Because it was built on clay and sand, materials that were not very firm to support a building of that size. Designed to house the bell of the cathedral of Pisa, in northern Italy, the tower was begun in 1173: its first three floors had barely finished being erected when a slight inclination was noticed, due to the subsidence of the land and the irregular settlement of the foundations. .
The engineer in charge of the project, Bonnano Pisano, tried to compensate for the tilt by building the remaining five stories slightly higher on the side where the structure leaned downwards – but the excess weight only made the tower sink even more!
Construction only ended in the second half of the fourteenth century and, over the centuries, several attempts were made to straighten the eight-story structure, but to no avail. In the 20th century, the tower began to lean about 1.2 millimeters per year. When this slope in relation to the axis reached 4.5 meters, in 1990, it was closed to the public, under the risk of collapsing. Since then, several proposals have been made to save the tower, until one of them, formulated by a commission of 14 specialists, was finally chosen. Work began in 1997. “The winning proposal was simple and, at the same time, extremely effective: gradually remove earth from the sloped side and reinforce the foundation with lead plates to avoid any danger of collapse while the work was carried out” , says civil engineer Henrique Lindenberg, from USP. In addition, cement was injected into the walls surrounding the tower.
The work cost 25 million dollars and was only completed in June 2001, reducing the tower’s inclination by 40 centimeters, which was reopened to the public on December 15 of the same year.
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– How was the Eiffel Tower built?
– How was Venice built?
1 – In 1292, still in the middle of its construction, the tower had an inclination of 1.5 degrees
2 – By 1817, the angle of inclination had grown to 4 degrees
3 – During the 20th century, the tower leaned 1.2 millimeters per year. In 1990, it was tilted 4.5 meters (about 5.5 degrees) from its vertical axis and ended up being closed to the public. Today, recovered and reopened, the slope has decreased by 40.6 centimeters
RESCUE OPERATION
Before the tower collapsed, special recovery work was carried out between 1997 and 2001: 1. The earth was gradually removed from the section of ground where the ground yielded further 2. The ground foundations were reinforced with lead plates to prevent the building to sink during the works
UNSTABLE BASE
The ground, formed by layers of sand and clay, could not support the 14,000-tonne tower. Result: it began to lean at the beginning of its construction, in the 12th century