Ep 35 Leaning Tower Of Pisa

Engineering News – James Webb Space Telescope (1:40))

This week's engineering failure is the Leaning Tower of Pisa (7:45). After taking almost 200 years to build, the tower has a few subsurface issues that contribute to its iconic lean (18:10). So much of a lean that it’s even won some world records (26:50). Luckily, to preserve the tower and prevent collapse, it underwent a stabilization project in the 1990s (31:05).


Engineering News

Leaning Tower of Pisa

Episode Summary

Hi and welcome to Failurology; a podcast about engineering failures. I’m your host, Nicole

And I’m Brian. And we’re both from Calgary, AB.

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This week in engineering news, the 10 billion dollar James Webb Space Telescope has been removed from its transport container in French Guiana in preparation for its launch in early December.

The James Webb Space Telescope will allow astronomers to look deeper into the cosmos (further back in time) than is possible with the Hubble Space Telescope. This will be possible due to the larger mirror size (6.5m diameter v. 2.4m for the Hubble Space Telescope) and instruments that are tuned to the infrared spectrum.

The JWST journey from final assembly and testing at the Northrop Grumman factory in California to French Guiana included a transit through the Panama Canal which we covered in episode 30.

Now on to this week’s engineering failure; the leaning tower of Pisa.

We couldn’t decide if the tower was an engineering failure or an engineering marvel, so we stuck this episode halfway in between the marvels we cover every 10th episode. Whatever it is, this is a really cool building.

  • Freestanding bell tower in the Italian city of Pisa

    • Almost 56m tall

    • The outer base is 15.5m in diameter

    • Walls are 2.44m thick at the base. I am only about 1.57m tall, so the walls are thicker than I am tall.

    • 296 steps to the top - the Calgary tower has 802 steps

    • Used by Nazi forces as an observation post during world war 2, the US Army sergeant decided not to call in an artillery strike against the tower’s position. This is good because it likely wouldn’t be here today if they did.

    • Declared a World Heritage Site along with the neighbouring cathedral, baptistery and cemetery in 1987.

  • 3 million visitors each year to the tower, plus 6 million visit the Miracles’ Square

  • Construction occurred in three stages over 199 years, starting in January 1173 with a few stones at the base of the tower. That’s a really long project.

  • The tower began to sink after construction progressed to the second floor in 1178. The Republic of Pisa was battling Genoa, Lucca, and Florence, so construction on the tower ceased for almost a century. Some experts now believe that without this pause, which allowed the soil to settle, the tower would have almost certainly toppled over.

  • In order to compensate for the tilt, the south side of the upper floors are taller than the others, so the tower is actually curved

  • The seventh floor was completed in 1319 and the bell chamber in 1372

  • There are 7 bells, one for each note of the musical major scale. The largest one was installed in 1655.

Reasons for leaning

  • The tower only has a 3m deep foundation, which is not very deep at all, and on top of this, the foundation is set in weak and unstable soil

  • On the south side of the tower, the soil was siltier and more clayey than on the north side, and the sand layer is thinner. The ability of different soils to support the weight applied by the tower differs between the north and south side.

  • There is also a high water table in this area that is said to deposit sediment and shells and vertebrae from small marine animals over time. Since the water table on the north side is higher, there is more sediment deposited on the north side and a higher soil on the north side, further contributing to the lean. As well, the higher water table on the north side leads to more impact from heavy rains during the winter months.

  • In 1838 a walkway was excavated, showing the column plinths and foundation steps. This impacted the water table as well as the soil strength on the south side, where the walkway was located. The tower nearly collapsed when this occurred.

Why hasn’t it fallen down?

  • So, how has this tower stood for almost 850 years and not fallen down yet? Despite many almost collapses and at least four strong earthquakes that have hit the region since 1280. The tower survived due to an effect called Dynamic soil-structure interaction (DSSI) - the height and stiffness of the tower and the softness of the foundation soil influence the vibrational characteristics of the structure in such a way that the tower doesn’t resonate with ground motion. This is not my area of expertise, so we’ll default to Professor George Mylonakis, from the Department of Civil Engineering at the University of Bristol, who took part in the study that looked into why the tower hadn’t collapsed. He said, “Ironically, the very same soil that caused the leaning instability and brought the tower to the verge of collapse, can be credited for helping it survive these seismic events.”

  • In other words, the tower’s height and stiffness combined with the softness of the soil means the structure is protected during an earthquake. It simply doesn’t vibrate during the tremors.


  • By 1990 the tilt reached 5.5 degrees. Luckily, the structure was stabilized between 1990 and 2001. In 2008, scientists announced the movement had finally stopped and the tower, now leaning at a modest 3.9 degrees, is expected to stay put for at least 200 years.

  • The tower used to hold the Guinness World Records for the unintentional farthest leaning tower, but after it was stabilized, the Church of Suurhusen in Germany took this record at 5.1939 degrees

  • According to a local historian, the Church of Surrhusen tower was built on marshy land on foundations of oak tree trunks preserved by groundwater. When the water was drained, the wood rotted and the tower tilted.

  • The Capital Gate building in Abu Dhabi, UAE is the world’s most tilted man-made tower, with an 18-degree slope, but it was deliberately built to slant

  • The Leaning Tower of Wanaka, another well know leaning building, is part of the Puzzling World attraction near Wanaka, New Zealand. It’s balanced on one corner and leans at an angle of 53 degrees to the ground

Stabilization Project

  • The tower was stabilized in the 90s. The project reinforced the soil under the foundation and made room for the tower to compress some soil and straighten back to the north

  • The cost of this project cost approximately 30 million Euro

  • The stabilization was led by John Burland an Emeritus Professor and Senior Research Investigator at the Department of Civil and Environmental Engineering of Imperial College London

    • He led 13 experts appointed to fix the tower.

    • Burland started his career in 1967 with the thesis called “Deformation of Soft Clay” so he was the perfect man for the job

  • They placed counterweights on the north side at the base to slow down the lean to the south

  • They used steel cables (comparable to those you find on a suspension bridge) to harness the tower and prevent it from collapsing and then to “pull it back” once the soil was prepared

  • They dug wells under the foundations of the tower - 60 m3 of clay were removed

  • They drained the water from the wells

  • They reinforced the foundations with 15m concrete pillars

  • And then they used the steel cables to straight the tower

  • Recovered over 38cm of lean from this project

    • Between 2001 and 2013, the tower leaned back an additional 2.5cm

    • That said, they believe it’s safe for at least another 200 years

  • The people of Pisa are happy that it’s stabilized, but not that it’s straightened. Leaning is part of its charm

So there you have it, the leaning tower of Pisa. What started out as just a bell tower, is now known worldwide for its iconic lean. The tower taught us a lot about geotechnical, civil and structural engineering and has been a tourist attraction to millions of people every year. Even though they straightened the tower, the people of Pisa are happy it still leans.

For photos, sources and an episode summary from this week’s episode head to Failurology.ca. If you’re enjoying what you’re hearing, please rate, review and subscribe to Failurology, so more people can find it. If you want to chat with us, our Twitter handle is @failurology, you can email us thefailurologypodcast@gmail.com, or you can connect with us on Linked In. Check out the show notes for links to all of these.

Thanks everyone for listening. The next episode is the one-year anniversary of the podcast. We’re going to do a year in review and talk about all of the things we’ve learned over the last year; about podcasting and engineering and failures. Bye everyone, talk soon!