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Megaproject spotlight: constructing the world’s longest railway tunnel

Megaprojects debunked: modernism, management & mishaps

Historic construction failures and how we avoid them- Part 1: Tower of Pisa

Historic construction failures and how we avoid them - Part 2: Tacoma Narrow bridge

Extensive, efficient, and well-running transport infrastructure is essential to maintaining a country’s prosperity and competitiveness. They’re also critical to connecting supply chains and allowing economic development through the mobility of people and freight. The more dense and well-connected transport systems are the greater the economic and social opportunities. This is precisely what the 

 hopes to achieve; narrow gaps, remove bottlenecks, and reinforce social and economic integration within the continent. The TEN-T can be characterized by the increasing development of transport infrastructure megaprojects of impressive scale and magnitude, and a standout is the 

The BBT comprises a flat railway tunnel with two parallel bores and an exploratory tunnel beneath them. The 1371-meter-high Brenner Pass is a mountain pass through the Alpine region that forms the border between Austria and Italy and is currently one of the most important transit routes between Northern and Southern Europe. Despite its winding roads, narrow valleys, and extreme gradients, over 2 million trucks cross the Brenner Pass annually, with upwards of 50 million metric tonnes of freight traveling through the existing rail line and roads.

Bordering two EU countries means that border enforcements rarely happen. Even with this freedom of movement, freight, and leisure traffic still cause long traffic jams resulting in significant air and traffic pollution. 

The solution? One of the most complex, ambitious, and groundbreaking megaprojects connecting traffic and freight between two sides of the content. The BBT  will eliminate the challenging route of the existing railway alignment and significantly improve sustainable mobility in the region and across the continent. An exploratory tunnel was first dug 12m deeper than the proposed tubes of the rail tunnel to explore the rock mass, once completed, it will also be used as a drainage and service tunnel.

Major specs of the Brenner Base Tunnel are summarised below:

Tunnels, particularly in urban areas, are critical to supplying, providing, and maintaining essential infrastructure like road and rail supporting our transport, water, sewer, and utility systems. Keeping up with the increasing demand for underground infrastructure means that modern tunnels are being built in increasingly constrained and demanding environments. The Brenner Base Tunnel is no exception.

What makes this record-breaking project even more momentous is that it is being built in Alpine regions. Alpine areas have tectonically deformed rocks and geologically challenging sections known as fault zones. The principal challenge is when excavation activities have to cross fault rocks. Fault rocks are characterized as weak, meaning they have a low compressive strength from about 2-20 MPa. The presence of fault zones intermittently throughout the composition of an underground structure can lead to significant challenges in delivery. 

Brenner Base Tunnel Longitudinal geological profile

When rock masses are weak or fractured, they can behave like soils and can easily be disturbed by underground works, heavy structural elements, and hydrogeological challenges. As a result, construction teams can often face all sorts of ground instabilities. Therefore, a comprehensive understanding of the site geology is essential to produce a safe and rigorous design of deep alpine tunnels.  

This is commonly done through specimen preparation, field investigations, and laboratory testing. In the case of the BBT, each advance of the tunnel boring machines (TBM) was documented particularly in geologically challenging sections like fault zones and in sections with homogenous geology, where every second advance of the TBM was documented. This is known as geological mapping. By mapping the rock face, critical physical parameters such as hardness, weathering, and groundwater conditions were measured, recorded, and managed through tunnel database software. Since different rock types require different cement specifications to stabilize the tunnel, geology must be documented and analyzed regularly. Information obtained here directly influences the safest excavation methods for the operational railway tunnels, the site's safety, tunnel support mechanisms, and long-term behavior and can allow for more accurate project costing.

So, in all matters crucial to megaproject delivery, designers are interested in considering the characteristics and properties of rock masses and faults at the earliest possible stage of underground construction. In one of the two exploratory tunnels, the Aica-Mules tunnel, along its 10.4 km span, 18 faults were found. At around 2.5 km into the tunnel, an unexpected fault was encountered, which caused some deformation and cracking in concrete segments. At about 6km into the tunnel, a 50 m deep fault was found along with significant water inflows. Deformations in certain areas along the tunnel axis almost caused a four-month shutdown of the TBM. Rock samples were taken continuously and sent for laboratory testing. From nearly 200 samples taken from this tunnel, an average rock density of 2.67 kN was determined with compressive strengths around 130 MPa. Samples were also taken of weaker rocks with an average compressive strength of 35 MPa. Whilst the boring of the exploratory tunnel set the project at an initial cost disadvantage, the comprehensive data and information on prevailing rock types, fault zones, and geologically hazardous sections are compensatory advantages [1]. 

I’ve scratched the surface of this project's day-to-day challenges, but it is worth mentioning just how experts and multi-disciplinary teams are rising to it. I get into that next.

Technical excellence & milestones reached

Once completed, the BBT will boast a complex system of rail tunnels composed of two main single-track tunnel tubes, each 8.1m wide and over 40m apart, connected with cross passages. The implementation of these tunnels is characterized by planning and construction lots, where exploration and construction occur. A third of the pilot tunnel is driven through by conventional blasting, with the remainder using either tunnel boring machines (TBM) or shield machines during exploration. 

By 2011, most of the preliminary work was undertaken, with approximately 15km of the exploratory tunnel driven through. By then, financing and the start of the building phase were approved, along with financing of preparatory construction work and further prospection. The main construction work for the main tunnels began in 2016.

A highlight of this megaproject was the use of a massive belt conveyor system to remove and relocate the excavated material. The conveyor system can remove 5,000 tonnes of rock per hour. Without this innovative feature, almost 200 trucks with a load capacity of 26 tonnes each would have had to drive in and out of the tunnel every hour to handle the amount of excavated material. Another win was that this conveyor system could be monitored and operated from a control center.

 of the project happened on July 6, 2020, with the breakthrough of the exploratory tunnel, a definitive milestone in part due to the speed of the TBM, which tunneled over 61 meters per day. This 26 m2 cross-section of the exploratory tunnel is unique in that it is crucial to both the construction and operational phases of the BBT. During construction, it provides essential geological and geotechnical information about the rock mass, weaknesses of fracture zones, and the location of any fault zones. This ensures that the main tunnels' construction risks are significantly reduced and provides information that can help optimize against areas of specific concern and sections where methods and materials can be altered, leading to potential cost savings.

Since stoppages on main tunnel construction can cost up to and over €40,000 a day, excavation work was also done to eliminate surprises when the main tunnels are bored. A team of geologists, geotechnical engineers, and hydrogeologists continually assess data from the exploratory tunnel and provide an updated design basis for the main tunnels. This forecast was initially done through surface mapping and point exploration through deep boreholes and then augmented with the information obtained from the exploratory tunnel. Particular attention is paid to considering fault zones that the exploratory tunnel encounters, with data, parameters, and essential geological, hydrological, and geotechnical information forming a fault catalog that is then used to form a model of the fault zones for the main tunnel forecast. 

This learning process is key to gaining significant insights into geotechnical conditions. Correlating this data with the parameters of the TBMs means that mechanized processes can be optimized more precisely during main tunnel excavation. This could be adapting the lining of the boring machines or even the partial omission of time-consuming probe drilling in areas of homogenous ground conditions.  It was found that after 50% of the exploratory tunnel had been dug, important optimizations of upcoming forecasts were made [2]. A major advantage of the exploratory tunnel today is that it can be the sole location for maintenance, cleaning, and inspection works completely independent from the main tunnels, without requiring their closure during operation. This would not have been possible had it not been for the findings gained during the construction of the exploratory tunnel.  

Another innovative technique used in this project for the portion of the tunnel under the Isarco River was a ‘

 adopted to address the unique complexities faced. One of the greatest risks during ground stabilization is that it can get washed away near a moving water source. These risks were mitigated by drilling the ground under the river and injecting it with chemicals and cement mixtures, which slowed the speed of the water. Drilled holes were then filled with liquid nitrogen, freezing the soil surrounding the injection hole for about 1 m on all sides. This makes the space between the river and the tunnel below impermeable to water providing greater resistance, strength, and stability to the ground surrounding the tunnel. 

The era of megaprojects is here, and many countries are accelerating major projects hoping for an infrastructure-led recovery from economic downturns. 

With more and more mega projects in the pipeline, the aims are often the same: spend big and spend quickly, create jobs, and stimulate the economy. Yet time and time again, history has shown that megaprojects take too long before they prove to be an effective stimulus and are often prone to ‘mega’ cost overruns. I’ve written an article about that, too, check it out 

Yet, the Brenner Base Tunnel has been in the pipeline for a very long time, with preliminary projects and prospection 

. Whilst all risk characteristics of megaprojects apply to this project, if forecasted and executed accurately and accordingly, the BBT would represent an important and strategic step forward for sustainable mobility in Italy, Austria, and Europe. The transfer of freight and leisure traffic from road to rail will significantly reduce CO emissions, increase transport safety, reduce noise exposure and air pollution, and be an iconic piece of infrastructure in Europe. Additionally, it creates a connecting axis for travel from Finland to Malta. It is one of the numerous sustainable mobility projects across the continent aimed at delivering systemic modal shifts in transport.

I’ll end on the myriad opportunities and innovations megaprojects like BBT can create. Researchers are looking into the prospect of using tunnel construction as an environmentally friendly energy source! When tunneling into a mountain, the structure often has to pass through or be designed to contend with groundwater and mountain water. The latter refers to rain or rainwater flowing from the mountain surface. This water seeps into a tunnel structure and has to be drained. Mountain water is warmed by the earth’s heat, creating substantial geothermal energy. Unlike solar panels, which achieve maximum efficiency when the sun is shining, or wind turbines which need wind to provide energy, geothermal energy is always there. Right now, discharged water from underground tunnels is usually cooled before being reintroduced into the natural environment. 

But what if we don’t cool it down? What if this heat is used as a source of energy? This is the challenge that researchers are 

. The prospect of using naturally heated tunnel water to provide energy for households and entire neighborhoods in cities like Innsbruck is groundbreaking. This is made more viable by how the project has undergone construction; an exploratory tunnel independent of the operational tunnel means that measures for heat recovery of drained water are realizable. But it is still early days, the tunnel structures in the BBT are still under construction, and its geothermal potential can only be estimated at this point, but as we often like to say: watch this space!

The global population is on the upswing, and with this growth comes mounting pressures on housing, transportation, and resource extraction. Not to mention the increasing impact on our natural and built environments. Super-sized projects to address rising demands have proliferated worldwide and are buoyed by growing economic, social, and environmental challenges. 

These massive ventures are better known as 

. Such megaprojects range from transport infrastructure to power plants, dams, power systems, ports, wind farms, and more! They’re typically contentious, hugely expensive, and can shape our modern landscape, igniting excitement and awe and eliciting criticism and even outrage.

So why are megaprojects so controversial? These projects are everywhere, on land, in the sea, in the skies, and beyond, dating back millennia. Yet, more recent endeavors seem to have ‘mega’ notorious track records for budget blowouts, schedule overruns, and broken promises upon delivery. So a list of failed megaprojects will probably be bigger than a list of successful ones based on these metrics alone. But despite this, the importance of these projects to cities, nation-states, and global and local communities is uncontested. Megaprojects are critical; they inspire, connect, redefine, and quite literally make and transform the environment in which they are placed. 

Safe to say that mega-projects are here to stay and will only get bigger. Egypt’s 

 with a price tag of US$40B, or the US$105B 

, are examples of huge mega-projects currently under construction. As aspirations grow and sites become increasingly constrained, project complexity and stakeholder considerations skyrocket, it’s critical that we know where these projects go wrong and figure out how to manage them better.

There are several ways to define a megaproject. It is not just scaled-up versions of a smaller project; it varies radically in complexity, stakeholder involvement, and lead times.

A standard definition is that megaprojects are large undertakings that cost US $1 billion and up, take years to plan, develop and build and involve diverse public and private sectors. Megaprojects are also complex and ambitious. Outcomes aim to be ‘transformational’, affecting millions and generating immense economic and social value. Megaprojects also typically span many domains; they simultaneously impact financial, economic, engineering, construction, and social and geopolitical dimensions. Positive outcomes typically strike a careful balance across all these domains.

The global market size for megaprojects is estimated at a whopping US $6-9 trillion annually, approximately 8% of the total global GDP [2]! To put that in perspective, it's equivalent to spending the US’s entire accumulated debt owed to foreign countries annually, which currently sits at US$7 trillion [3]. It’s big business!

Business sectors from infrastructure to industrial processing, water, and energy, IT, mining, supply chains, banking, defense, air, and space projects make business cases for megaprojects. They are increasingly becoming the preferred delivery model to meet the needs of these diverse and expanding markets. But the list could go on, and their scale might not always be apparent. Megaprojects also don’t only come in the form of massive civil infrastructure projects; for example, the setup and delivery of ICT infrastructure supporting national health schemes or pensions information is indeed a megaproject.

What factors make megaprojects alluring? From an engineering perspective, it’s likely technical aspects; projects of such scale excite engineers and technologists. Building the longest bridge, the tallest building, or the fastest aircraft are projects engineers build in their careers. But there are many more factors at work here. 

The most important factor, of course, is social. Developing, building, assessing, and maintaining infrastructure is often critical for governments and industries. For example, delivering water supply/treatment and sewage systems are crucial for sanitation and disease protection. Further, where would the Netherlands be if it weren’t for the ‘Zuiderzee Works’ or North-Sea and Delta protection works? Without this marvel of civil engineering, involving a man-made system of dams, land reclamation, and water drainage works, a lot of the country would be underwater! 

There are also economic incentives. Infrastructure projects at ‘mega’ scales can fuel economic activity for years. Consider Dubai’s International Airport; it is one of the busiest of its kind globally; it employs 21% of the Emirate and accounts for 27% of its GDP! [4]. Also, consider how much economic activity is generated in cities like Tokyo, Hong Kong, or Shanghai; a large portion of that is possible due to their efficient metro systems, all of which were constructed as a single or series of megaprojects. The economic allure and added value of job creation in construction and operation, combined with financial incentives for those directly involved, create a compelling case for megaprojects. 

This appeal is evident in the speed at which spending on megaprojects occurs. For example, between 2004 and 2008, China increased its expenditure twentyfold on infrastructure projects, spending more than it did during the entire 20th century [2]. 

Political motivations are also major factors in megaprojects. Endorsements and funding of major infrastructure development can generate excellent publicity for politicians and governments. Megaprojects have been credited with winning and losing elections on numerous occasions. 

Finally, there’s the aesthetic appeal; delivering something iconic, culturally significant, and beautiful is always a driving force in getting a project funded, designed, and delivered. 

Nevertheless, a Mckinsey Global Institute (2016) study warns that international investment figures in essential infrastructure such as transport, water, or communication systems are insufficient. It warns that investment will fall short of meeting expected population growth rates and increasing business demands.

‘Mega’ problems: pitfalls and challenges

What defines failure in the context of megaprojects? If we take it to mean falling short of meeting projected targets or planned objectives, then megaprojects have a substantial history of failure. Based on these metrics, megaprojects have a 65% failure rate! Indeed, the poor performance metrics don’t end there: research by Bent Flybjerg concluded that only 1 in 10 projects are delivered on schedule, and similarly, only 1 in 10 are completed within the projected budget. The most damning stat is that only 1 in 10 projects deliver expected benefits [5]!

‘Over cost, over schedule over and over again’ 💬 

These figures are pretty alarming, so why is this case? What are the root causes of such disappointment, and how can we improve these numbers?

First, we must delve into megaprojects' common challenges and pitfalls and how they translate to real-life scenarios. The first major feature and significant challenge of megaprojects are cost overruns. This is an extremely common denominator; you can see it clearly in the data. The graph below shows a list of global megaprojects by their respective percentage cost overrun; notice how far back in time it goes.

Similarly to cost overruns, there’s the big issue of benefit or demand shortfalls compared to targets and promises. There’s often a disconnect between the reality of megaprojects and the extensive analysis which forms the backbone of planning and decision-making. For example, major rail projects experience an average cost overrun and demand shortfall of 44.7% and 51.4%, respectively [2]. On average, roads and other transport projects experience a cost overrun of 20.4%. The magnitude of these errors suggests that business forecasts, cost-benefit analyses, and other relevant projections are often misleading and overly optimistic. 

Megaprojects are risky and uncertain because they take so long to complete. Dealing with this uncertainty in projections and analyses requires assumptions and assertions, so it’s no surprise there are a lot of surprises upon completion. But historical data would suggest that such assumptions are consistently overly optimistic and geared toward selling the project rather than accuracy. As a result, we’re left with strategic misinterpretations, optimism bias, overconfidence, and underestimation of timelines and budgets. This has undermined megaprojects time and time again. 

Clearly, megaprojects tend to repeat the same mistakes. But as megaprojects continue to be the go-to solutions for emerging economic and social problems, what should be our strategy for success?

We see time and time again the miscalculation of forecasted costs and an overzealous pitch of the benefits a major project can bring. The economic risk and consistent waste of resources that comes with megaprojects beg the question: would a scaled-back version of the project be a more viable option? 

Misallocations can be avoided through meticulous cost forecasting methods, especially in real-time, as forecasts on a project are made or updated. Furthermore, it becomes clear that there has to be a more effective alignment of stakeholder goals, strategies, and interests. This means that all stages of project development should engage internal and external stakeholders and be able to receive, send, and access the necessary information and data in real-time. What kind of forum or platform could facilitate this large-scale transfer, access, and coordination of stakeholders, technology, and data? We’ve written an article about it! Check it out 

There’s also the tried and true: learning from past mistakes. This seems to be overlooked or deliberately glossed over, and it is sometimes hard to determine why. The best estimate is that some megaprojects, in their scale and promise, are believed to be the first of their kind, so little weight has been put on aggregating past insights. However, there are often similarities between megaprojects that would allow some organizational learning, albeit differences in governance, standards, stakeholders, or sustainability commitments. Again data integration could solve this problem, creating an accessible repository of data, designs, processes, constraints, and solutions to allow project managers, governments, investors, etc., to make informed decisions from reliable and acquired insights. A problem we’re actively working on.

In Australia, the Grattan Institute issued a report that discussed how government spending on megaprojects has never been bigger, yet they have been poorly considered most of the time. Indeed, as populations and cities grow and more pressure is put on natural and built environments, the business case for megaprojects becomes increasingly compelling. Yet the reality is that investment spending is driven not by rigorous evaluation of the costs, benefits, social, environmental, and risk impacts but by political agendas. 

The report argues that governments are too quick to commit to major infrastructure projects before a thorough and independent evaluation of the proposals and a business case are appropriately developed. Not doing so is a precursor to all the challenges and pitfalls we’ve already discussed, and will find the same issues being rehashed in a costly and unfortunate feedback loop.

We want to see accurate social and financial projections, precise forecasts, and pragmatic and precautionary approaches that identify, define, and assess the risks and the expected benefits.

Given the accelerating support for increased ecological consciousness in governmental and regulatory actions, sustainable and environmental best practices are becoming significant concerns for megaproject stakeholders. One of the significant challenges facing megaproject development today is community member engagement and reliable business cases that prove that such massive undertakings are economically, socially, and environmentally sustainable. More and more stakeholders, including local, national, and even international communities, want to implement environmental sustainability into megaprojects’ development. Engaging with these stakeholders, a major social sustainability pillar provides organizations with tremendous insights into designing, building, planning, and executing megaprojects. This ensures that decisions are responsive to community needs and helps avoid benefit shortfalls. Another ‘collateral benefit’ [8] that can come from this is public approval and demand for the kinds of services that megaprojects can provide.

Like all projects, megaprojects must reach outcomes that find a medium between sustainability and economic value. Flybjerg contends, 

‘A megaproject may well be a technological success but a financial failure’,

 so striking a balance between technological, social, and financial success is paramount.

It begs the question of whether megaprojects should be a first or last resort. We benefit from being able to look back and learn from the past and to be able to see the magnitude and frequency of cost blowouts to make sound and studied decisions, yet there have never been more mega projects in the pipeline than now. In Australia, there are over nine major transport projects worth at least AU$5 billion under construction, a scale and number unthinkable a decade ago. These include the Inland rail, Sydney Metro City, and Melbourne’s North-East link. All initially cost almost half their current cost estimates [9]. Big projects are risky, and we’ve seen the pitfalls they can fall into. So, it leaves us wondering whether countries and governments would get a bigger bang for their buck by upgrading existing civil and social infrastructure instead of overcommitting on promises that megaprojects, just by nature of their scale, magnitude, and complexity, can’t let them keep. 

So, we’ve seen the pitfalls of poor project management and business process fragmentation; if only there were something to help! Look no further, CalcTree has the toolbox you need.

Bridges are a part of human history. Engineers have revolutionized bridges, from simple wooden log bridges to ultra-slender suspension bridges that span kilometers. This development is the accumulated lessons of endless experiments and failures. Before we touch on a particular failure that changed bridge design forever, let’s rewind and observe the evolution of bridges:

 of all types have been constructed or formed naturally and used by humanity for centuries. From the improvised and sometimes natural wooded log crossing over rivers and streams to modern 30+ meters wide structurally designed wooden bridges, this is one of the oldest forms of bridges in human history. 

2. Popular in ancient Rome from around 100 BC [1]. The Romans were famous for using 

 structures to design aqueducts. These marvels are still around after 2000 years and inspire current viaduct designs. With the appearance of steel and reinforced concrete, modern arch bridges can span anywhere between 20 meters to 200 meters. Some arch bridges can achieve even longer spans by incorporating suspension cables or truss designs. 

 was most likely constructed in the 13th century. But truss bridges didn’t rise to prominence until the 19th century when engineers and architects realized that trusses are easy to build, require fewer materials, and are efficient over long spans. It enabled some of the world's architectural marvels like the Eiffel Tower and Ikisuki Bridge. The Ikisuki Bridge in Japan is the world’s longest-spanning truss bridge, reaching over 400 meters in length.

 became popular in the early 1800s. This is due to increased economic demands for longer-span road connections, especially in the US. The longest-spanning suspension bridge is more than 2000 meters. This accolade belongs to the 1915 Çanakkale Bridge in Turkey.

Looking at the most modern suspension bridge types, are magnificent and elegant but also intimidating. Citizens or even engineers sometimes feel uneasy crossing that seemingly fragile thread and wonder why it has not been blown away by the wind yet. That is the case with the Tacoma Narrows bridge in 1940, a landmark structural collapse on a windy day. This has resulted in textbook lessons and stricter engineering tests for suspension bridges. 

With that short history lesson complete, let’s explore the valuable engineering lessons from this iconic structural failure.

A famous bridge and a famous bridge collapse

In 1940, the Tacoma Narrows Bridge was counted as one of the superstructures of the time. It was the world’s third-longest suspension bridge, with a total length of 1810 meters. 

During its short 4-month life span, the bridge had been swaying up and down noticeably even at average wind speeds. Despite several efforts to stabilize the bridge, it later collapsed, experiencing 70 km/h winds with a 40 cycles/minute gust frequency. 

On the day of its collapse, onlookers were shocked to note a drastic change in its deflection motion from up and down wave-like movements to rigorous twisting motions - When the left side of the structure went down, the right side would rise and vice versa. You can observe the twisting and oscillating motions of the bridge below.

Tacoma Narrows Bridge Collapse "Gallopin' Gertie"

These motions exposed the structure to two failure mechanisms; 

. Eventually, the main support of the suspension bridge, the cables, could not handle the sway and failed individually. The central deck fell into the water with the reduced capacity of key structural cables, bringing about a dramatic and phenomenal end! 

Through investigations, engineers learned that the bridge underwent overly aggressive ‘cost-saving’ efforts during design. Without these cost savings, key elements would have had more capacity and performed better under these extreme loads. If the designers put less emphasis on cost savings and aesthetics and more on dynamic analysis, the story could have been quite different!

Let’s take a moment to explore some of the key concepts at work here…

The continuous wave-like motion of the bridge can be explained by a natural phenomenon called resonance. This is where an external force imposed at a particular frequency excites an object at its 

. This excitation at the natural frequency causes an amplification of displacement. Every object’s natural frequency is determined by its shape, mass, and material properties. 

Natural frequency is also when an object will remain vibrating if it’s excited at the same frequency by an external force. 

This can happen when objects are subject to dynamic loads. Dynamic loads can be anything from gusts of wind to someone walking down a hallway. Unlike sustained static loads, dynamic loads cause vibrations in the subjected object or systems they’re imposed upon. In most instances, vibrations aren’t observable, but a great visual example of such vibrations occurs when a guitar string is flicked, and you can watch its resonance vibrations as its strings oscillate back and forth.

The charts below show that when the resonance reaches the natural frequency, the amplitude of vibration (how strong the object vibrates) is much higher than the normal amplitude.

In the case of a guitar string, the result of the vibration is a pleasing (or terrible) sound. In the case of the Tacoma bridge, the result was large-scale resonance, resulting in the violent swaying of the bridge deck and eventual structural collapse! 

Well, why was that the case? Surely a bridge can withstand design wind loads? Normally, yes, but in Tacoma's case, the wind's vibration frequency was the same as the bridge's natural frequency. As a result, resonance was induced in the bridge, and the experienced vibrations were amplified and greatly exceeded the expected vibrations. In turn, key structural elements were subject to a greater magnitude of forces and deflections than they were designed for. They then failed, and the entire bridge collapsed. 

Every structure has a natural frequency, and engineers ensure experienced vibrations under dynamic load are within allowable limits. And in modern design, great care is taken to ensure possible excitation frequencies do not converge on a structure's natural frequency. In the case of Tacoma, this didn’t happen!

The twisting motion of the bridge can be attributed to another phenomenon - aeroelastic fluttering. Wind airflow can be categorized into the turbulent flow (chaotic and unstable) and laminar flow (stable). 

In the case of Tacoma, when hitting the bridge from the side, the wind flow was separated above and below the bridge, which created vortexes and turbulent flows. This chaotic airflow further accentuated the bridge’s deflection, and the different vortexes below and above the bridge caused the bridge's two sides to move up and down violently. 

The substructure, cables, bridge deck, and towers were not designed for these intense movements, and the extreme forces and fatigue induced in all elements sent the entire bridge to the scrap heap! 

Why did other bridges with similar spans not encounter aeroelastic fluttering? Simply because the Tacoma bridge incorporated an ‘innovative’ idea that used plate girders to support the road deck instead of the originally proposed truss system. 

Wind can easily pass through the truss structure for open trusses, remaining laminar and imposing minimal load. At the time, the importance of this aspect of truss supports on bridges had not yet been realized. The original application of trusses was only to support the bridge's side-by-side movement and vertical deflection, not to accommodate the wind. Furthermore, typical plate girders bridges were usually much shorter in length. 

Plate girders, however, are an 8-foot solid web, not allowing airflow to pass through it and forcing wind flow to go around it. This generated turbulent flow and caused fluttering. 

Ironically, the Tacoma Narrows Bridge’s original design included trusses below the deck. The reason for the switch to the plate girder solution was not only for aesthetic reasons but to reduce costs! The result? An unprecedented slender bridge with a depth-to-span ratio of 1:350 and width-to-span ratio of 1:72 with a construction cost of $6.4 million. 

Comparatively, a suspension bridge with a similar span, the Bronx-Whitestone, cost $19.7 million to build and has a depth-to-span ratio of 1:209 and the width-to-span ratio of 1:31. One collapsed, and one didn’t…

Utilizing the plate girder on a longer, taller bridge meant that the Tacoma bridge engineers could reduce the complexity and cost of their design. But it exposed the bridge to a series of unknown risks. While the results were catastrophic, the. That is why the failure of the Tacoma bridge incident has opened up a whole new perspective for wind considerations in structural design.

How would modern engineers reconstruct the bridge?

As it turns out, modern engineers have already reconstructed the bridge twice! In 1950, the ‘new’ Tacoma Narrows Bridge was opened. In the 1990s, as the traffic increased, the government decided to build another next to the second Tacoma Narrows Bridge, completed in 2007. 

With the lessons from the 1940s disaster front of mind, both bridges have incorporated deeper trusses to let the wind pass through under the deck! Fortunately, both new bridges still serve local transportation safely to this day.

Some forgiveness should be given to the designers of the Tacoma bridge. After all, it was mostly designed by hand! But if we were to design it with today’s design methodology, what processes and tools would we use?

Wind tunnel testing is a common experiment to simulate the actual wind situations (speed, flow, directions) and reactions of a structural model (vibrations, deflections, creep) in the laboratory during the design process.

Wind tunnel tests are performed in addition to computer simulations when there is uncertainty in design, such as applying new construction techniques, new materials, or building large-scale constructions. The test is usually applied for structures exposed to strong wind, such as skyscrapers, airplanes, and suspension bridges. 

After the Tacoma bridge collapsed, the U.S. Government demanded that all bridges built with federal funds should have to conduct wind tunnel testing. Nowadays, U.S. engineers follow 

ASCE/SEI 49-12 Wind Tunnel Testing for Buildings and Other Structures

 to conduct the test. And all jurisdictions have similar codes that account for the local environment. You can see how Wind Tunnel testing was done in practice with the longest suspension bridge in the world, the 1915 Çanakkale Bridge, 

When the U.S. government decided to build the new twin Tacoma bridge next to the 1950 bridge in the 90s, both the new and existing bridges underwent 

. The purpose was to determine how the new bridge influenced the aeroelastic effects on the existing bridge. This consideration demonstrated how engineers have taken past lessons and applied them to subsequent designs. 

Generally, a wind tunnel test for a long-span bridge includes:

Before the historical Tacoma failure, engineers still used the ‘deflection theory’ to design suspension bridges. The theory only accounts for deflection under gravity and static wind loads, leading to the insufficient conclusion that the longer the span, the less stiffness is required for the deck. However, just because the bridge failed does not mean the theory was completely rejected. The deflection theory is still largely used in suspension bridge design regarding static load analysis and cable stress analysis. It especially helps Finite Element Analysis (FEA) develop as one of the most common tools for engineers to calculate deflections, deformations, stress, etc. 

Nowadays, FEA theory has been applied widely as a calculation mechanism behind engineering 

 with much higher accuracy than hand calculations. Almost every structure around the world in recent years, including long-span bridges, utilize FEA software for design.

Nowadays, in bridge design, engineering simulation plays a crucial part in the testing process. If an experimental test cannot be carried out, CFD software can simulate wind load conditions with high accuracy on a computer. 

Combining the CFD software to simulate the complex wind load and the FEA software to investigate the structural behaviors, the designs of the Tacoma bridge today would be much more thorough and result in a far more robust structure.

Not only have bridge designs evolved, but so have the way ways in which they are maintained. Vibration analysis is a method to detect bridge damage by monitoring vibration levels and investigating vibration patterns. Vibrations are collected using various sensor types installed throughout the structure. Sensors measure vibrations as a result of winds or traffic. 

This technique is commonly used in railway bridge maintenance because they experience stronger vibrations at a higher speed [2]. The sensor will present the vibrations under the amplitude-frequency graph, which can detect abnormal vibrating patterns. For instance, a crack developed on a roller bearing outer race will lead to periodic collisions with bearing rollers.

This allows maintenance teams to identify small local issues before they proliferate into large macro issues that impact asset performance.

How has the Tacoma bridge impacted structures today?

The Tacoma Narrows Bridge failure shocked engineers of the day. While costly, it did provide many lessons and sparked many studies that advanced structural design. Countless structures benefited from these lessons, so here are just a few close calls! 

This suspension bridge finished one year before Tacoma Bridge in the US and also used solid plate girders. After Tacoma collapsed, the Whitestone Bridge was immediately strengthened by adding trusses and openings below road decks to let the wind pass through. This decreased the oscillations and extent of deflections experienced by the structure, and the bridge is still in use even today.

The Tacoma collapse revealed how wind-sensitive long-span bridges could be. As a result, it kick-started the growth of bridge aerodynamics-aeroelastic study, an area of research that influenced the design of long-span bridges for years to come [3]. In the next three years after the failure, the designer of the 8,038-metre-long Mackinac Bridge, David Steinman, published a theoretical analysis of suspension-bridge stability problems. The paper recommended that future bridge designs include deep stiffening trusses to support the bridge deck or an open-grid roadway to reduce wind resistance. The Mackinac Bridge, finished in 1957, achieved both features in its design, and it is still the longest suspension bridge between anchorages in the Western Hemisphere until now. 

Every suspension bridge since then is either designed as aerodynamically streamlined, stiffened against the torsional motion, or both. This opened up new areas of study, new design methods, and testing regimes that have enabled modern bridges to reach farther and be safer.

Civil Engineering works have a history tracing back to 4000 years ago since the construction of the Great Pyramids. Until now, the field is still amazed by the magnificent structure in the past. And yes, people are still debating whether we can rebuild the pyramids in the modern day. This left us wondering, would it 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 in history

One exemplary case study of any structural failure is the Tower of Pisa, or as it has been famously known, the 

 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 not knowledgeable about 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 foundations being only 3 meters deep on soft soil, supporting 14,700 tons of masonry. This 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:

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 $27 million was spent to stabilize 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 and labour cost). So we could have built six straight towers of Pisa instead of trying to save one leaning tower of Pisa! 

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. 

The 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. When maintaining the tower, engineers 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 judgments about types of foundations and suitable construction techniques.

Modern machinery 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 large temporary loads on it before construction. Why? It is a way to compact the soil, or in other words, make it stronger, denser, and more stable. This is particularly useful for lightweight infrastructure projects such as roads and pipelines because the soil can be compacted with a similar load level before construction using construction machinery such as road rollers. For larger construction such as production plants, a hefty weight is lifted and thrown on the ground to make it compact through large dynamic impacts. This approach 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 it suddenly lean 10 mm towards the South. This proves how sensitive the tower was at the time, and any construction methods must 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:

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[2]. 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 on a tray. So making a deep and spread-out foundation might not be the answer.

A deep foundation is one of the more modern foundation types. Luckily, it is a technique that allows us to build taller, enabling slim but incredibly tall skyscrapers that typify our cities. 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, the load above is too much, or there are constraints on footing size. Sounds like the Tower of Pisa?

Engineers typically prioritize the use of pile foundations for high-rise structures. However, in the case of the Tower of Pisa, its weight exceeds that of other similarly sized structures, and drilling piles to the necessary 40-meter depth (depth of sufficiently hard material) for a 57-meter structure is impractical due to the lack of firm soil at that depth. By contrast, the Burj Khalifa, standing at a towering height of 848 meters, has a foundation depth of only 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. 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 are 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 

. 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, and made up of mostly stiff clay. This led engineers to overestimate the design capacity, which was supposed to be smaller because there was a soft clay layer below the stiff clay. This flawed assumption made the structure collapse on its self-weight when the soil was not as strong as people thought. If a deeper investigation of the soil strata had been carried out, the soft clay condition below would have become known, and more suitable building techniques could have been 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 and skills and learning from past mistakes to minimize construction errors in design. As a construction tech company, CalcTree’s vision is to be a platform where engineers can automate and standardize their design process to create a safer, more certain, and more time-saving design process.