Construction of Aqua Claudia and its Longevity  

Written by: Glen Dawson

Introduction

The ancient Roman Empire is famous for its many impressive achievements, and one of the greatest examples is their system of aqueducts. These large, complex structures were built to carry water from distant sources into cities, ensuring that people had access to fresh water for drinking, bathing, and other daily needs. Among the most remarkable of these aqueducts is the Aqua Claudia (figure 1), an ancient water supply system that stretches nearly 43 miles and once delivered over 6.5 million cubic feet of water to the city of Rome every day (Dembskey 2009a).

Built between AD 38 and AD 52 during the reigns of Emperors Caligula and Claudius, Aqua Claudia was the eighth of eleven aqueducts that Rome would eventually build (Aicher, 1995). At the time of its construction, it provided about one-fifth of Rome’s total water supply (Dembskey, 2009a). Today, nearly two thousand years later, sections of this aqueduct are still standing, with one of the most well-preserved parts being the viaduct that runs through the Park of the Aqueducts in modern-day Rome. This towering structure, reaching heights of 90 feet (figure 2), is a major tourist attraction and an iconic symbol of Rome’s engineering expertise.

But why is Aqua Claudia still standing after so many years? The answer lies in the construction methods and materials used by the Romans. This paper explores the design, construction techniques, and engineering principles that made Aqua Claudia so durable. By focusing on the viaduct through the Park of the Aqueducts, we will see how Roman engineers created a structure that could stand the test of time.

Research Question

Why is the nearly two-thousand-year-old Roman aqueduct Aqua Claudia still standing? This article will explore the construction techniques, materials, and engineering methods used to build this aqueduct, with a focus on the viaduct through the Park of the Aqueducts.

Methods

This study combines observations with historical research on Roman engineering methods. By looking at how the aqueduct was designed and built, and comparing these techniques to modern engineering practices, we can gain a better understanding of why Aqua Claudia is still standing.

To explore this topic, the following steps were taken:

1. Site Visit: I visited the Park of the Aqueducts in Rome to observe the Aqua Claudia viaduct in person. This gave me a chance to study the structure’s design, materials, and current condition.
2. Historical Research: I gathered information from historical texts and modern engineering studies about how the Romans built aqueducts. This included reading about the materials they used and the challenges they faced.
3. Engineering Analysis: I applied basic engineering principles to understand how the aqueduct’s arches and materials contributed to its stability and longevity.
4. Comparison with Modern Techniques: By comparing the construction methods of Aqua Claudia with modern engineering practices, I explored how these ancient techniques influenced the way we build today.

Results

Observations 

When I visited the Park of the Aqueducts in September, 2024, the Aqua Claudia aqueduct was impossible to miss. A long section of viaduct with multiple arches stretches across a wide open plain. It’s built from large stone blocks that form the pillars and arches holding up the water channel above (figure 3). Each block is about 2 feet tall, and the arches stand roughly 20 feet high from the ground to the top. The pillars are about 10 feet wide and 8 feet deep, making the structure look massive and solid. While much of it remains intact, there are sections that have collapsed or are missing, but you can also see repairs made with newer bricks and materials in some areas (figure 4). Despite its age, the structure still dominates the landscape, showing just how well it was built.

Not only is the structure impressive in size and scale, the gracefulness of the arches at Park of the Aqueducts gives it a stunning look, and the straight line of the top channel contrasts with the repeating semi-circles of the arches. Each arch acts like a doorway to frame the surroundings, and you can walk through them from one side to the other and view the viaduct from many angles.

Roman Aqueducts: An Overview

Aqueducts were one of the most important pieces of infrastructure in ancient Rome. As the population of Rome grew, so did the need for freshwater. At the peak of the Roman empire, about a million people called Rome home (Turismo Roma, 2024) and bringing in water for that number of people required a major public works system. Aqueducts were built to transport water from natural springs and rivers outside the city into Rome. The water was used for drinking, bathing, and in public fountains, baths, and toilets. Having an adequate supply of clean water (that helped constantly drain the sewers) also helped Rome be a notably clean city (Aicher, 1995).

Though the Romans didn’t invent the idea of aqueducts, they made significant improvements to the design of them, and built them on a massive scale. By the time Aqua Claudia was constructed, Rome already had seven aqueducts supplying water (Dembskey, 2009a). But Aqua Claudia was larger and more advanced than many of its predecessors. Eventually, Rome would have 11 aqueducts (Aicher, 1995).

Aqueducts were made up of a series of underground pipes and above-ground structures, like bridges and viaducts, to carry the water over uneven terrain. These structures were often built with arches, which provided strength and stability while reducing the amount of material needed. The water was carried in channels that maintained a gentle slope so gravity could pull the water from the source to its final destination.

Aqua Claudia: Historical Context

Aqua Claudia was commissioned by Emperor Caligula in AD 38 and completed by Emperor Claudius in AD 52. This aqueduct was built to bring water from springs near the town of Subiaco, about 43 miles away from Rome, to the city. The aqueduct crossed a variety of landscapes, including hills, valleys, and plains, which meant that it required both underground tunnels and above-ground bridges to carry the water.

One of the most famous parts of Aqua Claudia is the viaduct that runs through what is now the Park of the Aqueducts in Rome. This section of the aqueduct was built to cross a low-lying valley, and it consists of a series of tall arches that support the water channel above. Today, these arches are still visible and are one of the best-preserved parts of Aqua Claudia.  

How Arches Work

The arch is the key structural element that allowed Aqua Claudia to stand tall and strong for so long (figure 5). While the Romans didn’t invent the arch, they perfected its use in large-scale construction projects. An arch is a curved structure that spans an open space and supports weight by distributing the forces along the curve of the arch. Arches also allow the builder to use much less material than what would be required for a solid wall. This made this type of structure both strong and efficient. In Aqua Claudia, the viaduct’s arches support the weight of the water channel above. 

Arches are effective because they convert the weight of the structure (called load) into forces that are spread out along the curve. Instead of pushing down in one spot, the weight is spread evenly along the sides of the arch. This helps prevent any one part of the structure from carrying too much weight, which reduces the risk of collapse.

The most important part of the arch is the keystone (figure 6), the stone at the very center of the arch. The keystone locks all the other stones in place, making the entire structure stable. Its unique wedge shape, wider at the top and skinnier at the bottom, keeps everything in place through the use of compression caused by the load. In Aqua Claudia, the keystones and other stones were carefully cut and placed to ensure the arches could carry the load of the water channel above.

Materials Used

Aqua Claudia’s viaduct was built primarily from stone, a material chosen for its strength and durability. The Romans used locally available stone, which reduced the cost and effort of transporting materials. The stones were cut into precise blocks, which were then fitted together to form the arches and other parts of the aqueduct. For the most part the blocks were simply set in place without concrete. One exception to this was for the water channels which were coated in some form of waterproof cement (Deming, 2019). Lead pipes were also used in ancient Roman times, but these were incapable of handling high pressures. 

Tuff and Travertine

Two types of stone were commonly used in Roman construction: tuff and travertine. Tuff is a soft volcanic stone that’s easy to cut and shape, making it ideal for the core of the aqueduct. Travertine, a harder and more weather-resistant stone, was often used for the outer surfaces of the aqueduct. 

Tuff is soft volcanic rock that was plentiful around Rome. It is easily cut into shapes, such as the ashlar blocks with a rectangular shape used in Roman construction. Although tuff is soft, after being quarried tuff hardens with exposure to air (Kings College London, 2024). In Aqua Claudia, the Romans used tuff for the interior parts of the arches and the water channel, while travertine was used for the exterior, especially in areas exposed to the elements (Deming, 2019). This combination of materials helped the aqueduct withstand both the weight of the structure and the effects of weather and erosion over time.

Roman stonework was done in different styles for different purposes (Wright, 2009). For aqueducts, a style called opus quadratum was used (Aicher, 1995). This involved using square stone blocks with no mortar. Opus signinum was a process of making waterproof mortar utilizing pulverized pottery or bricks (Dembskey, 2009a). This was used for lining aqueducts.

Construction Process of Aqua Claudia

The construction of Aqua Claudia followed a well-organized process. First, the route of the aqueduct was carefully surveyed (Dembskey, 2009a) to ensure that the water could flow downhill at a steady rate (figure 7). This slope was crucial because the Romans relied on gravity to move the water—there were no pumps or mechanical devices to help. 

Once the route was planned, the foundations were laid. In areas where the aqueduct needed to cross valleys or other low-lying terrain, tall viaducts with arches were planned. The Romans used wooden frames to hold the stones in place while building the arches (Dembskey, 2009a). The stones were then cut and placed layer by layer until the arch was complete.

Once the arches were built, the wooden supports were removed, and the weight of the structure was transferred to the stones themselves. The water channel, or specus (figure 8), was then constructed on top of the arches (Dembskey, 2009a). The Romans lined the channel with a waterproof coating to prevent leaks.

The Romans were known for their ability to adapt their construction methods to different challenges. For example, in most areas, the aqueduct was built underground to protect it from weather and damage. In other areas, like the Park of the Aqueducts, large viaducts with arches were built to carry the water across low grounds.

One of the most impressive aspects of Aqua Claudia is how the Romans were able to maintain a gentle slope over such a long distance. This required careful planning and precise construction. The Romans used simple surveying tools, like the groma and the dioptra, (Dembskey, 2009a) to make sure the aqueduct followed the correct slope from the springs to the city. 

Modern Water System Construction Process  

Cities still need to bring large quantities of freshwater from distant sources to population centers. Today’s water systems rely on pipes that have the ability to withstand high pressures. Strong pipes allow water to be pumped, or to let gravity push water through low areas and back up inclines. The availability of electricity makes pumps possible, which allows water systems to convey water uphill over obstacles or to storage areas. The use of metal as a building material allows pipes and supporting structures such as bridges and trusses to use a minimum amount of material – much less than stonework. Metal pipes and trusses can also flex, allowing for some resilience during earthquakes. Pipes allow water to be conveyed through covered ducts and are therefore not exposed to possible contamination. 

Discussion

Aqua Claudia’s Longevity

Considering the things I learned about Roman construction, the observations I made, and what I know about engineering, there are several factors that have led to the longevity of Aqua Claudia. 

Overbuilding 

Without the use of modern day math and physics principles the Roman would have had no way of knowing how big to build structures. Without understanding the principles of physics, they could not calculate loads mathematically. In order to account for this they ended up overbuilding everything by a large factor to err on the side of safety. While having a margin for safety is still a good engineering principle, a modern structure to convey this amount of water would require far less material due to more sophisticated methods.

Earthquake Resistance

Rome is located in a region that experiences earthquakes, including several of magnitude 6 or greater (Earthquakelist.org, 2024). Over the centuries, many buildings in Rome have been damaged or destroyed by seismic activity. Harries (1996) states that, although the Romans may have come across certain earthquake resistant techniques by serendipity rather than any understanding of seismic forces, such practices as securing structures to strong foundations helped form the basis of what evolved over time to become earthquake resistant design. The standing portions of the Aqua Claudia aqueduct are evidence that its construction has been earthquake resistant. The design of the arches helped the aqueduct withstand the shaking caused by earthquakes, although modeling seismic forces on the structure show that it has limits (de Felice and Mordanova, 2024). 

When an earthquake occurs, the ground moves, creating forces that can cause structures to collapse. Precise stonework without mortar in Aqua Claudia could have helped the structure flex slightly, absorbing some of the energy from the earthquake. Also, the fact that the structure was overbuilt probably helped it survive over time. 

Use of Durable Materials

Aqua Claudia has been exposed to rain, wind, and changing temperatures for nearly two thousand years. These environmental factors can cause significant damage to stone structures, but the materials and construction techniques used by the Romans helped protect Aqua Claudia from erosion and weathering.

The choice of stone was critical to the durability of Aqua Claudia. By using tough, weather-resistant stones like travertine on the exterior (figure 9), and softer but strong tuff on the interior, the Romans created a structure that could stand up to the elements and resist wear over time. The use of travertine on the outer surfaces of the aqueduct helped protect it from weathering because travertine is highly durable (Angeles García-del-Cura, et al., 2012). Although travertine is dense and durable, it also is highly workable and can be shaped precisely. This allowed careful fitting of individual stones which also helped achieve durability. Additionally, the Romans lined the water channel with a waterproof coating, which prevented leaks and protected the stones from being worn away by water over time.  

Design

Careful design helped ensure that the structure would function as intended, transporting water for 43 miles without failure. This required sophisticated planning and quality of workmanship over the course of a long, complicated project. The project required a large number of workers organized into well-defined divisions of labor, and also required many skilled and trained people (Dembskey, 2009a). The project also spanned different reigns of emperors, and therefore required a high level of commitment to the goal. The high level of commitment also probably led to a desire to maintain the aqueduct over a long period of time. 

Load Distribution

The key to Aqua Claudia’s long life lies in the way it was designed to distribute weight. The arches spread the load of the structure evenly, which prevented any one part of the aqueduct from being overstressed. This helped to avoid collapse, even under the weight of the water carried by the aqueduct.

A side benefit to using arches instead of solid walls to cross valleys is that the arches provide a way for streams or flood waters to pass though. Having openings meant the aqueducts were less likely to fail from flood damage. Likewise, people could pass through, and so the aqueducts did not impede travel.

Regular Maintenance

Even though Aqua Claudia was built to last, it still required regular maintenance during the Roman Empire. Workers were employed to inspect the aqueducts, remove debris, and repair any cracks or damage. A puzzling aspect of Aqua Claudia is that there is evidence that, 8 years after it was completed, it went out of use for 9 years (Dempsky 2009b). Dempsky offers several possible explanations, including damage from natural disasters, but also notes that the repair was of high enough quality that the aqueduct was put back into use more than 700 years after it was first constructed. After the fall of the Roman Empire, many aqueducts fell into disrepair, but parts of Aqua Claudia were maintained and repaired over the centuries, ensuring its survival. I observed repairs that had been done with bricks (figure 10 and 11) well after it had been retired. 

Hydraulic Considerations

Romans built structures with certain considerations for the forces of water, which they may have learned through trial and error (Chanson, 2000). Chanson hypothesized that steep chutes, cascades and dropshafts were incorporated into aqueducts in ways that helped dissipate energy, and increase oxygenation, which helped maintain water quality.

Environmental Considerations

The Romans also paid close attention to the environment when building Aqua Claudia. The aqueduct was designed to withstand rain, wind, and temperature changes, with materials and construction methods chosen to protect the structure from erosion and damage over time. Holding tanks were added to the design to allow sediment to settle out, keeping the water clean. 

Lessons for Modern Engineers

Aqua Claudia offers important lessons for modern engineers. First, the importance of weight distribution in large structures is clear. By spreading out the load, engineers can create more stable and durable buildings. Second, the choice of materials is crucial to the long-term success of a structure. Quality and durability of materials used determine how long a structure that is exposed to the elements will last. Finally, the flexibility of a structure is important, especially in earthquake-prone regions. By building a sophisticated, durable work lasting for such a long period of time the ancient Romans have inspired countless generations of engineers.

Conclusion

Aqua Claudia is a powerful example of Roman engineering skill. Nearly two thousand years after its construction, this aqueduct remains a symbol of the Romans’ mastery of architecture and engineering. By using arches, durable materials, and careful construction techniques, the Romans created a structure that has stood the test of time.

For modern engineers, Aqua Claudia offers valuable lessons about the importance of design, materials, and maintenance. The principles behind this ancient aqueduct are still relevant today, showing that great engineering can create lasting structures that endure for centuries. As a student of engineering, studying Aqua Claudia has provided valuable insights into how ancient techniques can inform and inspire modern construction. 

References

Aicher, P. (1995) Guide to the Aqueducts of Rome. Bolchazy-Carducci. https://books.google.com/books/about/Guide_to_the_Aqueducts_of_Ancient_Rome.html

Angeles García-del-Cura, M., Benavente,  D., Martínez-Martínez, J., Cueto, N. (2012). Sedimentary structures and physical properties of travertine and carbonate tufa building stone. Construction and Building Materials, 28(1) 456-467. https://doi.org/10.1016/j.conbuildmat.2011.08.042

Chanson, H. (2000). Hydraulics of Roman aqueducts: Steep chutes, cascades, dropshafts. American Journal of Archeology. 104(1) 47-72. Retrieved from http://www.jstor.org/stable/506792?origin=JSTOR-pdf

de Felice, G., and Mordanova, A. (2024) Seismic Assessment of the Claudio Aqueduct. Roma Tre University Engineering Department. Retrieved from https://www.romatrestrutture.eu/ricerche/the-claudio-aqueduct/

Dembskey, E. (2009a). The Aqueducts of Ancient Rome. University of South Africa. Retrieved from https://uir.unisa.ac.za/handle/10500/2624

Dembskey, E. (2009b). The Aqua Claudia Interruption. Acta Classica, 52 73-82. https://www-jstor-org.offcampus.lib.washington.edu/stable/pdf/24592485.pdf

Deming, D. (2019) The Aqueducts and Water Supply of Ancient Rome. Groundwater, 58(1) 152-161. https://ngwa.onlinelibrary.wiley.com/doi/full/10.1111/gwat.12958

Earthquakelist.org. (2024) Rome Earthquake Report. Retrieved from https://earthquakelist.org/italy/lazio/rome/

Harries, K. (1996). Earthquake Resistant Construction in Classical Rome. Elsevier. Retrieved from https://www.iitk.ac.in/nicee/wcee/article/11_548.PDF

Kings College London (2024). The Art of Making in Antiquity: Stoneworking in the Roman World. Retrieved from https://artofmaking.ac.uk/explore/materials/24/Tuff

Turismo Roma. (2024) Park of the aqueducts. Retrieved from https://www.turismoroma.it/en/places/park-aqueducts

Wright, G. (2009) Ancient Building Technology, Volume 3: Construction. Brill. Retrieved from https://books.google.com/books?id=CQHsKG6g5zwC