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Leaning Tower of Pisa

Pisa's leaning tower is perhaps the best known bell tower in the world.

Construction took nearly 200 years, starting in 1173. The aim was to create a structure that would be the envy of the rest of the world.

The difficult clay and silty sand ground, overlain with river deposits, immediately began to affect the structure. By the time it reached the third storey in 1185 the tilt was clearly noticeable.

The tower is constructed from large blocks of local stone cemented together and clad in carved marble up to 600mm thick. It rests on pad foundations only 3m below ground.

Historical analysis suggests that even with only three storeys in place the load on the soil was close to bearing capacity. But it was war rather than engineering judgement that interrupted construction.

By the time work resumed in 1275 the soil had consolidated, increasing bearing capacity and allowing construction to carry on up to the seventh storey by 1319.

Ground movements continued to plague the tower during this phase and in 1298 engineers discovered that it was 1.43m out of plumb - this time leaning to the south.

The final stage was to add the bell tower at the top. This was completed by 1350. The bell tower is built with a pronounced correction to the lean, giving the whole 58m tall tower a peculiar 'banana' shape.

Since its completion, the lean has continued to increase. By the time Gallileo's students were dropping cannon balls from the tower in the late 16th century, it was already off-centre by about 4m. It is now 5.2m out of plumb, a tilt of about 5degrees.

Despite its handicap, the tower is beautifully built, displaying fine design and craftsmanship. Each storey precisely matches that below and is finely decorated and detailed.

Perhaps this explains the passion and politics surrounding attempts to make the structure safe.

Following decades of debate the latest phase in the tower's construction is now under way. Under the guidance of Professor John Burland, Imperial College London's head of soil mechanics, more than 800t of lead has been added to the north side of the foundation to arrest the tilt.

Soil will be extracted from beneath the foundations on the north side to allow the tower to reverse the tilt by 10%. Before this is done stabilising cables will be attached to the tower to steady it. If it succeeds tourists will see the tower in its glory for the next nine centuries and be able to walk the 293 steps to the top.

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