Stabilizing the leaning tower of Pisa


In this blogpost we outline how engineers have been stabilizing the leaning tower of Pisa. We consider the history, techniques and end-result.

The leaning tower of Pisa

One of the major problems in civil engineering is getting the foundation of the building correct. Especially when the material which you are planning on building the building on consists water adhesive materials such as silt and clay. The problems with these types of characteristic materials particularly include differential settlement.

Differential settlement of the leaning tower of Pisa

Differential settlement is the procedure on which foundation weights presses water out of specific soils which adhere a large amount of water. This water is usually not easy to leave the soil of the foundation as this watery soil content, which is expressed through excess pore-volumes, contains water which when pressed over time, cause a slowly changing consolidation of the material.

The soil underneath foundations thus experience what is considered differential settlement as the water is pressed out unevenly across the foundation when pressured consistently over an excessive amount of time.

This causes the building to settle unevenly which in worst cases can form cracks along the foundation and in best situations cause lasting damages to the entire structure.

The geological composition underneath the leaning tower of Pisa

One, if not the most famous case when considering differential consolidation is the leaning tower of Pisa. The case of the leaning tower of Pisa is famous exactly because of this foundational engineering problem and have transformed from being a problem for frightening people, to be an memorable historic landmark which is cherished worldwide, see Figure 1.

The core ‘problem’ behind the leaning tower of Pisa, is the geological composition and the water tables high occurrence just below the foundation. The sandy clayey-silts which are the main source of concern is located just below the foundation. This particular composition of grain sizes is the root of the differential settlement observed.

The next layers consist of clay, followed by a small layer of intermediate sand and then once again clay. The sand is particularly problematic as water can be pressed out of the clays draining underneath water from clays to the sand content. The flow of water from the differential settlement of the leaning tower structure, allows the compression of the soil layers to easily form underneath the silty clayey top layer. Now to make things worse, the history of the leaning tower of Pisa, makes the stabilizing procedures a lot more challenging.

Figure 1: Left, the leaning tower of Pisa. Right, a cross-section of the geotechnical soil characteristics underneath the leaning tower of Pisa

History of the leaning tower of Pisa

The tower started out as a general massive construction planned to stand for multiple years. Construction of the main tower began in 1173 with expectations to finish later on during that century. However due to political unrest, recessions and expensive wars the construction of the tower was paused after finishing just four orders out of the originally planned eight cornices.

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Financial and political unrest

This periodic influence of macroeconomic tendencies are typical of middle age construction projects where the first of the different projects are started in good periods, while most activities are cancelled or delayed during times of recessions.

The delay in construction of the at the time, Tower of Pisa, was quite significant as compared to similar buildings at the time. The delay was more than 100 years before financial and political stability was sufficient as for the funding of the building to continue onwards.

Related read: Natural materials – clay, Natural materials – sand, Natural materials – silt.

Second construction period of the tower of Pisa

The second construction period began completing three more cornices of the originally planned eight, before another interruption happened again due to political unrest and financial instabilities occurred.

Now by this time, the tower had already started tilting as the geological layers underlining the foundation had been exposed to excess pore-pressures for already 100 years more than any typical pre-consolidation theory usually encounters.

The now leaning tower of Pisa

Now the tower was leaning and construction continued until only the last order and top tower cornice needing to be finished. Work was then paused once more, due to political and financial unrest before finally the leaning tower of Pisa was finished with the tower and eight cornice completed.

It thus took almost 200 years from start to finish of the leaning tower of Pisa, a period which by many standards is quite significantly larger than any other major project under construction.

For comparison, the modern day one world trade took ‘only’ 13 years to complete but is simultaneously 104-storeys, 417 m’s high. This is in stark contrast to the 8’th orders corresponding to 57 m’s high leaning tower of Pisa.

For an example of the history of the leaning tower of Pisa, see Figure 3 from [1].

Figure 2: The history of the leaning tower of Pisa with indications of building mass and construction phases.

Modern day famous leaning tower of Pisa

The completed tower of Pisa remained standing for centuries, without any catastrophic events to occur. The result is that the tilt of the leaning tower of Pisa had reached levels which experts deemed to be critical, furthermore advanced measurement systems for monitoring the increase in the leaning tower of Pisa showed that the tilt was in matter of fact, accelerating slowly and if no measures were taken would lead to catastrophic failures within a matter of a lifetime.

Therefore it was decided to begin stabilization work in the 1990’s. At this point the the tilt was approximately 5.5 degrees, resulting in an overhang of the top cornices with about 4.1 m’s compared to the bottom of the structure, see Figure 3.

Figure 3: Cross-section of the Leaning tower of Pisa with inclination, cornices and geometric instances.

Modern day technologies have allowed for the restoration of the tilt ensuring that it is not accelerating as previously experienced. A number of different technologies were investigated. These include; placing heavy rocks on the side of the leaning tower of Pisa’s foundation, placing anchors on the leaning side and installing man pillars on the side of the leaning tower of Pisa.

The architectural challenges

It was of utmost importance not to completely fix the tilt of the tower as this was now considered a historical monument whose perseverance was valuable.

Furthermore most of the proposed solutions such as installing pillars on the side of the leaning tower and subjecting the far most side to extra load resulting in a stabilizing moment were disregarded due to the imprint on the architectural expression the building incurs in its users.

The solution to these architectural constraints was an expensive horizontal drilling action, where specific amounts of soil were excavated deep under the existing foundation in amounts carefully outlined as to provide sufficient stabilizing actions to the now already strained foundation.

Stabilization measures for the leaning tower of Pisa

Most of the proposed remedies to the tilting of the leaning tower of Pisa were tossed in the bin, as a result of the architectural expression of the leaning tower of Pisa. However some of them were utilized with great success. These measures are outlined in the following.

Horizontal directional drilling

The horizontal directional drilling equipment utilized for the procedure is outlined in figure 4-5. This type of equipment is specialized towards the excavation of small exact amounts of soils deep underneath the foundation.

Furthermore, it is also possible to see the stones and counterweights placed on the side of the foundation as a counterweight ensuring that any potential miss excavations which could resolve in destabilizing of the entire building would be counteracted by the weight of the blocks and tower combined.

The surgical procedure allows engineers and scientists to control exactly how much of the soil underneath which needed to be excavated before the tilt-acceleration is stopped.

Figure 4: Horizontal directional drilling boreholes underneath the leaning tower of Pisa. Counter weights placed away from the tilt providing a stabilizing moment. Source: Wikipedia commons.
Figure 5: Pan view of the restoration process of the leaning tower of Pisa. The view is showing the horizontal drilling action and the counterweights placed on the tower. Source: Wikipedia commons

Drainage of foundation soils

The stabilization of the leaning tower of Pisa is furthermore complicated by the excessive amount of water filled soil contents which caused the differential settlement to begin with.

The differential settlement thus occurs as a result of where the water content is pushed out from the inner core samples. This water leaks from the soil as the excess pressure from the entire building is pressing on the soil causing consolidation.

Drain pumps installed

In order to combat this phenomena, drain pumps was installed allowing the excess water contents to spill out from the soil and drained away before causing problems. The result is increased strength of the soil as water is drained causing compaction and consolidation.

The drainage pumps have been constantly pumping throughout the entire restoration process of the leaning tower of Pisa. This allowed for engineers and scientists to easily calculate and ensure safe soil consolidation while simultaneously ensuring that equipment was kept dry at all operation phases.

Cable strays for temporary stabilization

As a final installment of the renovation operation procedures ensuring that the tilting of the leaning tower of Pisa was stabilized in a sufficient manner was cables.

The cables wrap around the construction in the lower decks of the tower and was afterwards pre-strained ensuring that the Tower had gotten sufficient support ensuring that the tower was stabilized to a degree that catastrophic failure was not an issue.

Open for public usage

All the above measures were sufficient as to stabilize the tower tilt by approximately 50 cm’s from the top of the tower. The stabilizations incorporated ensured that the leaning tower of Pisa could safely be opened to the public after being closed for more than a decade in the 1990’s.

The safety of the structure was increased significantly while simultaneously ensuring that the leaning tower of Pisa remained a leaning tower. This was important as a catastrophic failure event was absolutely unacceptable by authorities while authorities simultaneously recognized the need for keeping the leaning tower of Pisa leaning.

In fact, this purposely insufficient recovery of the leaning tower of Pisa’s tilt, was incorporated with the purpose of keeping the beloved tourist attraction alive without destroying the historical value which the tower had amazed throughout its history of more than 800 years.

In conclusion

Visitors can now climb to the top of the tower and witness the beautiful views of the city of Pisa without fear or concern for their safety. The restoration project has helped to preserve the tower for future generations to enjoy, ensuring that the historical significance of this iconic structure is not lost.

Thanks to the diligent efforts of the team behind the stabilization project, the leaning tower of Pisa will continue to lean for many years to come, delighting tourists from all over the world with its fascinating story and captivating beauty.


[1] Burland, John & Jamiolkowski, Michele & Viggiani, C.. (2003). The Stabilization of the Leaning Tower of Pisa. Soils and Foundations. 43. 63-80. 10.3208/sandf.43.5_63.

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