JAES Learning

To straighten the leaning tower of Pisa, the engineers came up with a brilliant solution in 1999 - under excavation. If the under excavation is not done correctly, a disaster is guaranteed. Here are the two cases of underexcavation. What do you think? Which one will save the tower? To answer this tricky question we should have a clear understanding about the tower’s foundation. Did you notice a strange structure called Catino around the foundation? Also some other strange things around the foundation? The support from the Catino in fact saved the tower from toppling. The foundation constructed for the tower was a mere 3-meter deep, which was too shallow for the loose and marshy soil of Pisa. When the construction reached 3 storeys, they had to stop the work due to war. During the period of inactivity, the building developed a lean - a lean due the phenomenon called ‘leaning instability’. Most importantly, the tower settled down due to its own weight, squeezed and removed a lot of water out from beneath the foundation. This increased the strength of the soil and prevented any further lean. They resumed construction after almost 100 years. Unfortunately, the leaning instability kicked in again. The lean kept increasing. The engineers tried to solve this problem by using lighter construction material for the top floors. Moreover they constructed the tower vertically, not normal to the previous floor. They thought this technique would produce a visual illusion that the tower was not leaning. As a result of these efforts, the tower attained a banana-like shape. Despite this attempt, the lean kept increasing. After 6 years of construction, when seven levels were completed, they had to stop the construction again due to war. The third stage of construction began in 1360. The bell chamber was constructed at this time. The artisans continued their trick of creating an illusion of a straight tower. For example, at the bell tower region, the north side has 5 steps and the south side has 6 steps. Also the architects brought the bell chamber a few degrees closer to the center axis. Again, despite these strong attempts, the lean kept increasing. The construction of the Pisa Tower was finally completed in 1372. It is estimated that the lean at this time was 1.6 degrees. Did you know that the Leaning Tower of Pisa looked like this in the 1800s? The foundation was completely below ground. In 1838, the architect Alessandro Della Gherardesca took the initiative to dig out the ground and create a walkway called the Catino. He believed this would allow people to walk around and appreciate the intricacies of the architecture and observe the foundation's details. However, this excavation caused the tower to tilt by more than 0.25 degrees. Interestingly, according to many engineers, the Catino actually provided structural support against leaning. One significant improvement Della Gherardesca made was adding a 0.7 meter thick concrete ring beneath the Catino, possibly to reduce water inflow to the foundation. We will learn it later, once the ‘leaning instability’ starts, it will never stop. By 1934, the lean was around 4.5 degrees. Benito Mussolini, the dictator of Italy at the time, didn’t like this lean. He considered this imperfection an insult to Italian engineering. Mussolini had a brute solution to this problem. He drilled many holes in the north side of the foundation and poured concrete in the hole. They injected concrete with a total weight of 90 tonnes into the foundation. This is what Mussolini thought would happen. What happened was exactly the opposite. The soil in the south side got more compressed, and the tower’s tilt increased. The lean reached 5 degrees. In the coming years, the lean of the tower kept increasing, and in 1990, the lean reached a dangerous level of 5.5 degrees. The tower was closed to the public for the first time. The authorities removed the tower’s bells. They believed that doing so could reduce the stress on the tower. It was a ticking time bomb, waiting to fall at any moment. To save the tower from an immediate collapse, the engineers kept lead counterweights on the north side. They had to first construct a concrete ring at the base to keep these weights in place. They added a total of 600 tons of lead counterweights on the north side. This effort reduced the tilt by 1 by 100th of a degree. For the first time, the tower leaned towards the correct direction. Remember, the lead counterweight method was only a temporary method. The engineers still had to find a permanent solution. Now, it’s time to learn about the phenomenon of ‘leaning instability’ - the phenomenon of which tilted the tower of Pisa. You might have observed this. On a soft ground, you can never build a tower more than a specific height. It doesn’t matter how straight you build it, the tower always fails after reaching a specific height. That too is a slow failure. The tower is tilting very slowly, and it failed. This is known as the ‘leaning instability’. Let’s introduce a small perturbation to this perfectly straight tower while it is settling. Is the reaction torque larger on the right or left side? This will decide the stability of the tower. The right side of the ground has compressed more than the left side, producing more reaction force. Here is the trick, the left side’s reaction force has more distance from the center of gravity than the right side. This makes predicting the bigger reaction torque values a tricky affair. At higher tower heights, the left side reaction torque becomes greater than that of the right side, causing the tower to fail. This is exactly what happened to the Leaning Tower of Pisa. At a height of 56 meters, the tower has already crossed the height of leaning instability and slowly and gradually the tower began to lean. With increase in lean the reaction torque on the left side continued to overpower the right side’s reaction torque. The engineers came up with an innovative solution in 1995 - the liquid nitrogen method. This straightening method was such an elegant solution. The idea was to go 40 meters deep and connect the foundation of the tower with the hard strata using anchor bolts. Tightening anchor bolts seemed like a perfect solution to straighten the tower. Pisa’s soil is so watery and loose, not ideal for such construction activities. Strengthening the soil by injecting liquid nitrogen is a proven method in construction engineering. However, the liquid nitrogen method backfired in the soil of Pisa.