The Leaning Tower of Pisa

Group 10

La Torre di Pisa

• The Tower of Pisa is the bell tower of the Cathedral

• Construction began in August 1173

• Work completed by 1360

• Total number of bells: 7, tuned to musical scale

Technical Details

• The bell tower rises to a height of 53.3 m above ground level

• Comprises of eight levels • Weight estimated at 14,500 tonnes• Outer diameter of base: 15.484 m • Inner diameter of base: 7.368 m • Angle of Slant: 13 degrees • The tower has 294 steps

Galileo and the Tower

Galileo Galilei is said to have dropped two cannon balls of different masses from the tower to demonstrate that their descending speed was independent of their mass.

Galileo showed that Aristotle was wrong, but could not explain why, so the University of Pisa decided not to renew his contract.

Other Historical Stuff

• Benito Mussolini ordered that the tower be returned to a vertical position. Concrete was poured into the foundation which resulted in the tower actually sinking further into the soil

• During World War II, the Allies discovered that the Nazis were using it as an observation post. A decision not to call in an artillery strike saved the tower

• In 1987, the tower was declared as part of the UNESCO World Heritage Site along with it’s neighbouring buildings

• Evil Superman apparently straightened the Tower in the movie Superman III

What Went Wrong?• Originally conceived as a

straight building.• Began leaning to the

southeast soon after the onset of construction in 1173.

• This was caused by the particular morphological characteristics of the ground underneath, composed of various strata formed of deposits and clayish material, interlayered, at about a metre of depth, by strata of subterranean waters.

• Poorly laid foundation and loose substrate allowed the foundation to shift.

Attempts At Straightening

• 1838 - Decided to remove the blanket of earth from the base of the Tower which had been covered for centuries

• Tower loses equilibrium• Increased inclination by about 20

centimetres

• 1934 – Mussolini orders 200 tonnes of concrete be poured into the foundation.

• Inclination increases by a tenth of a degree, 10mm at the top of the tower

Attempts At Straightening

• 1970’s - groundwater extraction from the lower sands increased the movement by about 12 mm.

• These responses showed how very sensitively poised the tower was.

• 1989 - Commission established to make recommendations regarding the safety of the tower and to implement stabilisation measures

Attempts At Straightening• 1993 - 600 tonnes of lead weights

applied to the north side of the foundation via a post-tensioned, removable concrete ring cast around the base of the tower.

• This caused a reduction in inclination by about one minute of arc. Of greater importance, it reduced the overturning moment by about 10 percent

• 1995 - The load was increased to 900 tonnes to control the movements of the tower during an unsuccessful attempt to replace the unsightly lead weights with temporary ground anchors

How The Tower Was StraightenedThe final solution to correcting the lean was to remove 38 cubic metres of soil from underneath the raised end.

40 drills were used to excavate the soft estuary silt.

An artificial subsidence was then created as the drills were withdrawn one metre and the tower moved towards the vertical.

The Tower was steadied by steel cables attached to the first tier and anchored to the ground.

The Tower Today• The Tower stands stable

• The inclination remains but solely as a tourist attraction

• The tower is one of the Seven Wonders of the Modern World

• The tower has now been returned to the angle of inclination it exhibited in the 1830s

Lessons Learned

Ground Investigations prior to construction would have saved a lot of time and effort