The Leaning Tower of Pisa (thing)

The fact that the Leaning Tower of Pisa, Italy (it. La Torre di Pisa) is still standing eight hundred years after commencement of its construction, is remarkable. The 58 m (191 ft.) structure leans off to the south at an inclination of approximately 5.5 degrees. This means that the seventh (top) cornice protrudes an astonishing 4.5 meters (14.8 ft.) over the first cornice. Indeed, since its construction the tower has been at an ever increasing risk of either buckling, or tipping over.

The real history of the tower starts a few thousand years before its construction; actually even before urbanization of the area that would later become the city of Pisa. Geographical surveys have shown that between the second half of the third millennium, and the VIII century B.C.. (when the earliest settlers moved in), a river ran through the area where the Cathedral Square and adjoining tower would be built. Over time, the river bed moved eastwards, leaving a loose soil of soft river sediments. This unsturdy foundation is the primary reason of the inclination of the Tower.

In 1172, the widow Berta of Bernardo leaves 60 coins in her will to the Santa Maria institution to purchase stones for a tower; a campanile (bell tower) associated with the cathedral. One year later, construction commences. It is not clear who the original architect of the tower is; classical texts identify Bonanno Pisano as the architect, although recent studies show that either Diotisalvi or Biduino are more likely candidates. However, what is known is the initial construction date; August 9, 1173, as evidenced by the inscription in the foundation: A.D. MCLXXIV. CAMPANILE HOC FUIT FUNDATUM MENSE AUGUSTI (note that the historic Pisan calendar system differs one year from our current calendar system). The foundation of the tower was built in a circular ditch, about five feet deep, on ground consisting of clay, fine sand and shells. The tower itself was constructed from marble, lime and other stones.

Construction of the tower was halted several times. The first halt was already in 1178, due to a war with Florence. During that time, the tower was 10.6 m. (35 ft.) tall, and consisted of three stories. At this height, the incline of the tower was already apparent to the constructors. A second war with Florence in 1185 added another interruption to the construction schedule, which would last until 1272. In 1284, construction was halted again; this time due to a war with Genoa. These unscheduled stops are the main reason the tower still exists today. Modern analysis has revealed that the interruptions allowed the underlying soils to consolidate. Had this not happened the tower would certainly have toppled.

Over time, the tower began to lean over at an ever increasing angle. Initially, the incline was only 0.2 degrees, but in 1278 the incline was already 1 degree, with an increase to 1.6 degrees during the following 90 years.

Around 1370, the tower is officially completed. Seven bells are added to the tower, and in 1655 the largest bell is installed, weighing around three and a half ton. Without doubt, this further adds to the inclination of the tower. Nowadays, the bells are silenced, since they could easily trigger a collapse of the tower. In 1817, the incline is measured by two British architects. The tower now leans over at around 5 degrees.

In 1838, the architect Alessandro Della Gherardesca nearly kills the tower by digging a trench (the catino) around the base of the tower. The architect's intention was to expose the foundation and bottom of the columns to the public. In a matter of days, the tower leans over an additional 0.5 degrees (approximately 1 m, or 3 ft.). Since the trench was below the water table, it triggered an inrush of water on the south side, resulting in additional instability.

In 1839, a scientific conference commemorates Galileo Galilei, with the installation of a memorial stone that reads: "From the top of this Tower, Galileo used to do his experiments on the fall of heavy bodies.". Although Galileo lived and worked in Pisa for several years, this probably never happened.

Serious efforts begin in the early 20th century to measure the inclination and its rate of rotation, and plans are proposed to halt the tower's potential collapse. However, those initial attempts have adverse effects. In 1934, engineers drill holes in the foundation, and inject grout to strengthen the masonry. The tower immediately leans over an additional 31 arc seconds. Additional drilling in the soil and masonry in 1966 and 1985 further worsen the situation. In 1990, the government officially closes the Leaning Tower of Pisa.

In the nineties, several more scientifically sound efforts are undertaken to stop a further deterioration. But these efforts have a mixed success due to technical complexities, and also political obstacles. First, the base of the tower was wrapped with steel belts, to reduce the danger of buckling. Next, a concrete foundation was laid down on the north side with several counterweights to tilt the tower's axis back up. This work has a positive effect on the stability of the tower. In 1995, engineers try to replace the ugly counterweights with anchored cables. In order to anchor the cables, engineers treat the loose soil with liquid nitrogen. The results of this procedure were disastrous, and the tower was again heading for earth...

In 1996, Engineers evaluated a soil extraction method below the tower's foundation. Holes were drilled on the north side of the tower, and soil was removed. This method proved to be the most successful so far, and further soil extraction was conducted in 1999. The engineering team managed to reduce the incline angle back to where it was in 1970, and stabilize the inclination. On June 17, 2001 the tower will be opened again for the public.