Stabilizing the Leaning Tower
The Original Construction
In the 11th century, church leaders in Pisa decided to place three buildings in the Campo dei Miracoli, or Field of Miracles. These buildings were the Duomo, the magnificent cathedral, the circular Baptistery, and the bell tower itself.
The foundation stones for the Tower of Pisa were laid in 1173. Constructed mainly of marble and lime, the tower was built in a circular ditch, about five feet (1.5 meters) deep, on ground consisting of clay, fine sand, and shells.
In 1178, with only three stories of the tower built, work stopped because of politics and debt, but the tilt toward the south was already evident. If work had continued before allowing underlying soils to settle, the tower would certainly have collapsed.
Work recommenced in 1272 and, to compensate for the lean to the south, heavier building materials were added to its north side.
By 1278, when workers reached the seventh cornice, work stopped again with the tower tilting to the south by about one degree, or roughly 2.5 feet (0.8 meters).
Work began on the bell chamber at the top of the tower in 1360. To compensate for the southward lean, workers added six steps from the seventh cornice up to the bell chamber's floor on the south side, and only four steps on the north side. In 1370 the tower, eight stories and 200 feet (60 meters) high, was officially completed.
The cause of the lean is the composite of clay, fine sand, and shells the tower is built on, which is more compressible on the south side. Over the years as the tilt increased, the tower stopped sinking and began to rotate, the north side moving up toward the surface.
Finally, in 1838, the architect Alessandro Della Gherardesca dug a walkway called the catino around the base of the tower to expose the buried foundation steps and column plinths. Because the catino was below the water table on the south side, the excavation triggered an inrush of water, with a subsequent half-degree increase in the lean.
Based on theodolite readings since 1911, the tower would have fallen by 2050 if not sooner.
How has it remained standing? Before remedial work began, the tower was leaning so far that a computer model could not replicate the actual position (5.5 degrees off perpendicular) because the model collapsed at 5.44 degrees.
Burland, who has worked on similar projects such as the Metropolitan Cathedral in Mexico City and the Big Ben clock tower in London, said that one of the reasons it had not fallen was its sheer mass; it weighs 16,000 tons (14,500 tonnes).
The leaning tower was closed in 1990 for fear of it collapsing. It is subject to two different risks: structural failure of the fragile masonry and collapse due to the breaking up of the subsoil around the foundations.
In 1993 a lead counterweight of about 660 tons (600 tonnes) was placed on the north side of the tower's base in order to stop the southward rotation.
In September 1995 attempts were made to stabilize the tower by freezing the ground and inserting steel cables. Unfortunately this increased the lean, which was stopped by increasing the counterweight to about 960 tons (870 tonnes).
For years, rescue efforts were based on an assumption that the tower needed support on its south side. But Burland saw that the solution was below ground on the north side. He concluded that soil extraction was key to bringing the tilt back to the goal of five degrees.
Soil is extracted primarily from two layers of earth: the top layer of sandy soil and the second of marine clay. As the sandy soil is removed, the ground compresses and the clay firms, giving a stronger foundation.
The drills, each 8 inches (21 centimeters) in diameter, extract soil from inside a drill casing without disturbing anything outside it. The cavity shuts very gently when the drill is retracted. The ground then settles, forming a cradle that cushions the tower as it shifts slightly to the north.
In 1998, long cables were secured to the tower as a precaution during excavations. In February 1999, engineers tentatively began extracting soil. The tilt began to recede, and they knew they had the answer.
By using this method engineers have reduced the lean back toward the center by 20 inches (50 centimeters), where it was in 1838. The top of the tower now leans just over 13 feet (4 meters) off center.
While Burland worked on the soil extraction solution, Giorgio Macchi, a structural engineer, dealt with the structural weaknesses of the building.
The marble facade is only 10 inches (25 centimeters) thick, and behind it there is a conglomerate of stones and lime, which has severely weakened the structure.
Holes in the marble walls created by the original builder's wooden scaffold have further contributed to the weakness. The stress created by the lean is borne mainly by this thin facade.
The largest stresses are on the south side at the second story, where a doorway meets the staircase. If the tower were to collapse, this is where it would begin. In 1992 a dozen plastic-coated steel tendons were wrapped tight around the critical second story to hold the cracks shut.
The tower has moved northwards by about 20 inches (50 centimeters), not visible to the naked eye. The lead counterweight and the safeguard cables have been removed. To improve stability, the ancient concrete ring in the floor of the catino has been connected to the foundations.
As John Burland said, "Only time will tell whether the previous progressive seasonal movements have been eliminated. Even if they have not, it is likely to be hundreds of years before the tower returns to its 1990 inclination, and if necessary, the process of soil extraction could be repeated."
If you are wondering: yes, it could have been brought back to the vertical, but it would not then be the Leaning Tower of Pisa. Has anyone tried to put the arms back on Venus De Milo?
Don Barker is a freelance writer and photographer in London, UK, who has lived and worked in Europe, Australia, Thailand, Sri Lanka, Hong Kong, and Singapore.