because the tilt isn’t the  same all the way up. And actually, that fact makes  

it possible to track the history of the lean back  before it was being monitored. The tower started  

construction in 1173 and reached about a third of  its total height by 1178 when work was interrupted  

by medieval battles with neighboring states.  When work started back up nearly a century later,  

the tower was already tilting. But the  masons didn’t tear it down and start over;  

they just made one side taller than the other  to bring the structure back into plumb. By 1278,  

the tower had reached the seventh cornice, the  top of the main structure minus the belfry,  

when work was interrupted again. One short  century later, the belfry was finally built,  

and again with a relative tilt to the rest  of the structure to correct for the continued  

lean. On the south side of the belfry, there are 6  stairs down to the main tower; on the north side,  

only four. The result of all this compensation  by the builders is that the Leaning Tower of  

Pisa is actually curved. Knowing the timeline  of construction and how the tilt varies over  

the height of the structure allowed historians  to estimate how much sinking and settling the  

foundation underwent over time. By 1817, when  the first recorded measurement was taken,  

the inclination of the tower was about  4.9 degrees, and it just kept going.

The new committee charged with investigating  the issue first spent a lot of their time simply  

characterizing the situation. They drilled  boreholes and tested the soil. They estimated  

stability using simple hand calculations.  They built a scale model of the tower and  

tested how far it could lean before it toppled.  They developed computer models of the tower and  

its foundation to see how different soil  characteristics would affect its stability.  

All of the analysis and various engineering  investigations all pointed toward the same result:  

the tower was very near to collapse. 

In 1993, one researcher estimated the factor of safety to be 

1.07, meaning (generally) that the underlying soil  could withstand a mere 7 percent more weight than  

the tower was imposing on it. There was basically  no margin left to let the tower continue its lean.  

A similar tower in Pavia had collapsed in 1989,  and the committee knew they needed to act quickly.

To start, they installed a modern monitoring  system that could better track any movement  

over time, including surveying benchmarks and  inclinometers. I have a video all about this  

type of instrumentation if you want to learn  more after this. The committee also opted to  

take immediate temporary measures to stabilize  the tower with something that could eventually  

be removed before developing a permanent fix.  They built a concrete ring around the base of  

the tower and gradually placed lead ingots, about  600 tons in total, on the north side to act as a  

counterweight to the overhanging structure.  As they added each layer of counterweights,  

they monitored the tilt of the tower. It was ugly,  but it worked.

For the first time in history,  

the tower was moving in the right direction. A few  months after they finished the project, the tower  

settled into a tilt that was about 48 arcseconds  or a hundredth of a degree less than before.

In fact, it worked so well, the committee decided  to take it one step further. To reduce the visual  

impact of all those lead weights, they proposed to  replace them with ten deep anchors that would pull  

the northern side of the tower downward to the  ground like huge rubber bands. This fix didn’t  

go quite so smoothly.

The engineers had assumed  that the walkway around the base of the tower,  

called the Catino, was structurally separate  from the tower. But what they found during  

construction of the anchor solution was that  some of the tower was resting on the Catino.  

The project required removal of part of the  catino to make room for a concrete block,  

and when they did, the tower started tilting  again, this time in the wrong direction,  

and fast (about 4 arc seconds per  day, enough for serious concern that  

the tower might collapse). They quickly  abandoned the anchoring plan and added  

350 more tonnes of lead weights to stop the  movement and focus on a permanent solution.

Engineering ANY solution to a structure  of this scale with such a severe tilt is  

a challenge in the best circumstances. But  adding on the fact that the solution had  

to maintain the historical appearance of the  building (including leaving the right amount  

of lean!) made it even tougher. And after  the near disaster of the temporary fix,  

the committee knew they would have to be  extremely diligent. They ultimately came  

up with three ideas to save the tower. The  first one was to pump out groundwater from  

the sand below the north side of the tower,  but they didn’t feel confident that they could  

predict how the structure would respond over  the long term. Another idea was electroosmosis.

If you’ve seen some of my other videos about  settlement, you know that it’s hard to get water  

out of clay, and there are quite a few clever  ways engineers use to make it happen faster. One  

of those ways involves inserting electrodes into  the soil and passing electric current through it.  

Clay particles have a negative surface charge, so  the majority of the ions in the water between the  

particles are positively charged. Electro-osmotic  consolidation takes advantage of this by applying  

a voltage across the soil, causing the water to  migrate toward the cathode where it can be pumped  

to the surface. The idea seemed promising because,  by carefully choosing the location of electrodes,  

engineers hoped they could selectively consolidate  the clay below the north side of the tower,  

reducing its overall tilt. They even performed  a large-scale field test near the tower to shake  

out some of the kinks and gather data on  the effectiveness of the technique. But,  

it didn’t work at all. Turns out the soil was  too conductive, so things like electrolysis,  

corrosion, heat, and all the other  effects of mixing electricity and  

saturated soil made the process pretty  much useless for this particular case.

So, the committee was down to one last idea:  underexcavation. If they couldn’t get the soil  

below the tower to consolidate, they could just  take some out. And again, they would need to test  

it out first. So, in 1995, they built a large  concrete footing on the Piazza grounds not far  

from the Tower. Then, they used inclined drills to  bore underneath the footing and gradually remove  

some of the underlying soil. Guide tubes  kept the boring in the right direction,  

and a hollow stem auger inside two casings  was advanced below the footing. The outer  

casing stayed in place while the inner casing  moved with the auger. The auger and the inner  

casing were advanced past the outer casing to  create a void, and when they were retracted,  

the cavity would gently close. At first, it wasn’t  looking good. After an initial tilt in the right  

direction, the test footing started leaning  the wrong way. But the crew continued refining  

the process and eventually got it to work, even  finding it was possible to steer the movements  

by changing the sequence of underexcavation. It  was finally time to try it on the real thing.

Knowing the risks and uncertainties involved,  the engineers first designed a safeguard system  

for the tower if things started to  go awry. Cable stays were attached  

between the tower and anchoring frames. 

The cables could each be tightened individually,

giving the engineers opportunity to stop  movement in any undesirable direction if  

the drilling didn’t go as planned. In 1999, they  started a preliminary trial with 12 holes. And  

the plan went perfectly.  

Over the course  of 5 months, the underexcavation brought 

the tilt up by 90 arcseconds, and after a few  more months, it settled in at 130 arcseconds,  

about four hundredths of a degree. This gave the  committee confidence to move on to the final plan.

Starting in 2000, 41 holes were drilled  to slowly tilt the tower upright. Over  

the course of a year, 38 cubic meters of  soil were removed from below the tower,  

roughly 70 tonnes. The lead counterweights  were removed. A drainage system was installed  

to control the fluctuating groundwater levels  that exacerbated the tilt. And, the tower was  

structurally attached to the Catino, increasing  the effective area of the foundation. In the end,  

the project had reduced the tilt of the tower  by about half a degree, in effect reversing  

time to the early 1800s when its likelihood  of toppling was much lower. Of course, they  

didn’t straighten it all the way. The lean isn’t  just a fascinating oddity; it is integral to the  

historical character of the tower. It’s a big part  of why we care. Tilting is in the Campanile’s DNA,  

and in that way, the stabilization project was  just a continuation of an 850-year-old process.  

Unlike the millions of photos with tourists  pretending to hold the tower up, the contractors,  

restoration experts, and engineers actually  did it (for the next few centuries, at least).

The logistics of the operation to fix the  Leaning Tower are really the most fascinating  

part of the whole project. This idea of inclined  drilling to carefully remove soil with so much  

precision is an insider story at its core. On  the surface, it’s not really that interesting,  

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Maybe you’ve noticed what I  have over the past few years,  

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