Leaning Tower of Pisa

The Leaning Tower of Pisa is a white marble bell tower standing beside the cathedral of the Italian city of Pisa, recognizable worldwide as an architectural landmark for its unintended tilt towards one side.

The construction of the tower began in 1170 and continued (with two long interruptions) for about 200 years. The structure, in essence, is a hollow masonry cylinder with columns and vaults rising from the base cylinder.

The tower began to lean southwards during the construction stage, and thereafter its inclination continued to increase. In 1993, the tower had tilted to the maximum angle of 5.5° towards the south.

The leaning tower of Pisa is constructed on the alluvial sediments over an unstable soil surface, which gradually caused the tower to lean towards one direction. The continuous increase in the foundation’s inclination originated due to the combined effects of the soil creep and the groundwater fluctuation.

In 1993, the Italian government, concerned about the progressive increase of the rate of inclination and the risk of sudden structural collapse, appointed an international committee to safeguard and stabilize the leaning tower of Pisa.

After the formation of the committee, temporary and permanent stabilization measures were taken up to improve the stability of the tower. Currently, the tower is stable after the implementation of several stabilization measures. However, the future behavior of the tower will depend to a large extent on the effectiveness of the drainage system on the north side.

This article describes the construction of the tower, the ground conditions on which it was constructed, the history behind the tilt, and the temporary and permanent measures taken to stabilize it.

Construction of Tower

The following points describe the construction of the tower:

  1. The tower was constructed as a hollow cylinder. The walls of the tower are clad with marble and the cavity between the outer and inner wall is filled with a rubble and mortar mix.
  2. After five years of construction, work came to a halt due to the scarcity of funds.
  3. The construction works started again in 1272, and a total of seven floors were constructed. However, in 1278 the construction work was hindered again due to a massive earthquake that had occurred 160 km away from the Pisa city.
  4. Once again, in 1360, the work began on the bell chamber of the Pisa tower. It required the building of six steps on the south side of the tower, compared to four steps on the north side between the seventh cornice and the flooring of the bell chamber. However, the tower had started to lean considerably to the south during that phase.
  5. Construction of the tower was completed in 1370. It took almost around 200 years to construct the tower.

Tilting of the Tower

The following points describe the reason behind the tilt of the tower:

  1. During the preliminary construction of the tower, when a total of four storeys were constructed, the tower established a tilt to the north. Consequently, when the work had recommenced in 1272 (after a break of 100 years), the tower was leaning at an angle of 0.27°.
  2. When the seventh story of the tower was completed, it started to lean towards the south at an angle of 0.67°.
  3. After another break of 90 years in construction, the tower continued to lean towards the south. In 1360, the inclination of the tower went up to 1.6°.
  4. In 1817, the leaning angle of the tower was an astonishing degree of 4.87.
  5. The condition worsened in 1838 when architect Alessandro dug a pathway around the foundation of the tower. Unfortunately, as the excavation extended below the groundwater level, water entered into the excavation such that the 4.87° southerly inclination of the tower increased by some 0.575°.
  6. In 1928, four leveling stations were installed near the foundation area to measure the tilt of the tower.
  7. The tower has been very sensitive to ground disruptions and modifications in groundwater conditions.
  8. In 1934, 361 holes were drilled into the foundation masonry and 80 tons of grout was injected to strengthen the stonework. At that time, as a result of ground disturbance and the temporary lowering of the groundwater, there was a sudden increase in the tilt of the tower.
  9. In 1970, pumping of water from the alluvial sands triggered subsidence and the tower slanted more towards the south.
  10. In 1990, it was observed that the tower was inclining towards the south and had reached a maximum angle of 5.5°.

Causes of Tilting of Tower

The rapid increase in the leaning process of any building or tower towards the end of the construction process is known as leaning instability. Leaning instability of a tall, narrow structure occurs at a critical height when the overturning moment generated by a small increase in inclination is equal to, or larger than the corresponding resisting moment generated by the foundations.

Leaning instability is not due to lack of strength of the ground but due to insufficient stiffness. It is apparent that the combination of the very soft ground and the geometry has actually resulted in the tower of Pisa reaching its critical height.

The factors that have contributed towards the leaning of the Pisa tower are:

  1. Change in the groundwater table causing subsidence in the alluvial sand
  2. Heavy rainstorms
  3. Temperature variation in the summers caused the change in the leaning stability of the tower
  4. Stabilization process of the tower



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