Completed in 1350, the leaning tower of Pisa was built in stages over 177 years. It began to lean during its construction as it subsided into the soft ground beneath, and it was only relatively recently that eff orts were made to correct it.
The tower sits on soft clayey and sandy silt over a thicker layer of highly compressible Pancone Clay at a depth of 10m to 25m. This in turn rests on a layer of denser sand.
Compression of the ground and fluctuations in the water table are thought to be the main causes of settlement.
The structure’s distinctive tilt began at the construction phase but gradually worsened, and in 1990 it was closed to the public because of safety fears.
Professor John Burland of Imperial College London’s soil mechanics department was a member of the 14-strong international commission set up that year to save the tower.
Burland proposed a soil extraction technique to stabilise it. First, the north side of the tower’s foundations was loaded with 600t of lead weights to temporarily pull the tower back from the brink and gain time for permanent works.
These weights were to be replaced by a ring beam and ground anchors. But in September 1995, the contractor - a consortium of Bonifica, Ismus, Italsonda, Rodio and Trevi - dicovered the foundations were not as reported.
The tower subsequently lurched further south, causing more concern until ground freezing and more lead saved the day.
Shortly after this, the commission’s jurisdiction ran out and the team was disbanded, halting work.
A new committee including eight of the original members was set up by the Italian government in early 1997 but it had to wait six months before holding its first meeting.
Preliminary works were agreed after a trial of the soil extraction method on a specially constructed test tower. This technique relied on undermining the north side of the tower.
With stay cables attached to the tower as a precaution, the project finally started. But initial results were not as planned. The tower appeared to be going the wrong way.
Fortunately, it turned out that this was a glitch as a result of seasonal temperature variations, and the northward trend continued.
The tower reopened to the public in December 2001, and has been declared stable for at least the next 300 years.
In May 2008, after the removal of another 70t of ground, engineers announced that the tower had stopped moving for the first time in its history.