The Leaning Tower of Pisa is one of the most famous constructions in the world. Its fame not only comes from the original trigger for its construction – to show the importance of this city after successfully attacking the city of Sicily, but also from its tilt, which has lasted for more than nine centuries. During this time, numerous people have tried to find the reason why the tower is tilted and how to fix it.
It goes without saying that since the very beginning of the tower construction process, there were already existing factors that could cause the tilt. The most important factor at this point in time, and the one that would ultimately contribute the most to the tower’s tilt, is the engineers’ lack of understanding of the soil profile at the tower’s base. Construction began at 1173. At that time, those engineers and architects knew far less about the ground they would construct on than we do now. Ancient Romans used massive stone pillars- piles rested on earth’s stable bedrock. Those architects believed that a three-meter foundation was deep enough as the Tower of Pisa was going to be a relatively short structure. However, the complexity of the soil profile would require more than that in order to achieve stability at the base of the tower.
As figure 1 shows, the stratigraphy of the soil under the Tower of Pisa is quite complicated. Architects needed to be considerate of the properties of the different soil layers in order to avoid any construction accidents. The upper horizon, Horizon A, from 3 to 10 m, was quite dependent on tidal conditions. The geological environment of this tower contributes to the chaotic nature of the soil in this layer. The construction location of the tower is located near the seashore, and the high-water table results in subsoils soaked in sea water. Because of this, sedimentation would occur and be deposited in the soil. Marine organisms, especially those animals with shells, were one of the components of the sediments in the estuarine environment. As those sea organisms, which came from the Arno and Serchio rivers located on the north side of the tower, accumulated over time, the northern base of the tower grew higher and higher. This further contributed to the uneven height of the soil.
Figure 1. The soil profile of the Tower of Pisa.
Source: M.B. Jamiolkowski. “The leaning tower of Pisa: End of an Odyssey”. https://www.issmge.org/uploads/publications/1/30/2001_04_0071.pdf, Fig. 3 - Leaning Tower of Pisa - Soil Profile.
In addition to the geotechnical factors, the original soil conditions are also important to consider. As figure 2 shows, there was an obvious elevation of the distribution of the soil layer in boreholes a,b,c and d. Borehole e also shows that bending and subsidence of the different soil layers also exists in the soil under the Tower of Pisa despite the sedimentation coming from the rivers. These boreholes were performed in the subsoil in other parts of the city of Pisa in 1950 and 1953, and were performed to a depth of 220 m so that the information collected from those boreholes would be very close to the subsoil properties of the Tower of Pisa.
Figure 2. Soil samples collected by boreholes.
Source: Lo Presti, Diego & Jamiolkowski, Michele & Pepe, M.. (2003). Geotechnical characterization of the subsoil of Pisa Tower. Characterisation and Engineering Properties of Natural Soils. 2. 909-946, Figure 2. Typical structures of sedimentation in Horizon A (MPW 1971)
Based on the analysis of the soil samples, engineers found that at the south side of the tower, the soil was siltier and more clayey than the north side and the sand layer is thinner. Because of this, the ability of different soils to support the weight applied by the tower is different. Looking at the subsoil profile shown in figure 3, there was a thick clayey layer, layer B, under the chaotic layer A. This upper clayey layer consisted of a soft, sensitive clay called Pancone clay. As the sea level was high, this layer was saturated so that it became undrained. A large amount of load was applied. The soil when behaved under undrained conditions, resulting in excessive pore pressure. Because of the low permeability of the clay, this excessive pore pressure within the clay was not easily dissipated. This caused the Pancone clay to enter a non-steady state. Under this non-steady state, the elevated pore pressure in the clay layer led to a lower factor of safety, hence the failure occurred. This was also an important factor in causing the tilt of the tower.
Figure 3. The subsoil profile under the Tower of Pisa.
Source: Burland, J. B., "The Leaning Tower of Pisa Revisited" (2004). International Conference on Case Histories in Geotechnical Engineering. 3. https://scholarsmine.mst.edu/icchge/5icchge/session00f/3, Fig. 3. Ground profile beneath the Tower.
As the construction of the tower went on, a leaning instability continued to increase the tilt of the tower. “Leaning instability” is a phenomenon that occurs at tall, narrow structures. When structure reached a critical height/width ratio, the overturning moment produced by a tiny increase in height will be equal to or larger than the resisting moment produced by the foundation. This will make the structure in a high risk of toppling down. As mentioned earlier, the tower was constructed on soft, compressive soil. As the consolidation of the soil under the tower occurred during the stop period of construction, the compressibility of the soil gradually increased. The stiffness of the soil was not linear due to the existence of sediments and natural effect. The high compressibility of the soil and the non-linear soil stiffness caused the resisting moment produced by the soil to not be able to balance the large overturning moment produced when the tower was constructed. Hence a self-driving instability was produced. This led to a progressive increase of the tilt of the tower. Figure 4 shows the trend of the tilt of the tower, and this trend is most likely due to tower’s leaning instability.
Figure 4. The leaning trend of Tower of Pisa.
Source: M.B. Jamiolkowski. “The leaning tower of Pisa: End of an Odyssey”. https://www.issmge.org/uploads/publications/1/30/2001_04_0071.pdf, Fig. 10 - Inclination of Leaning Tower of Pisa.
Another factor that worsened the tilt of the tower was excessive construction around the tower. In 1838, under the instruction of a famous architect and engineer- Alessandro della Gheradesca, a walk-way called catino was excavated. The goal of this excavation was to show the column plinths and foundation steps for everyone to see as it was intended before. However, this excavation resulted in raising the water table on the south side. At south side, the excavation was under the high water table, which largely affected soil strength, hence made the tilt more severe. The excavation resulted in an increase of the tower’s inclination by more than 0.25°. The tower nearly collapsed at this point.
Since the moment the tilt of the tower was first found, people tried many different ways of fixing it. Some of them, like the excavation of the catino, made the tilt even worse. But some of them also successfully addressed the tilt of the Tower of Pisa.
One method of restoring the tower of Pisa was the consolidation of the soil layers. The construction of the tower begun at 1173, and was temporarily suspended at 1178. This was the first phase of construction. The reason why the construction was stopped was unknown, but this coincidentally helped the Tower of Pisa. At that time, the soil compressibility was very low. If the tower construction had not stopped, the soil layer was not strong enough to support the load of tower, an undrained bearing capacity failure would have occured, and the tower might have fallen over. Luckily, the resumption of the construction was not until 1272, led by the Architect Giovannii Di Simone. This, roughly 100 years of time, gave the soil layer enough time to consolidate. The strength of the soft clay layer was increased under the weight of tower itself.
Another method used to correct the tilt of the tower was the usage of obliquely cut stones. As Figure 5 shows, the tilt of tower became severe during the second period of construction. When architects found that the tower was tilting, they started to adjust the thickness of the stone layers. They used unevenly cut stones and gradually changed the thickness of the stones to keep each floor horizontal. As Figure 6 shows, the stone is thicker on the northern side and thinner on the southern side.
Figure 5. Deduced history of inclination of the Tower.
Source: Jamiolkowski, M.; Lancellotta, R.; and Pepe, C., "Leaning Tower of Pisa — Updated Information" (1993).
International Conference on Case Histories in Geotechnical Engineering. 5.
https://scholarsmine.mst.edu/icchge/3icchge/3icchge-session15/5, Fig.5. History of rigid tilt.
Figure 6. The obliquely cut stone.
Source: Soga Kenichi. Geotechnical and Environmental Engineering, Lecture 27-32. “7. Compression and Consolidation”.2019. PDF download, Slide 5.
Figure 7. The shape of Tower of Pisa.
Source: M.B. Jamiolkowski. “The leaning tower of Pisa: End of an Odyssey”. https://www.issmge.org/uploads/publications/1/30/2001_04_0071.pdf, Fig.9- Shape of the Tower.
The tilt of tower went on without a break, and the tower correspondingly became more and more brittle. In the year 1989, a scarce safety margin of the monument was declared by Government Committee, saying that the tower was in a danger of toppling. The tower shaft was weak as a result of the opening of the stair. The bond strength between the infill masonry and the facing was not adequate, and numerous cracks and cavities of masonry were visible. The stress was mainly focused on the bedding joints of the marble stone. Additionally, the occurrence of another omen drove the government in panic. During that same year, the 12th-century Civic Tower of Pavia collapsed. This tower had similar masonry with the Tower of Pisa. With all things considered, the Tower of Pisa was closed to public. The Committee was charged to figure out plans for saving the tower. A wide range of experts in fields such as architecture, geotechnical engineering and construction were assembled. Their goal was to stabilize the foundation of the tower, strengthen the masonry, and renovate the tower. Also, because the tower was so fragile, any modifications should be kept to the absolute minimum. Any visible external supports were forbidden.
Under such harsh situations, the Committee still figured out a good plan. The plan carried out by the Committee consisted of two parts: temporary stabilization and final stabilization. The execution of the temporary stabilization was due to the dangerous situation of the tower mentioned in the safety margin of the monument, so that this stabilization plan was completely reversible in order to both not worsen the bad situation of the tower and reduce the tilt of the tower. The main component of this temporary plan was to add weight on the higher side of the tower. In the year 1993, the stabilization began. 600 tons of lead ingots was added to the north side of the foundation of the tower as a counterweight. The heavy lead ingots generated a stabilizing moment of 450 tons that effectively decreased the inclination angle of the tower. As a result. the overturning moment of the tower was reduced by 10% and the inclination was reduced by 1 minute of arc. This was the first time in the history where the movement direction of tower was reversed as figure 8 shows.
Figure 8. The tilt towards northside as weight applied.
Source: M.B. Jamiolkowski. “The leaning tower of Pisa: End of an Odyssey”. https://www.issmge.org/uploads/publications/1/30/2001_04_0071.pdf, Fig.20- Tilt towards North as result of counterweight application.
After these successes, people tried to replace the lead weights, which lacked aesthetics, with 10 deep anchors. Those anchors would be deeply planted into the ground to carry the load of the tower. Each of the anchors had a workload of 1000 kN. However, when the lead weights were removed, the tower started to tilt to the southside again at a rate of 3’’ to 4’’ per day. This plan of replacement had to be stopped. Later on, the lead ingots were increased to 900 tons in order to prevent further movement as a result of that unsuccessful attempt.
Even though the tilt of tower had stopped, a permanent solution was still required. There were three possible methods that could be used for the final stabilization.
In order to ensure that this type of soil extraction, named underexcavation, was feasible for stabilization, engineers took many years to study this method. First, physical models were built. Then, numerous models were generated. And finally, many large-scale trials were conducted. These large-scale trials were necessary to demonstrate the effectiveness of underexcavation. Figure 9 shows the locations and mechanisms of those large-scale trials. A long excavation drill will be planted in the soil for excavating. The most important factor for these large-scale trials lies in how the drill was implanted. The drill is a hollow-stemmed continuous flight auger drill planted inside a 180 mm diameter casting. According to the mechanism shown in figure 10, when protruding the drill to make a hole, an instrumented probe could measure the closure of the hole. Analyzing the measured results from the drill, there was a 0.25° rotation under the direction control even though the soil properties were not quite uniform. The cavities dug by the drill were fixed gently under multiple excavations. Those results proved the feasibility of this method.
Figure 9. Soil extraction location.
Source: John B. Burland (2002) The Stabilization of the Leaning Tower of Pisa, Journal of Architectural Conservation, 8:3, 7-23, DOI: 10.1080/13556207.2002.10785324, Figure 2 Diagram showing location of extraction tubes beneath the Tower.
Figure 10. Mechanisms of operating the testing drill.
Source: John B. Burland (2002) The Stabilization of the Leaning Tower of Pisa, Journal of Architectural Conservation, 8:3, 7-23, DOI: 10.1080/13556207.2002.10785324, Figure 3 Diagram showing principles of operation of the extraction drill.
Before starting the final stabilization process, safeguard cables needed to be tied to the tower in order to apply a horizontal force. With the application of these safeguard cables in 1999, the preliminary underexcavation started. Using 12 boreholes and 220 mm castings, the total volume of soil extracted was 7 m3. After the preliminary underexcavation, the tower rotated northward by 90 seconds, and reduced the inclination by 132 seconds. The result of the first excavation was pleasing.
Finally, full soil extraction began. All the lead weights were gradually removed from the tower. The soil extraction counteracted the overturning moment produced by the removal of the lead ingots. In the end, a total of 1834 seconds of arc were further deducted. As figure 11 shows, the whole excavation process was successful without any doubt.
Figure 11. The rotation of tower during excavation process.
Source: M.B. Jamiolkowski. “The leaning tower of Pisa: End of an Odyssey”. https://www.issmge.org/uploads/publications/1/30/2001_04_0071.pdf, Fig. 32 - Tower, rotation during uncerexcavation.
Generally speaking, there are two scenarios predicting the behavior of the tower in the future. The positive one says that the rotation of tower will halt aside from tiny cyclic changes due to seasonal changes of groundwater. The negative scenario indicates that the tower will remain stable for a while, after which the southwards rotation will resume again but at a lower rate. It would take hundreds of years until the next intervention would become necessary. Both of those scenarios are indicating that the tower of Pisa is safe so far, and that the effects of the stabilization will last longer.
1. Lo Presti, Diego & Jamiolkowski, Michele & Pepe, M.. (2003). Geotechnical characterization of the subsoil of Pisa Tower. Characterisation and Engineering Properties of Natural Soils. 2. 909-946.
2. Burland J.B., Jamiolkowski M.B., Viggiani C., (2009). Leaning Tower of Pisa: Behavior after Stabilization Operations. International Journal of Geoengineering Case histories, http://casehistories.geoengineer.org, Vol.1, Issue 3, p.156-169
3. M.B. Jamiolkowski. “The leaning tower of Pisa: End of an Odyssey”. https://www.issmge.org/uploads/publications/1/30/2001_04_0071.pdf, p.2980-2996.
4. Burland, J. B., "The Leaning Tower of Pisa Revisited" (2004). International Conference on Case Histories in
Geotechnical Engineering. 3. https://scholarsmine.mst.edu/icchge/5icchge/session00f/3
5. Burland J.B., Jamiolkowski M.B., Viggiani C., (2003). The stabilization of the leaning Tower of Pisa. Soil and foundations, Vol 43, No.5, 63-80.
6. John B. Burland (2002) The Stabilization of the Leaning Tower of Pisa, Journal of Architectural Conservation, 8:3, 7-23, DOI: 10.1080/13556207.2002.10785324
7. Jamiolkowski, M.; Lancellotta, R.; and Pepe, C., "Leaning Tower of Pisa — Updated Information" (1993).
International Conference on Case Histories in Geotechnical Engineering. 5.
8. Soga Kenichi. Geotechnical and Environmental Engineering, Lecture 27-32. “7. Compression and Consolidation”.2019. PDF download.