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.
1. Geology of Site
The following points describe the geology of the site:
- The variety of rocks ranges from Paleozoic to tertiary in Pisa city. Due to tectonic activities, the city’s strata have undergone severe deformation. Moreover, there is a fault passing through the bedrock beneath the town of Pisa.
- The town of Pisa is located on the alluvial sediments and the mean sea level is only 3-4 m.
- Soil profile below the tower consists of three horizons.
- Horizon-A consists mainly of estuarine deposits. This stratum is laid under tidal conditions. Therefore, the amount of sandy and clayey silts varies under the strata. The thickness of horizon-A is 10 m.
- Horizon-B consists of marine clay and sandy soil. Sandy soil is sandwiched between the layers of clayey soil. The upper clay layer is very sensitive, whereas the lower clay layer is stiff and less sensitive. The thickness of horizon-B is 40 m.
- Horizon-C consists of dense sand. The thickness of horizon-C extends to a considerable depth.
- The amount of silt and clay is more on the south side of the tower. Also, the sand layer is much thinner than on the north side of the tower. This is one of the reasons that led to the tilting of the tower towards the south side.
- The natural groundwater level is 1-2 m below the ground surface area. However, in the past, water extraction from the lower sand has led to downward seepage from the horizon-A layer. As a result, the pore pressure distribution is slightly below hydrostatic pressure.
2. Construction of Tower
The following points describe the construction of the tower:
- 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.
- After five years of construction, work came to a halt due to the scarcity of funds.
- 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.
- 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.
- Construction of the tower was completed in 1370. It took almost around 200 years to construct the tower.
3. Tilting of the Tower
The following points describe the reason behind the tilt of the tower:
- 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°.
- When the seventh story of the tower was completed, it started to lean towards the south at an angle of 0.67°.
- 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°.
- In 1817, the leaning angle of the tower was an astonishing degree of 4.87.
- 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°.
- In 1928, four leveling stations were installed near the foundation area to measure the tilt of the tower.
- The tower has been very sensitive to ground disruptions and modifications in groundwater conditions.
- 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.
- In 1970, pumping of water from the alluvial sands triggered subsidence and the tower slanted more towards the south.
- In 1990, it was observed that the tower was inclining towards the south and had reached a maximum angle of 5.5°.
3.1 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:
- Change in the groundwater table causing subsidence in the alluvial sand
- Heavy rainstorms
- Temperature variation in the summers caused the change in the leaning stability of the tower
- Stabilization process of the tower
4. Stabilization Process of the Tower
Two primary aspects have been affecting the stability of the tower. The first one is associated with the structure itself, and the second one is associated with the structure sub-soil. The approach for stabilization was divided into two stages. Firstly, to increase the margin of safety by providing some temporary measures and secondly, to provide a permanent solution.
4.1 Temporary Stabilization
The following points describe the temporary stabilization measures taken to improve the stability of the tower:
- Marble cladding around the outer wall of the tower started showing signs of cracking. Therefore, to reduce the failure of marble cladding, temporary lightly pre-stressed plastic-covered steel tendons were installed at a suitable interval up to the second story in the year 1992.
- With time, not only the southern side of the tower was tilting, but also the northern side was steadily rising. Therefore, the researchers suggested to put weight on the northern side of the masonry foundation. As a result, it would counteract the continuous tilting of the tower and possibly reduce it to some extent.
- For the above purpose, a temporary pre-cast concrete ring was constructed around the plinth level of the tower. For loading, lead ingots were placed on the pre-cast concrete ring at a suitable time interval.
- The loading on the northern side was imposed in four phases to observe its effect on the tower.
- The first lead ingot was placed in July 1994 and the last lead ingot was placed in January 1995. One month after the application of loading, the tower started to tilt towards the northern direction.
- Significant changes in the inclination were observed. However, due to the application of loading, the tower settled at around 2.5 mm relative to the surrounding ground level.
4.2 Permanent Stabilization
To permanently rotate the tower back in the northern direction, the committee looked for a permanent solution and conducted trials for several methods. These methods included drainage below the north foundation using wells, consolidation below the north foundation by electro-osmosis, and loading the ground around the north foundation using a pressing slab loaded by ground anchors. However, none of these methods proved to be satisfactory.
Following this, the committee decided to adopt the induced subsidence method on the northern side of the tower. The following points describe the induced subsidence method to rotate the tower back in the northern side:
- This method involves the installation of several soil extraction tubes just beneath the north side of the foundation.
- This method was conducted first on the trial footing installed near the tower area. Trial footing was successfully rotated to 0.25°.
- In August 1998, the actual work of permanent stabilization started on the northern foundation of the tower. At first, a small volume of soil was extracted from the northern side of the foundation, which resulted in the formation of a cavity. The cavity started closing automatically due to the overburden pressure and resulted in the small surface subsidence.
- The process mentioned above was adopted for the full width of the northern side of the foundation. Around 41 extraction holes were installed on the northern side of the foundation at a spacing of 0.5 m.
- After the fully induced subsidence, the tower significantly rotated towards the northern side.
4.2.1 Additional Foundation Stabilization Measures
In addition to the above measures, two more permanent solutions were adopted for reducing the inclination of the tower.
Firstly, a 0.8 m thick cement-conglomerate ring was provided at the base of tower and was connected to the foundation of the tower through steel reinforcement. Also, the ring was strengthened by post-tensioning method. As a result, the effective area of the foundation was increased, which also increased the safety against leaning instability.
Secondly, to reduce the groundwater level fluctuation during the rainy season, a drainage system was installed. This system consists of three wells. These wells were constructed with radial sub-horizontal drains running beneath the north side of the foundation. The water level in the outlet pipes control the water level in the wells. The installation of this drainage system induced a further northward rotation of the tower.
5. The Future of Leaning Tower of Pisa
While the monitoring of the tower continues to date, the major concern is how the tower will respond in the future. Due to complex phenomena and distinct soil-structure interaction, an indisputable answer is not possible. However, the researchers have given two following possible circumstances:
5.1 Optimistic Scenario
The phenomena of the leaning instability and continuing rotation have stopped, just leaving some small movements brought on by seasonal oscillations of the groundwater. This case suggests that the dominant system driving the leaning instability was due to changes in the groundwater level in the vicinity of the foundation.
5.2 Pessimistic Scenario
As soon as the effects of the induced subsidence method completes, the tower will stay stationary for a couple of years. Moreover, it can be followed by a possible rotation in the southward side. But the rotation rate will slowly increase and may once again approach close to 5.5° angle.
In the worst-case scenario, the stabilization work will bring the tower inclination back to the condition that existed at the end of the 12th century. The researchers suggest that the tower will take at least 200 years to return to its 1993 inclination.
The height of tower of Pisa is 56 m.
The natural groundwater level is 1-2 m below the ground surface area.
In 1990, it was observed that the tower was inclining towards the south and had reached a maximum angle of 5.5°.
The following are the factors that have contributed towards the leaning of Pisa tower:
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
5. Sensitivity of the foundation materials