Historic construction failures and how we avoid them - Tower of Pisa
Civil Engineering works have a history tracing back to 4000 years ago since the construction of the Great Pyramids. Up until now, the field is still amazed by the magnificent structure in the past. And yes, people are still debating about whether we can rebuild the pyramids in the modern day. This left us wondering, would it really be safer, stronger or faster if modern technology were to replicate a past construction? Would past construction failures still happen? What could we have done differently if we actually rebuilt something?
Let’s explore the valuable engineering lessons from construction failures from history
The famous leaning Tower of Pisa
An iconic failure:
One exemplary case study for any structural failure is the Tower of Pisa, or as it has been famously known as, the Leaning Tower of Pisa. Most tourists may not consider this as an engineering disaster, but no! The tower’s tilt was not intentional! Without appropriate intervention, like the structure enhancement project in the 2000s, the tower might have continued to tilt and eventually collapse. To geotechnical engineers, it is an interesting case study as one of the earliest and most exemplary soil capacity failures.
In short, the main cause of the famous lean can be attributed to the composition of the soil on the building site. This soil profile is multi-layered, which comprises mainly sand and clay. This soil profile is not very strong, but in 1173, engineers were obviously not knowledgeable about the soil stratigraphy since proper scientific studies about soil mechanics were not around until the 18th century. Before that, we can find spectacular foundation failures when engineers tried to build foundations only by assumptions and experiments. That led to the tower’s foundation being only 3 metres deep on soft soil, supporting 14,700 tons of masonary, which is like balancing a stone statute on soft bread dough. Of course, the ‘statute’ subsided and swayed.
To summarize, the main reasons behind the tower’s tilt are:
- North of the Tower, the Arno and Serchio rivers bring sedimentation from sea organisms such as shells that build up in the north soil of the tower. This results in the tower’s northern base growing higher and higher.
- Below the tower, the uneven and soft soil layer below the ground leads to an uneven settlement between the North and South of the tower. The bow shape at the upper clay layer of the sand (See the figure below) indicates that the tower has settled for more than 3 meters, which shows how compressible is the underlying soil.
- With tidal waves, the water level fluctuates, making the soil extremely unstable. That is why modern engineers have to create a drainage system and seal all the wells in the local area to avoid water pumping in and out of the area when maintaining the tower.
The modern Pisa tower
Now we know about the tower itself, what lessons have we taken with us and how might modern engineers approach the same problem today?
Do you know that $27 million was spent to stabilise the Leaning Tower in 2001? The cost of building the Tower today would be approximately $4.1 million in the modern age (including the cost of marble, labour costs…)! So we could have built six straight towers of Pisa instead of trying to save one leaning tower of Pisa (joke).
One of the more straightforward renovations is to build the Tower of Pisa somewhere with more stable soil strata. But that doesn’t mean civil engineers have not dealt with construction on soft soil before.
Construction phase all starts with a site investigation. The current technology has permitted engineers to drill deep into the soil to make boreholes and bring back soil samples from different soil layers. This is how engineers were able to generate the above soil profile. The truth is, when maintaining the tower, engineers already made several boreholes with a depth of 220 m around the city of Pisa to collect information on the subsoil properties of the Tower of Pisa in 1950 and 1953. Geotechnical engineers used the data gathered from soil layers to make judgements about types of foundation and suitable construction techniques.
Modern machinery already permits engineers to do more large-scale tasks, such as soil improvement. That includes various construction methods like soil compaction, grout injection and using a stronger type of footing.
Let’s go with the basics first! Construction workers usually improve soil strength by putting temporary large loads on it before the construction. Why? It is a way to make the soil more compacted, or in other words, stronger, denser and more stable. This is particularly useful for light-weight infrastructure projects such as roads and pipelines because the soil can be compacted with a similar load level prior to construction using construction machinery such as road rollers. For larger construction such as production plants, a very heavy weight is lifted and thrown on the ground to make it compact through large dynamic impacts. That can’t be used near residential areas and does not perform well on all kinds of soils.
Another construction method is grout injection, which injects mortar materials into the voids of loose soil layers, densifying the soil below and increasing its bearing capacity. Ironically, when this technique was used to strengthen the Tower in 1934, it caused complex deformations around the tower and made the tower suddenly leaned 10 mm towards the South. This proves how very sensitive the tower was at the time and any construction methods have to be well-considered.
The key is also in the choice of the foundation itself. There is a consensus among engineers that the Tower's footing needs to be deeper or larger when the current foundation is only 15 meters in diameter and 3 meters deep. Here, there are two choices:
- Use bigger footing and reinforce it with steel:
Modern construction has huge advantages over materials compared to the commonly used limestone footings of the time. Today, the introduction of reinforced concrete allows significant structural enhancements. A bigger footing can better support the load transfer from the tower to the ground, spreading the gravity from the tower onto the footing and to the ground. Of course, the depth of the footing needed to be increased. This spread foundation was very popular in the 1800s in the United States. Unfortunately, there are some cases in which historic buildings suffered from excessive settlement when using the spread foundation. Otherwise, the base area of the tower of Pisa is only 15 meters in diameter, so it won’t permit the footing to spread out a lot. Or else, it would look like having a candle put on a tray. So making a deep and spread out foundation might not be the answer.
- Use deep foundations (piling)
A deep foundation is one of the more modern foundation types. Luckily, it is a technique that allows us to build further up, opening the possibility of slim but incredibly tall skyscrapers that we are seeing today. Deep foundations consist of several piles (concrete or steel) drilled or driven straight to the ground until it reaches a prescribed depth or touches the bedrock (hard rock). This foundation permits the load transfer to go to deeper soil. Hence, this type of foundation is best used when the soil is weak at shallow depths or the load above is too much, or there are constraints on footing size. Sounds like the Tower of Pisa?
Engineers already consider the application of pile foundations first if there is a high rise structure. The Tower of Pisa, otherwise, is deemed to be heavy compared to other similar-sized designs. And as there is no hard soil until 40-meter depth, it is unrealistic to drill the piles to that depth for a 57-meter structure. Burj Khalifa, with a height of 848 meters, only has a foundation depth of 50 meters.
Stepping back, a group of friction piles (floating piles) for the foundation will be sufficient. If the usual end-bearing pile is the bridge to transfer the load from the structure to the bedrock, the friction pile will resist the load by friction force developed in its skin. The groups of friction piles will be connected by a concrete cap on top.
According to building codes, a driven concrete pile (0.5 meters in diameter and 15-meter depth in dense undrained sand) can endure up to 500 - 600 tonnes per pile. In theory, the tower would have needed a group of 30 piles to handle the load above. Of course, the actual situation is more complicated than that with pile group effects, various soil layers and fluctuating water conditions, which is not very good for floating foundations.
If you are curious about how to calculate the bearing capacity of the piles by building codes, CalcTree will soon offer our engineering calculation for deep foundations and piles. Meanwhile, stay tuned and join our waitlist to be one of the first to use the first construction platform.
One example of successful floating pile construction is the Burj Khalifa, built in Dubai, where the soil strata are predominantly sand and siltstone. The structure used 192 giant concrete piles. Each can endure 3000 tonnes of weight. The piles also contribute significantly to the building’s ability to resist lateral load from wind and earthquakes.
Wind loads are very important for towers! If you are curious about how an engineering catastrophe can happen due to wind loads, you can head to Part 2 of the ‘construction failures’ series: The Tacoma Narrows Bridge.
You might think Civil Engineering failures belong to the pre-modern era. That was not the case with the Transcona Elevator. This is also an engineering disaster because of the lack of soil inspection. The engineers who took part in the project assumed that the soil was homogenous, made up of mostly stiff clay. This led engineers to overestimate the design capacity, which was supposed to be smaller because there is a soft clay layer below the stiff clay. This flawed assumption made the structure collapse on its self-weight when the soil is not as strong as people think. If a deeper investigation of the soil strata was carried out, the soft clay condition below would have become known, and more suitable building techniques could be applied.
In 1173 Tower of Pisa’s engineers ‘assumed’ a 3-meter-deep foundation was enough. In 1911 Transcona Elevator’s engineers ‘assumed’ the soil below the construction was stable. Both ended in catastrophes. That is why engineers are responsible for continuing to develop their knowledge, skills and learning from past mistakes to minimise construction error in design. As a construction tech company, CalcTree’s vision is to be a platform where engineers can automate and standardise their design process to create a safer, more certain and more time-saving design process.
What is CalcTree, we hear you say?
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 Burland, B. "The Leaning Tower of Pisa Revisited" (2004). International Conference on Case Histories in Geotechnical Engineering. 3.
 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.
 Brazelton, R. (1948). History of building foundations in Chicago: a report of an investigation. University of Illinois. https://www.ideals.illinois.edu/handle/2142/4215