Fixing the Leaning Tower of Pisa
Pisa’s Torre Pendente, or “the Leaning Tower of Pisa”, in the Campo dei Miracoli, the grounds of Pisa’s Duomo, was in trouble right from the beginning.
Begun in 1173, the tower was built on weak sandy subsoil, which was once covered by the Ligurian Sea. It had already started to lean when work was abandoned in 1178 when they had only reached the third storey of the planned eight. In 1272, engineers attempted to correct the lean that over the 100 years had become already 3 feet from vertical by changing the thickness of the marble as they built. By 1301 the tower the full eight stories of the tower was completed. But by 1350 the tower was leaning 1.4 meters, 4 feet, 6 inches from vertical. In 1370 the upper belfry was added off-centre in another attempt to straighten the tower. The result, on the words of one engineer, was a structure that “curved like a banana”
On January 7, 1990 the Leaning Tower of Pisa was closed to visitors. The 196-foot, 60-meter, tower had reached 5.4 meters, or 17 feet, 6 inches from vertical. A lean of 5.5 degrees! The worst news was that the rate of lean was accelerating. At the rate of lean in 1990 experts foresaw two likely scenarios; One. The tower would implode at any moment, collapsing in on itself. Or, within the next 25 years gravity would win its battle and the tower would simply fall over.
An international panel was convened to find a solution. Endless theories and solutions were proposed. One was to cut off the base of the tower at the 3 storey mark where the first straightening solution took place in 1272, another was to completely pull down the tower and rebuild it so it would be straight.
While a permanent solution was being sought engineers fastened 18 narrow steel belts around sections of the tower to prevent it from falling in on itself and a concrete ring around the base to slow the lean. In 1993, 661 tons of lead weights were fixed to the tower’s north side to counterbalance the tilt to the south. The weights did have the desired effect as in six months the tower did begin to correct itself, but by just half an inch (1.25 cm).
The solution chosen to fix the dangerous lean of the tower was to gradually remove a 26 inch, 66 cm high wedge of sub-soil from the north side of the tower, which in theory would allow the structure to settle back towards the vertical. Work was started in 1998 where thick steel cables were attached to the tower 20 meters, 65 feet, up the tower and anchored in two huge concrete blocks set 90 meters, 100 yards, away from the structure and 10 meters apart in a “v” pattern. With the stabilizing cables and blocks in place the real work of correcting the tower could begin. A drill shaft with a corkscrew drill bit was placed on the north side of the tower and the soil was removed a little at a time. After 5 months of drilling the tower had moved back to 5 inches, 12.5 cm, off of vertical. A position the tower had not seen for over 110 years. A total of 53 cubic yards, 30 cubic meters, of soil was removed and the towers achieved a 10% correction or 45 cm, 17 inches of movement back to vertical.
The final stages of the fix engineers reinforced the foundations and removed the reinforcing cables, concrete anchors, lead weights and other external eyesores. The Leaning Tower of Pisa still leans but the iconic tower has become stable enough that engineers have stated that the tower no long moves to the south in a gradual rate of decent and should remain at it current degree of lean for the next 200 years and in 2001 the Tower re-opened for visitors to climb.
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