The Leaning Tower of Pisa tilted due to soft ground, but modern geotechnical engineering techniques were used to reduce the tilt and stabilize it. Lessons learned from this process have been incorporated into modern architecture, leading to innovative structures such as intentionally tilted buildings and skyscrapers.
The Leaning Tower of Pisa is a structure located in the western Italian province of Tuscany, an annex of the Pisa Cathedral and a world-famous tourist attraction made famous by the anecdote that Galileo Galilei conducted free-fall experiments from it. The leaning tower is not a typical structure, and its tilted shape makes it a unique sight for tourists. Despite its severe tilt, the Leaning Tower of Pisa has managed to maintain its position without falling down. Today, the leaning angle has been reduced from 5.5 degrees at its most severe to around 3.9 degrees thanks to ongoing repairs.
The Leaning Tower of Pisa began to lean after the first of three construction periods (first: 1173-1178; second: 1272-1278; third: 1360-1372). Engineers tried various methods to address the southward tilt, including hanging heavy materials such as bells on the north side of the tower and injecting chemicals to strengthen the ground on the south side, but they failed to stop the tilt. As a result, the leaning continued to accelerate over time. In 1990, when the distance from the plumb line to the center axis of the leaning tower exceeded the limit of 4.5 meters, the Italian government closed the tower to the public and embarked on a major renovation. Construction companies and scholars from various countries proposed various solutions, but none of them were successful. Eventually, however, a method of cutting away the ground on the north side was chosen, which stopped the leaning, and by 2010, the tilt had been reduced from 5.5 to 3.9 degrees.
So what caused the Leaning Tower of Pisa to start leaning, and how was it stopped?
The Leaning Tower of Pisa’s tilt was not caused by a structural defect in the building itself, but rather in the ground supporting it. In general, the ground has a solid layer of rock at the bottom and a mixture of different types of soil, such as groundwater, sand, and clay. Depending on the nature of the soil, the ground is categorized as soft, hard, clay, or sandy, and these conditions must be taken into account when building a structure. Soil has voids between its particles, called pores, which compress under load when a structure is built. This process is called ‘settlement’ and is not a problem if the ground supporting the building is settling evenly. However, when the ground settles differently at different locations, ‘unequal settlement’ occurs, which is a dangerous phenomenon that can cause buildings to tilt or even collapse.
The Leaning Tower of Pisa was built on soft ground, a mixture of mineral deposits and clay, with groundwater flowing underneath. In addition, the foundation was incompletely laid, causing the leaning tower to lean to the south.
Engineers tried to solve the problem by using heavy materials to reverse the tilt, using the principle of a seesaw, but the ground on the south side of the tower was unable to support the weight of the materials and the tilt worsened. In the 1930s, Benito Mussolini tried the “grouting method” of injecting concrete into the ground, but this also disturbed the ground and exacerbated the problem. In the 1960s, increased groundwater use lowered the groundwater level, which accelerated the subsidence and tilting of the leaning tower.
The “north shoring” method, which was finally implemented, succeeded in solving the problem of the leaning tower. The large-scale reinforcement work, which involved digging out about 70 tons of soil from the northern foundation and pouring cement, stopped the unequal subsidence of the ground and stabilized the tilt of the pagoda. As a result, the leaning of the pagoda was reduced by 48 centimeters compared to 1990, and it is no longer tilting.
The principles of geomechanics between the building and the ground played a key role in solving the Leaning Tower of Pisa’s problem. Rather than reinforcing the soft ground, the amount of settlement was controlled by digging out the soil on the opposite side to prevent differential settlement. Modern geotechnical engineering techniques have made it possible to design and construct intentionally tilted structures, unlike in the case of the Leaning Tower of Pisa, where technical difficulties prevented sufficient geotechnical investigation.
An example of this is the Capital Gate Building in Abu Dhabi, which is listed in the Guinness Book of World Records as the world’s most tilted man-made tower. The building is 35 stories tall and has a slope of 18 degrees, which is 3.9 degrees more inclined than the Leaning Tower of Pisa. The building has a unique design that goes vertically up to the 12th floor, and then from the 13th floor onwards, each floor is sideways by 30 to 140 centimeters. The building was also designed to resist gravity, high winds, and earthquakes by placing 490 2-meter-thick piles 30 meters deep in the center of the building and on the opposite side of the slope. These piles act to resist any rotational movement that would cause the building to tip over to one side. This design allows the Capital Gate Building to remain stable despite its tilted shape and, unlike the Leaning Tower of Pisa, has earned it a reputation as an intentionally designed structure.
Modern advances in geotechnical engineering are making these tilted structures, as well as skyscrapers, undersea tunnels, high-speed rail, and other engineering challenges, a reality. Thanks to the ability to realize creative designs while ensuring the stability of the structure, we are able to see forms of architecture that were once unimaginable.
These technological advances aren’t just about creating beautiful and unique landmarks, they’re also leading to the design of safe buildings that can withstand natural disasters and ground changes. For example, in earthquake-prone regions such as Japan, it is essential to thoroughly analyze the interaction between the ground and the building to design earthquake-resistant structures. These methods are not only making buildings safer, but they are also playing an important role in solving geological problems that were previously difficult to overcome.
The Leaning Tower of Pisa has long been recognized for its tilted form, but modern geotechnical engineering techniques no longer perceive it as a problem, but rather as an artistic and technical challenge. These engineering advances, which solve problems of the past while exploring new possibilities, are raising expectations for the future of architecture.
Just as the Leaning Tower of Pisa has stood for centuries, buildings built with modern technology will, over time, become iconic symbols of history and culture in their own right. And who knows, maybe one day, something even more innovative and challenging than the Leaning Tower of Pisa will be built and become a symbol of a new generation.
