A Case Study on Leaning
Tower of Pisa: Its Engineering
Geological Failures
Submitted by:
Chavez, Sean Andrie
Roxas, Mark Louie
Manumbas, Nina Kyla
Reyes, Alliaah Lalaine
Submitted to:
Mery Liza Villar
Course / Section:
CE011 – C06
EXECUTIVE SUMMARY
The human race has a magnificent cultural history that serves as a reminder of our civilizations'
achievements and a crucial repository for memory. However, historical sites have endured both natural
and man-made tragedies throughout time. The latter ones have a direct link to insufficient human action in
the geological environment. As a result, the rocks and soils, groundwater, and air underwent a series of
gradual, long-lasting, as well as sudden, quick, and generally unfavorable alterations. The iconic medieval
building known as the Leaning Tower of Pisa, or Torre Pendente di Pisa in Italian, is located in Pisa, Italy.
An Italian bell tower made of white marble and situated next to the city's cathedral, is famous across the
globe for its unintentional lean to one side. In the late 20th century, shifting of its foundations forced it to
lean 5.5 degrees, or roughly 15 feet (4.5 meters), off the perpendicular. The tower was then extensively
straightened, and its lean was eventually decreased to less than 4.0 degrees. The construction of the
Leaning structure of Pisa on top of an unstable soil surface led the structure to progressively slant in one
direction. Due to the combined impacts of groundwater fluctuation and soil creep, the foundation's
inclination has been continuously increasing.
Under the Tower of Pisa, the stratigraphy of the earth is exceedingly intricate. To prevent any building
mishaps, architects have to take into account the characteristics of the various soil strata. The top horizon,
Horizon A, which extended from three to ten meters, was very tidally sensitive. The chaotic character of
the soil in this stratum is influenced by the geological setting of this tower. The tower is being built close
to the water, and because of the high water table, the subsoils are saturated with salt water. As a result,
sedimentation would take place and be dumped into the soil. One of the elements of the sediments in the
estuary environment was marine life, particularly creatures with shells. Due to the presence of sediments
and natural forces, the soil's stiffness was not linear. Due of the soil's high compressibility and non-linear
stiffness, the soil's resisting moment was unable to counteract the significant overturning force created
during the construction of the tower. A self-driving instability resulted as a result. This caused the tower's
tilt to gradually rise.
BACKGROUND
One of the most visited tourist attractions on earth is the Leaning Tower of Pisa. It is not only a
UNESCO World Heritage Site but also one of the Seven Wonders of the World. Benenato, the tower's
architect, realized that it was tilting even more in 1234. The fourth story's south side was now precisely 6
inches shorter than its north side. Again making the south columns taller than the north ones, he
constructed a fifth level. Benenato gave up after adding that one storey, and the tower's development
ceased once more. The sixth and seventh storeys of the Tower of Pisa were added by William of
Innsbruck to the structure some thirty years later, in 1260. Tommaso Pisano began building the bell
chamber, which is located on the eighth storey of the tower, in 1350. He discovered that the tower was
still sinking, so he raised the wall of the bell chamber and the spiral staircase inside the tower on the south
side compared to the north. After over 200 years of construction, the tower was finally finished in 1372,
although it was still sagging. After thorough research and work by the university's archaeologists, it has
been determined that Bonanno Pisano is the true designer of this tower. After reading two lines of the
severely damaged text, the researchers came to this conclusion. Additionally, it has been speculated that
Bonanno's inscription may have been hastily removed because the tower had structural issues from the
beginning.
In the early years, Pisa was a well-liked stopover for European pilgrims traveling to Jerusalem. And
their military fleet of Pisa had the greatest success. Also, The fleet assisted in defending Salerno in 871,
and in 1015, the Pisans drove the Arab armies from the islands of Corsica and Sardinia. The city of Pisa in
Italy grew more and more powerful as their wealth and influence grew. In 1077, the island of Corsica
came under Pisan rule, and in 1113, the Balearic Islands near Spain went as well. The Pisans lived in
tremendous affluence throughout that time. The final component in the complement of the ceremonial
complex of monuments that enhance the Piazza dei Miracoli (Square of Miracles) is the construction of
the leaning Tower of Pisa, and particularly its completion. The Cathedral of Pisa (Il Duomo di Pisa), the
Baptistry, the Bell Tower of Pisa, and the Monumental Cemetery were among the four representative
monuments that were a part of the project.
The massive weight of this skyscraper, constructed of solid white marble, caused the ground beneath it
to shake. A remarkable engineering miracle, the tower mysteriously remained upright while experts
labored to fix the tilt at a point when it should have also fallen over. While it was being constructed, the
structure had already begun to tilt. Due to the fragile sand and clay that lie beneath its groundwork, the
building began to tilt as soon as the third story was completed. Despite the obvious lean manufacturing,
construction continued and was finished in the second half of the 14th century. The tower continued to
move even after construction was finished, and by the early 1990s, the tilt had increased to more than five
degrees from vertical. The ground was dug up, steel bracing was attached around the third story, and it
was secured to the ground with steel cables in order to correct the tilt. The primary causes are reportedly
disruptions brought on by earthquakes, wind, changes in ground water levels, transportation, and tourists.
Now, any disruption causes the tendency to rise, which is followed by an apparent steady condition.
In the city of Pisa, the spectrum of rock types spans the Paleozoic to the Tertiary. The construction of
the Leaning structure of Pisa on top of an unstable soil surface led the structure to gradually tilt in one
direction. Due to the combined impacts of groundwater fluctuation and soil creep, the foundation's
inclination has been continuously increasing. The Italian government established an international
commission to protect and stabilize the leaning tower of Pisa in 1993 out of worry for the steadily
increasing rate of inclination and the possibility of a catastrophic structural collapse. The strata of the city
have suffered substantial deformation as a result of tectonic activity. Additionally, a fault runs through the
bedrock under the town of Pisa. On the south side of the tower, there is more silt and clay. The sand layer
is significantly thinner than it is on the tower's north side. This is one of the factors that caused the tower
to lean to the south. Leaning instability is the fast acceleration of any structure or tower's leaning process
at the conclusion of construction. When the overturning moment produced by a slight increase in
inclination is equivalent to or greater than the corresponding resisting moment produced by the
foundations, leaning instability of a tall, narrow building develops at a critical height. Lack of rigidity
rather than a weak foundation is what causes leaning instability. It is clear that the tower of Pisa has really
reached its critical height as a result of the design and the extremely soft ground. However, once it
reaches a specific critical angle, the tower will collapse quietly. These attempts helped to straighten the
structure to some extent. In fact, by 2001, these efforts had roughly 45 centimeters (18 inches) of lean
corrected.
CASE EVALUATION
It goes without saying that there were preexisting elements that may induce the tilt from the very
beginning of the tower construction process. The engineers' lack of knowledge of the soil profile at the
tower's base is now the most significant component and the one that will ultimately contribute the most to
the tower's tilt. At 1173, construction started. Compared to today, those engineers and architects knew
much less about the land they would build on. The gigantic stone pillars that the ancient Romans utilized
rested on the solid bedrock of the planet. Lime mortar and limestone were used to build the foundation.
Although it was constructed on solid clay, it was only 3 meters deep, making it less stable than other
potential locations for a 14,500-ton tower. The soil began to compress as the weight applied pressure,
until it located the weakest area and began to sink in on one side. But something strange happened: the
structure didn't fall. It was instead laying on the limestone. Because the limestone was so malleable, it
could withstand the Tower's pressure. The building will move into a new postposition because of the
weather and soil. Today, builders and other foundation repair firms take this into account, resulting in
buildings that are better constructed and have a lower risk of moving. It was less common to account for
settling in 1178. This is why it is so important to start with the right foundation. If we wanted to construct
something similar today, we would make sure your foundation was strong. A deep foundation uses piles
that are driven far into the earth to anchor a building. Typically, driven piles or helical piles are used to
give these anchors.
Engineers discovered that the soil was siltier and more clayey on the south side of the tower than on
the north side, and the sand layer was thinner, based on the study of the soil samples. As a result, different
soils have varying capacities for bearing the tower's weight. The tower's tilt grew more pronounced as
construction progressed due to a leaning instability. At tall, thin constructions, a phenomenon known as
"leaning instability" takes place. The overturning moment caused by a slight increase in height will be
equal to or greater than the resisting moment caused by the foundation once the structure reaches a critical
height/width ratio. This increases the structure's risk of collapsing. As was previously indicated, the tower
was built on soft, compressible earth. The soil's compressibility gradually rose as the consolidation of the
soil beneath the tower took place during the construction's hiatus.
SOLUTION
Massive engineering efforts were made to stabilize the Leaning Tower of Pisa to prevent the structure
from collapsing. Since the soil underneath the foundation of the leaning tower is weak or not strong
enough to carry its own weight causing it to tilt and grew overtime with an angle of 5.5 degrees recorded
in the late 20th century. The solution to the problem in the leaning tower was to improve its stability,
reinforcing and compressing the soil under the foundation since it has an unstable ground. Engineers used
a rectification method called “Soil Extraction” in order to remove small amounts of earth by digging a
network of tunnels on the tower's north side. Steel cables assisted in bringing the tower back to its former
position because it sags to the south. For the foundations to correct themselves, approximately 2 tractortrailer loads of soil had to be removed. The project team also discovered that the tower tipped more
during the winter because when it rained, the north side's water table was higher than the souths. The
north side was thus raised much higher, and the drains were dug by engineers to allow water to drain into
wells.
Later engineers who worked on the skyscraper constructed additional stories with one side shorter
than the other to make up for this. This indicates that the building is leaning and curving. Due to Pisa's
frequent wars with the neighboring city states of Florence, Genoa, and others, construction was sluggish
and occasionally abandoned for decades. In 1372, the tower's construction was completed. Throughout
the tower's existence, tilt correction attempts were made. Some of them exacerbated the issue, and by the
1990s there were worries the tower might completely collapse. After all the engineering work for the
improvement in stabilizing the tower was finished, the structure was straightened by 38 cm and has
maintained its correction ever since. Until in the year 2001, it was re-opened to the public. There are
various engineering methods in rectifying leaned building and structures these include the Compaction
Grouting Method, Chemical Grouting Method, Underpinning Method, and the Micro-Tunneling Method.
These rectification methods are used by uplifting over tilted buildings and structures.
CONCLUSION / RECOMMENDATION
A variety of bracing and reinforcing techniques have been used to bolster the foundations, but as of the
late 20th century, the building was still at risk of collapsing due to the structure's continued subsidence at
a pace of 0.05 inches (1.2 mm) per year. The tower was shut down and all the bells were muted in 1990
while engineers worked on a significant straightening operation. The lean was reduced by 17 inches (44
cm) to 13.5 feet (4.1 metres) by siphoning earth from beneath the foundations. The work was finished in
May 2001, and the building was reopened to the public. Without any additional excavation, the tower kept
straightening until May 2008, when sensors indicated that the motion had finally halted after a total
improvement of 19 inches (48 cm). The tower was designed to be steady for at least 200 years. The
working group concluded that altering the vulnerable foundations may make matters worse, therefore
popular ideas like cement injections had to be disregarded. They decided on an expensive dirt extraction
operation instead. The engineers utilized steel cables to hoist the tower upright after removing dirt from
the foundation's upper side. The crew also discovered that the structure's water table was uneven, which
contributed to its tendency to tilt even more during winter rains. To help level the foundations and remove
some of the water, drains were built. We also suggested conducting more research and development to
look at ground-breaking and novel stabilizing techniques. With the aid of developments from engineering
and geotechnical investigations, new methods for reinforcing the tower's foundation, enhancing ground
support, and improving the drainage system may be developed. It will be crucial to work together with
experts from a variety of fields, such as engineering, geology, and preservation, to develop solutions that
are effective and can maintain the tower's historic and architectural integrity for a very long time.
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