 
The Leaning Tower of Pisa is located in the Italian city of Pisa, and is one of the most recognizable buildings in the world. The tower is famous for its lean, which is caused by the soft soil on which it was built. Utilizing many columns and arches, it is both beautiful and an advanced demonstration of the architect's understanding of weight and load characteristics. What he architect forgot to account for, however, was the base of the tower being built on a dense section of clay.Begun in 1173, the tower was designed to stand 56 metres tall, and was constructed of white marble. Three of its eight stories had been completed when the uneven settling of the building's foundations in the soft ground became noticeable. At that time, war broke out between Italian city-states, and construction was halted for almost a century. This pause allowed the tower’s foundation to settle and likely prevented its early collapse. When construction resumed, builders tried to compensate for the lean by making the new stories slightly taller on the short side, but the extra masonry caused the structure to sink even further. 
The tower was finally finished in the 14th century. Twin spiral staircases lined the tower’s interior, with 294 steps leading from the ground to the bell chamber at the top. One staircase has two additional steps to compensate for the tower’s lean.Over the next four centuries the tower’s seven bells were installed; the largest weighed more than 3,600 kg. However, by the early 20th century, the heavier bells were silenced, as it was believed that their movement could potentially worsen the tower’s lean. The foundations have been strengthened by the injection of cement grout and various types of bracing and reinforcement, but in the late 20th century the structure was leaning 5.5 degrees (about 4.5 metres) from the perpendicular. It was still subsiding at the rate of 1.2 mm per year, and was in danger of collapse. In 1990 the tower was closed and all the bells silenced as engineers undertook a major straightening project. 
First, three hundred tons of lead ingots were added to the north side as a temporary fix. This worked at stabilizing things while another solution was developed. Holes were drilled at an angle beneath the north side of the tower and steel cables were attached. Using hoses, dirt was removed from below that side of the tower over a period of eleven years. The effect of the lead and the dirt removal was to reduce the angle of the tower to 4.5°.At this point the lead could be removed, and tourists allowed to re-enter. After the lead was removed, the angle continued to decrease. It’s now at about 4 degrees, and stable. 
Picture at left shows lead weights on north side |