This blog post examines how the Leaning Tower of Pisa overcame its structural flaws and how modern architectural technology has advanced as a result.
The Leaning Tower of Pisa is a structure located in the Tuscany region of western Italy. It is an annex to the Pisa Cathedral and a world-famous tourist attraction, made even more renowned by the anecdote that Galileo Galilei conducted his free-fall experiments there. Unlike typical buildings, this tower leans, offering visitors a unique sight. Despite its severe tilt, the Leaning Tower of Pisa remains standing and maintains its status. Its current inclination has been significantly reduced from its most severe angle of 5.5 degrees to approximately 3.9 degrees through ongoing restoration work.
The Leaning Tower of Pisa began leaning after the first phase of its three construction periods (Phase 1: 1173–1178, Phase 2: 1272–1278, Phase 3: 1360–1372). Engineers attempted various solutions to correct the southward tilt, such as suspending heavy materials like bells on the north side of the tower or injecting chemicals to reinforce the southern foundation. However, these efforts failed to halt the tilt. Consequently, the tower continued to lean more rapidly over time. In 1990, when the distance from the vertical line of the tower to its center axis exceeded the critical threshold of 4.5 meters, the Italian government restricted public access and launched a major restoration project. Construction companies and scholars from various countries proposed diverse solutions, but none yielded significant results. Ultimately, a method involving excavating the northern foundation was chosen, halting the tower’s tilt. By 2010, the inclination had decreased from 5.5 degrees to 3.9 degrees.
So, what caused the Leaning Tower of Pisa to start tilting, and how was its tilt stopped?
The cause of the Leaning Tower of Pisa’s tilt lay not so much in structural flaws within the building itself, but rather in the ground supporting it. Typically, the ground consists of a solid bedrock layer at the very bottom, overlaid by a mixture of various types of soil such as aquifers, sand, and clay. Depending on the properties of this soil, the ground is classified as soft ground, hard ground, clay ground, sandy ground, etc., and the condition of this ground must be considered when constructing a building. Soil contains empty spaces called ‘voids’ between its particles. When a building is constructed, these voids are compressed by the load. This process is called ‘settlement’. If the ground supporting the building settles uniformly, there is generally no major issue. However, if the settlement varies in different locations, ‘differential settlement’ occurs. This is a dangerous phenomenon that can cause the building to tilt or even collapse.
The Leaning Tower of Pisa was built on soft ground composed of mineral deposits mixed with clay, with groundwater flowing beneath. Furthermore, the foundation was laid imperfectly, causing the tower to lean southward.
Engineers attempted to counteract the tilt using heavy materials, applying the seesaw principle. However, the southern foundation couldn’t withstand the added weight, causing the tilt to worsen. In the 1930s, under Benito Mussolini’s orders, the ‘grouting method’ was attempted, injecting concrete into the ground. This too disturbed the ground, worsening the problem. Furthermore, in the 1960s, increased groundwater usage lowered the water table, accelerating both ground subsidence and the progression of the tower’s tilt.
The ‘cutting the northern foundation’ method, ultimately implemented, successfully resolved the tower’s issues. A large-scale reinforcement project involving excavating approximately 70 tons of soil from the northern foundation and pouring cement prevented differential settlement, stabilizing the tower’s tilt. As a result, the tower’s tilt has decreased by 48 cm compared to 1990 and is no longer leaning further.
The principles of soil mechanics between the structure and the ground played a crucial role in solving the Leaning Tower of Pisa’s problem. Rather than strengthening the soft ground, the settlement was controlled by excavating soil on the opposite side to prevent differential settlement. Unlike the situation during the construction of the Leaning Tower of Pisa, where insufficient ground investigation was conducted due to technical difficulties, modern soil engineering technology now enables the intentional design and construction of leaning structures.
An example is the Capital Gate building in Abu Dhabi, listed in the Guinness Book of World Records as the world’s most leaning man-made tower. This building stands 35 stories tall with an inclination of 18 degrees, leaning 3.9 degrees more than the Leaning Tower of Pisa. Its unique design involves rising vertically up to the 12th floor, with each floor from the 13th floor outward protruding sideways by 30 to 140 cm. Additionally, 490 piles, each 2 meters thick, were driven 30 meters deep at the building’s center and on the opposite side of the tilt. This design ensures the structure can withstand gravity, strong winds, and earthquakes. These piles also prevent rotational movement that could cause the building to topple over. Thanks to this design, the Capital Gate Building remains stable despite its tilted form, gaining fame as an intentionally designed structure unlike the Leaning Tower of Pisa.
Advances in modern geotechnical engineering are making diverse engineering challenges a reality, including not only tilted structures but also super-tall buildings, undersea tunnels, and high-speed railways. Thanks to technologies that ensure structural stability while enabling creative designs, we can now encounter buildings in forms unimaginable in the past.
These technological advancements go beyond simply creating beautiful and unique landmarks; they lead to the design of safe structures capable of withstanding natural disasters and ground changes. For instance, in earthquake-prone regions like Japan, thorough analysis of the interaction between the ground and the structure is essential for seismic design that can withstand earthquakes. These construction methods not only enhance the safety of buildings but also play a crucial role in solving geological problems that were difficult to overcome in the past.
The Leaning Tower of Pisa has captivated people for centuries due to its tilted form. However, modern geotechnical engineering no longer views tilted structures as problems but rather embraces them as artistic and technical challenges. Such engineering advancements, solving past problems while exploring new possibilities, heighten expectations for the future of architecture.
Just as the Leaning Tower of Pisa has stood in its place for centuries, structures built with modern technology will, over time, become symbolic entities embodying history and culture themselves. And someday, a more innovative and challenging structure than the Leaning Tower of Pisa may emerge, becoming the symbol of a new generation.
