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Tower of Pisa
Table of Contents
Brief History of the Tilt
Causes of Failure
Attempts to Prevent the Tower from Collapse
In 1173, Bonnano Pianso had begun construction of the Tower of Pisa. The job carried on until 1178, and then put on hold once the masons reached the fourth level. When construction resumed a century later, a slight tilt to the north was noticed, and from this moment on, the Tower had begun to lean, shifting from north to northeast, and finally south. The Tower of Pisa’s purpose was to house the bells of the cathedral of the Piazza dei Miracoli, a site with buildings dating back to the Middle Ages. The Tower’s quality of stone is excellent: most have no mortar between them because the bond is dependent on the high quality of workmanship. The foundation block is made of masonry cemented with San Giuliano mortar.
The Leaning Tower of Pisa also helped Galileo during his years of exploration. He used it as an example and experiment for his work on gravitational forces. The Tower’s lean helped prove that everything falls at the same rate, regardless of weight, as long as they have the same air resistance.
Since 1911, the Tower has shifted approximately one twentieth of an inch each year, with inches adding up each year. During the winter of 1999, the Tower shifted more than a millimeter south in one day due to the intense weather. The Tower has been closed to tourists since 1989.
Where a river meets the sea.
Underground surface beneath which earth materials, as soil or rock, are saturated with water.
A mass of metal cast in a convenient form for shaping, remelting, or refining.
San Giuliano Mortar:
The product of a medieval technique for preparing high-strength mortars.
Brief History of the Tilt
Standing tall 185 feet in the air, the
Photo Credit: Paul Parfitt
Leaning Tower of Pisa is tilting at an angle of five and a half degrees southward: approximately sixteen feet out of vertical. Over the years, many experts have searched to find the cause of the lean of this world wonder. It has been found that about 10,000 years ago, the location of the Tower was a river estuary. Therefore, the water and the tide were constantly flowing in and out of the site, depositing layers of soft sand and silt. The reason the Tower has sunk into the ground is due to the soft, sand-like texture of the soil. The reason the Tower tilts southward is because the soil under the south side of the monument is more compressible than on the north side. Since 1911, when measurements of the lean began, the Tower has shifted approximately one-twentieth of an inch each year. Engineer and site manager of the restoration project, Paolo Heiniger says, “the Tower has begun tilting at a rate of about one-tenth of an inch per year and accelerating. At this rate, the tower would be in danger to fall in twenty to twenty-five years.” (Popular Science, 2000, pg. 72) The Tower was built in stages over two hundred years and the builders realized from the beginning the structure was shifting. Attempting to correct the lean, the builders put taller pieces of stone on the south side and shorter on the north side. For example, the bell tower has six steps up to the base on the south side and only four steps on the north side. As a result, the tower is slightly shaped like a banana.
While many strategies to stabilize the Tower have been under consideration, there have also been actions taken which have pushed the Tower closer to collapse. The base of the columns on the ground level had sunk below ground and in 1838 Alessandro Gherardesca dug into the ground in order for visitors to see the columns as they were intended to be seen. He dug down about one and a half to two meters down straight into an area where a water table existed. Water came spouting out of the ground and the top of the Tower moved three quarters of a degree south. In 1934, when Benito Mussolini came to power, the Tower was interfered with once again. His regime rejected the leaning of the Tower as an “inappropriate symbol” and they drilled holes through the floor and poured almost two hundred tons of concrete into the foundation. As a result, the Tower lurched southward a tenth of a degree.
Causes of Failure
The failure of the Leaning Tower of Pisa is due primarily to the location of the tower and the instability of the ground underneath the foundation. During its 200 year construction the tower had begun to show signs of instability but the builders persisted and as time lapsed the tower leaned more and more resulting in a fifteen foot vertical discrepancy at the peak. The weight of the tower compressed the ground beneath beginning with a seven meter layer of silt and thirty meters of clay under the foundation. An area of clay under the south side of the structure is softer than its counterparts causing the infamous lean towards the south. Further depression of the clay below the foundation is a result of consolidation.
The foundation is not the sole factor of instability in the Leaning Tower of Pisa. Full of debris and mortar and peppered with cavities, the circular walls of the tower are weak and a contributor to its precariousness. Some of these vacancies in the walls are the result of scaffolding put in place either during construction or during the attempts to right the tower. The stones consisting primarily of limestone are of low resistance and the marble facing of only 25 centimeters creates an unstable framework for the tower. Although the walls have a thickness of nine feet in most places, they taper to just over a yard in some places to accommodate the spiral staircase that winds up the tower.
Various strategies were put into place to save the tower from collapse but many of them worsened its condition. Since the tower had begun to lean before construction was complete, the bell tower was added at an opposing angle in an attempt to decrease the rate of compression on the south side. Unfortunately the added weight caused the tower to lean even more, and without addition of the bell tower it would have remained at an angle of 1.5 degrees. Because it began to lean during construction, the builders were prompted to use larger stones on the southern side. This, coupled with the addition of the bell tower at an angle, is what gives the tower its banana shape. Many attempts to save the tower from collapse have been unsuccessful and at times detrimental to the overall structure—at best they have been temporary fixes.
Over the years, there have been many strategies of how the tower could be righted. Some suggested ideas involved digging out portions beneath the tower, which would allow the weight of the tower to balance itself out. Other schemes were more complex and involved steel buttressing and jacks to physically lift the tower and move it back to center, or even in another case these jacks would be used to rock the tower until it is righted. Some believe that the best way to stabilize the structure is through the use of bolts, while some prefer pre-tensioned rings. There are many different opinions on what is the best plan of attack to right the tower, some of which have been met with controversy while others have been accepted and implemented.
Attempts to Prevent the Tower from Collapse
Drawing Credit: Caitie Hutton
While many strategies and attempts have been made to prevent the tower from ultimate destruction, none have been entirely successful. The Leaning Tower of Pisa is the most monitored building in the world; approximately 120 devices are used by scientists to not only support the Tower but to measure its movement and responses. For example, a circular sensor runs around the inner walls and is used to register vibrations caused by something as small as a footstep while metal clamps detect any change in the Tower’s large cracks. Pendulums measure tilt and strain gauges monitor stresses on the walls. Also, at select moments, steel bands have been wrapped around the Tower, like a belt, to support the stones. While these various instruments monitor the Tower at all times, there have been many attempts to straighten the Tower. However, the Leaning Tower of Pisa is one of the largest tourist attractions in the world, and the visitors cannot be upset with a visible difference in tilt. In July 1993, engineers loaded 600 tons of lead ingots on the north side of the Tower to counteract the lean, a method that has successfully brought the Tower back almost an inch. Yet, these lead piles are extremely unattractive and hinder the splendor of such a beautiful monument. In order to remove the lead weights and replace with them invisible cables a concrete ring will be wrapped around the base of the Tower and attached to anchors. These anchors will be secured in the solid bedrock 130 feet down and will replace the push of the unattractive lead weights. Yet, at 3:30 A.M. on September 6, 1995, the Tower reacts and lurches a 16th of an inch south, moving what it would normally move in a year. Consequently, an attempt to rid of the ugly lead weights resulted in the addition of more lead weights.
Another effort to save the Tower from collapse has been through soil excavation: removing soil from the north, higher, side. The final goal of this endeavor is to reduce the lean by 10%, or a half of a degree, north which will add at least 300 years to the Tower’s life. This endeavor has proved to be successful.
Attention was focused on the Tower of Pisa after the nearby Pavia Civic tower collapsed in 1989. The Civic Tower was another historical tower and collapsed due to weakening of the old masonry along with natural shrinkage of the mortar over time. This greatly concerned the Italian government, and made them extra cautious of Pisa’s condition. Pisa was then closed off to the public for safety reasons and testing.
Cracking of the Tower’s masonry has also been a concern ever since the 1902 collapse of the St. Mark bell tower of Venice.
**Transcona Grain Elevator**
failure is often associated with the Tower of Pisa because it is a very similar foundation failure. The Grain Elevator was also placed upon clay, and three years after its construction, it began to shift. Also like the Tower of Pisa, it moved as a monolithic structure and only received minor cracking. However, the Elevator did not come to rest until the tilt had reached 27 degrees, way more severe than Pisa’s 5.5 degrees. When research was done to figure out why the failure had happened, the compression of the soil beneath the structure was to blame just like at Pisa.
There are also less severe cases of leaning structures than the Transcona Grain Elevator that can be related to Pisa. Structures in China and India are less famous but do have similar tilt to Pisa’s. Some buildings tilt but the angle is so slight and the building remains stable that it goes unnoticed. Towers can have a slight lean as long as the structure is tilted evenly throughout its axis and remains stable, and not tilting starting out at higher point on the tower which could throw it off of its axis.
Drawing Credit: Colette Aboussouan
In addition to the structural and planning fallacies, the Leaning Tower of Pisa has experienced a plethora of environmental obstacles and threats to its stability. The most obvious environmental factor is the danger of wind forces. The tower is not a candidate for wind engineering evaluation because it is heavy, short, and rigid. However, due to the inclination of the structure and the existing soil pressures, building collapse due to wind is a viable threat.
Winter is the most dangerous time of year for the Leaning Tower of Pisa because the cold temperatures cause the stones on the south side to compress, increasing the tower’s tilt. On the contrary, summer temperatures cause the stones to temporarily expand allowing the tower to straighten ever so slightly. Daily changes in temperature move the tower in a circular motion of roughly a 100th of an inch across in diameter, however this has been known to increase during rainstorms. Increased precipitation during autumn and winter as well as heavy storms cause the water table to rise more on the north side, pushing the tower further towards the south. This contributes to the fluctuating instability of the building’s foundation.
Earthquakes are another possible cause of stress on the structure that could lead to collapse. Seismic tremors pose a threat to most structure including those damaged during an earthquake in Assisi. The Leaning Tower of Pisa is under an even greater threat because of the stress caused by its inclination, and the volatility of its foundation. Although the region of Pisa has moderate seismic activity, it would not take much to collapse the tower.
"Acts of God: The Symbolic and Technical Significance of Foundation Failures." Morley, Jane. Journal of Performance of Constructed Facilities Feb. 1996: 23-25.
"Acts of God: The Symbolic and Technical Significance of Foundation Failures" Online PDF
Discusses the significance of foundation failures, and goes into detail on the failures of the Leaning Tower of Pisa along with the Transcona Grain Elevator.
July 2000: 70-73. Print.
This article discusses the last step in the process of slow soil removal over several months to restore the Leaning Tower of Pisa into a more upright position. This procedure is not intended to set the tower completely upright but save it from the inevitable effects of gravity. Following the history of the tower’s tilt and how the engineers have attempted to fix the problem, the article gives numerical calculations for values such as soil excavation and the acceleration of tilt per year. How the engineers have gone about removing the soil and saving this famous monument is also discussed.
Campbell, Peter (2001). “Learning from Construction Failures: Applied Forensic Engineering”. Whittles Publishing, Scotland, UK.
"Learning From Construction Failures: Applied Forensic Engineering" Online Text
A text book with a chapter called “Propping up Pisa” which gives a brief introduction, details of the tower and ground profile, diagrams, a history of the construction, a history of its tilting, and other useful information. (available as a preview on Google books)
Cheney, James A., Abbas Abghari, and Bruce L. Kutter. "Stability of Leaning Towers." 117.2 (1991): 297-318 . Print.
"Stability of Leaning Towers" Online PDF
This article talks about the possibility of complete collapse and how safe it is today. It also mentions other monuments and towers that experience a similar effect and how some of them seem to be stable.
"Fall of the Leaning Tower."
. PBS. 5 Oct. 1990. Television.
This broadcast begins posing the idea of losing one of the worlds wonders to an earthquake or a storm and continues to recount the history of attempts to revive the Leaning Tower of Pisa. The narrator shares how engineers tried to support the tilting structure, such as by plastic-coated metal tendons on the second story.
Rampello, Sebastiano, and Luigi Callisto. Canadian Geotechnical Journal. A study on the subsoil of the Tower of Pisa based on results from standard and high-quality samples. Web. 10 Feb. 2010. <
The International Committee appointed by the Italian Government in 1990 prompted numerous studies in order to understand the mechanism of deformation of the soil beneath the foundation of the Tower of Pisa. In these works, detailed information was given on the history of the tower, the structural features, and the subsoil conditions.
Shrady, Nicholas (2003). “Tilt: a skewed history of the Tower of Pisa.” Simon & Schuster, New York, NY.
"Tilt: a skewed history of the Tower of Pisa" Online Text
“A detailed chronicle of the famous architectural and cultural structure notes its century-long construction postponement after the erection of the first four stories in 1173; its survival of earthquakes, war, and tourism; the 1360 addition of its bell tower; and the contributions of such figures as Galileo, Machiavelli, and da Vinci.” (Available through PSU Libraries or Preview in Google Books; Overview provided by Google books)
"Scientific American Frontiers: Science Italian Style- Special From Italy Show 503."
Scientific American Frontiers
. PBS. PBS.
. Web. 10 Feb. 2010. <
This program gives a comprehensive history of the structure, and various attempts to right the tower over time. The narrator speaks with the man responsible for keeping the tower upright today—Paolo Heiniger. He gives us a first-hand perspective of the history and possible future of the famous building.
Solari, Giovanni, Flora Livesey, and Timothy A. Reinhold. Wind and Structure. Vol. 1. Investigation of the wind and effects on the Leaning Tower of Pisa. Web. 10 Feb. 2010. <
Bonanno Pisano stared the Tower of Pisa in 1174 and finished it in 1350. Like the other buildings in the Piazza dei Miracoli, the Leaning Tower was erected during the Middle Ages. Normally, this type of building – short, stiff, and heavy – would not be a candidate for detailed wind engineering evaluations. However, because of extremely high soil pressures caused by growing inclination, there has been increasing concern that environmental loading such as wind actions could combine with existing conditions to cause the collapse of the tower.
“Stabilization of the Leaning Tower of Pisa.” G. Macchi, ASCE Conf. Proc. 171, 152 (2005), DOI:10.1061/40753(171)152
"Stabilization of the Leaning Tower of Pisa" Online PDF
This paper discusses the structural aspects and attempts for stabilization of the Tower of Pisa. Mostly deals with the period after the tower was closed.
Steen, Joan. "The Leaning Tower is Falling Down."
Sept. 1960: 71-73.
. Web. 10 Feb. 2010. <
tower of pisa&f=false>.
This article expresses the dire need in the sixties for a solution to the tower’s problem. It provides a brief background of the tower, and the reasons it began to lean. There are also useful pictures of the tower as well as a section cut and axonometric view of the structure.
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