Welcome back to Eye for Engineering for, at long last, another post. It’s been a while but here it is.
Having looked to the future with the previous entry, Reach for the Sky, for this post I will be looking to the past with a tribute to some of history’s foremost engineers: the Romans.
The Roman Empire
It is impossible to study ancient history without acknowledging the influence of the Romans. With their civilization having spanned over a thousand years, their advancements in the fields of warfare, engineering, medicine, and culture would not be seen again until at modern times.
There are many ways to judge a civilization, though. The roman legions are considered to have been the most effective fighting force in history until the advent of gunpowder. With them, Rome rose from a single city state in the 4th century BC to an empire that controlled the entire Mediterranean Sea by the end of the 1st century AD, and remained the dominant military power for centuries afterwards. Yet other cultures had accomplished equally impressive military feats before. Alexander the Great had conquered his entire empire, stretching from Greece and Egypt to the borders of India, in just over 10 years. Rome’s dominant culture and religion was derived from its subject nations – mainly Greece, with Rome adopting the entire Greek pantheon and mythology under different names. Similarly, much of Rome’s medical knowledge was influenced by the Greeks’.
The Romans did, however, excel over all their predecessors and contemporaries in one area. In this area they would be unparalleled until the modern day, and their developments would give inspiration even from the Renaissance period. That field is engineering: the Romans had exceptional engineers and built roads, buildings, bridges, aqueducts, and siege engines like the world had never seen and would not see again until the Renaissance. History is awash with empires rising through conquest, and scientific discoveries have been rediscovered and disproven, but for any nation to be strong it requires infrastructure, and this the Romans had.
Pont Julien bridge, Calavon River, France. The old roman bridge (foreground) dates back to the first century BC and carried traffic, even cars, up to 2005 when the new bridge (background) was constructed
De Architectura:
We owe most of our knowledge of Roman engineering and architecture not only to numerous surviving examples but to the architect Vitruvius, who compiled the treatise De Architectura for his patron Augustus Caesar, the emperor of Rome. De Architectura was written in ten volumes, each of which discussed a topic of engineering important to the empire:
- Book I: Town Planning
- Book II: Building Materials
- Books III and IV: Temples and orders of architecture
- Book V: Civil/Public Buildings
- Book VI: Domestic/Private Buildings
- Book VII: Pavements and plasterwork
- Book VIII: Aqueducts and water
- Book IX: Architectural sciences (geometry, sundials…)
- Book X: Machinery
De Architectura was written to serve as a guide for Augustus Caesar to all the types of engineering and architecture that would be of use to him in building his empire. He would not be the only one to make use of it: Vitruvius’ work was the reference for all further architectural treatises until the 18th century, and is the only surviving work of its kind from the classical era, thus serving as modern historians’ only reference to classical architecture and engineering. The principles illustrated in De Architectura can be found in hundreds of surviving Roman works around the Mediterranean.
Building Materials:
Roman architects and engineers were able to construct amazing structures by utilizing several architectural structures little used previously: namely the dome, the arch, and the vault. These structures can carry immense weight by transmitting load through the curved axis (or circumference in the case of domes) to the supports, with little deflection or bending because the structure is already leaning on itself. As structurally and aesthetically superior structures, vaults, domes, and especially arches found their way into most roman structures: palaces, temples, amphitheaters, bridges, and aqueducts to name a few. The trend is named the Roman Architectural Revolution by historians, but it also goes by another name due to the revolutionary new material the Romans used to make these structures even stronger.
Internal Forces of Arches and Domes
While discussing building materials in book II, Vitruvius describes this new material as “a kind of powder which, by nature, produces wonderful results. It is found in the neighbourhood of Baiae and in the lands of the municipalities round Mount Vesuvius. This being mixed with lime and rubble, not only furnishes strength to other buildings, but also, when piers are built in the sea, they set under water… when three substances formed in like manner by the violence of fire come into one mixture, they suddenly take up water and cohere together. They are quickly hardened by the moisture and made solid, and can be dissolved neither by the waves nor the power of water”. The powder he describes is pozzolona, volcanic ash and sand. While Vitruvius does not name the durable mixture of lime, rubble, and pozzolona, modern historians acknowledge it as ancient concrete.
Roman Pantheon
Roman concrete has long puzzled historians and engineers. The Pantheon in Rome features the world’s largest unreinforced concrete dome, ranging from 6 to 1.5 meters (approx. 19.66 to 5 feet) in thickness and capable of containing a 43 meter (143ft) sphere. The Pantheon has lasted nearly 2000 years without steel reinforcement that would be required for modern concrete. In addition, many Roman bridges, aqueducts, and harbors still stand undeterred by thousands of years of weathering and erosion. Modern Portland cement, which has been in use since the 1800s, can only last 50 years of erosion in seawater. You can easily understand why Roman concrete has kept scientists scratching their heads.
Recently, it was identified that the mixture of lime and volcanic ash and rock undergoes a chemical reaction with seawater. The hydrated lime, with water molecules incorporated into its structure, reacted with the ash to form the cemented mixture. When used for underwater structures, the materials were mixed into basic mortar and packed into wooden forms, after which the surrounding water would cause the mixture. For building structures on land, the materials were mixed in a mortar box with little water to keep it dry and stiff (better for strength). Then the mixture was transported to the work site, placed over existing layers of stone, and pounded into the cracks. These ancient practices are being brought once again into the mainstream to create strong, lasting, and more eco-friendly concrete. As an example, in the construction of Utah’s Upper Stillwater Dam in 1987, the Bureau of Reclamation used a concrete mixture of 40% Portland cement, which contains the calcium component of lime, and 60% fly ash from power plants, which was found to contain the same amorphous silica compounds as volcanic ash. Bonding gel was used to bind rock pieces together, and the concrete was spread by vibrating rollers.
While modern concrete structures may not survive our century, the Roman concrete structures outlasted their civilization and are still sources of wonder. Its significance and widespread use in Roman engineering and construction is the reason some historians call the Roman Architectural Revolution “the concrete revolution”.
Roads:
“All roads lead to Rome,” it was said in the ancient world, and truly it would be impossible to mention the Romans’ building skills without mentioning their roads. The Romans borrowed the art of road-building from their neighbors the Etruscans, but like with everything else they assimilated the art was improved upon and used to its fullest potential.
Ancient Roman Road connecting Antioch with Chalcis – Tall Aqibrin, Syria
Roads existed prior to the Romans, but most were more paths than roads, routes taken along the paths of least resistance between settlements. When travelling through mountains, deserts, and swamps, this could mean circuitous routes through dangerous country. It also placed limits on the movements of armies and merchants, who would have to choose routes their pack animals and carts could negotiate. The Romans were the first to build a substantial road network to provide direct, safe travel between cities. They also had the largest road system until the construction of modern highways, with roads spanning over 400,000 kilometers (250,000 miles), over 80,000 km (50,000 miles) were paved with stone. Roman roads were famously wide enough that an entire decanus, a legionary unit of ten men, could march shoulder to shoulder. Under the law of the Twelve Tables, established in 450 BC in the early Roman Republic, a standard road was required to be 8 feet (2.45 m) wide in straight sections and 16 feet (4.90 m) when curved. Over the centuries many roads differed from the standard, ranging from 4 to 30 feet (1.2 to 9m).
The majority of Roman roads were constructed by the legions. As the legions were already formed of able-bodied men and travelled constantly into new territory, it was convenient that they be put to work for the good of Rome. Amongst the legion’s ranks were a special unit of officers called the architecti, or chief builders, responsible for any military construction required of the legion. Under them were the agrimensores, land surveyors, and libratores, levelers, responsible for the layout of the road. Actual construction was done by the legionnaires, who in addition to carrying their armor, weapons, and rations were required to carry a set of building tools, including a saw, shovel, pickaxe, and hatchet. This strenuous work kept soldiers busy even in times of peace and helped build strength and stamina, although frequently local contractors and extensive slave labor supplemented the legionnaires’ work.
Roman Road Layers – From Shropshirehistory
Road construction varied depending on terrain, soil conditions, and available materials, but Roman roads used layered roadbeds bordered by drainage ditches to siphon away rainwater that would otherwise damage the foundation of the road. The foundation itself could be a deep excavated trench (fossa) or a raised earthen embankment (agger). The foundation trench would be filled with gravel, stones, and rubble. If sand was available a layer of it would be put down. At about a meter (approx. 3.3 ft) from the surface a layer of gravel would be added and tamped down to form a pavement surface, called the pavimentum, which itself could function as a road surface or could support another layer of flat stones (statumen) set in cement to support further layers. Agger roads generally sufficed with their own earthen embankment of sand and gravel. Either of these two foundations would then support three further layers of metalling (broken stones): coarse concrete or rudus with larger stones, fine concrete or nucleas with smaller stones or compacted gravel, and finally the paving stones or crusta which made up the paved surface. The central paving stones were always taller than the surrounding ones, to give a curved surface that would channel rainwater to the drainage ditches.
(A). Native earth leveled and occasionally rammed tight.
(B). Statumen: hand-sized stones.
(C). Audits: rubble or concrete of broken stones and lime.
(D). Nucleus: kernel or bedding of fine cement made of pounded potshards and lime.
(E). Dorsum or agger viae: surface or crown of the road (media stratae eminentia) made of polygonal blocks of silex (basaltic lava) or rectangular blocks of saxum qitadratum (travertine, peperino, or other stone of the country).
(F). Crepido, margo or semita: raised footway, or sidewalk, on each side of the via.
(G). Umbones or edge-stones.
Even after the fall of the Roman empire and the cessation of their maintenance, the Roman roads were utilized through the Medieval era for transportation and are still usable today, though not with comfort. The concrete cementing the paving stones together has eroded over the centuries and left very uneven surfaces, but in their prime the roman highways were as important to the empire as the US highway system is to its country and users. Using the roads, a legion could march up to 20 miles (32 km) a day and constructed a fortified camp (castra) at the roadside whenever the day’s march ended. Many frequent castra sites became permanent forts. Travelling officials and dignitaries, however, would not have entire legions to build stopping points nor would be satisfied with military accommodations. For this reason every 25 to 30km (approx. 16 to 19 miles), approximately the average distance travelled by an ox cart per day, mansiones were constructed as inns or villas that offered hospitality for travelers. These were normally reserved for those on official business, who were required to present passports for identification. Near the sites of the mansiones the less formal (and often disreputable) cauponae often served commoners, while more well-to-do but still unofficial travelers could stay at tabernae or hostels.
Remains of a Roman Mansio in Staffordshire, England
Another type of way station commonly found along Roman highways were the mutationes, or changing stations, which serviced vehicles and animals. Located every 20 to 30 kilometers (12 to 19 miles) these establishments featured cartwrights and wheelwrights to repair chariots and wagons, veterinarians to treat animals, and spare animals to replace exhausted ones. With this system, official messengers and couriers could deliver messages quickly by riding hard and changing horses at the mutationes. A message could be transported an average 80km (50 miles) a day, but greater distances were theoretically achievable: the emperor Tiberius supposedly travelled 500 miles (800km) in just 24 hours to reach his dying brother.
Just as boom towns sprang up along American railroad lines, and the modern highway system is peppered with stores and service stations, infrastructure and businesses often sprang up to support these types of inns. Often permanent forts were constructed to defend the mansiones, and sometimes villages and even cities could be founded around them. The cities of Rheinzabern in Germany and Saverne in France are held to have begun as Roman stopping points. In this way, the Roman roads permitted their users to build and expand their empire.
Bridges:
As a general rule, the Romans preferred to build their roads as straight as possible. When faced with rocky terrain, hills, or ravines, the usual response was to cut through with excavations and tunnels. Detours were often made, but Roman roads were famed for their straightness, and weren’t bothered by little obstacles like rivers and mountains.
Alcantara Bridge, Spain
Romans were not the first to construct bridges, but they revolutionized bridge-building with their use of arches. As mentioned above, an arch transmits loading along its axis to the supports, enabling great load to be carried by simple columns. It also allows for wide gaps between tall supports, allowing floodwater to pass without rising over the top of the bridge, while giving the bridge the ability to carry up to its own weight. Single arches were sufficient for short spans, but for long distances the Romans used several arches in a single bridge. Segmented arches (not full semi-circles) were used to increase the length even further. The Limyra bridge in Turkey was constructed with 26 segmental arches over 330 meters (approx. 1089 feet) with an average span-to-rise ratio of 5.3 to 1, resulting in a flat profile that would not be repeated for over a millennium.
Bridge near Limyra, Turky
Stone and mortar were not the only materials the Romans used to build bridges. In addition to over three hundred stone bridges, engineer Colin O’Connor lists 34 timber bridges and 54 aqueduct bridges from the Roman era. A number of amazing Roman bridges have survived from antiquity are fully functional. Many, like the Pont Julien in France, are still capable of carrying car traffic. Others have even more impressive achievements: the Roman bridge at Vaison-la-Romaine, France, has survived bombing in World War II and flooding over two meters above the bridge deck (Ancient architecture: Roman bridges).
Roman bridge at Vaison-la-Romaine
Aqueducts:
Roman Aqueduct of Segovia, Spain – 28 meters tall
Aside from the roads, the most famous example of Roman engineering is the aqueduct system. Like with the roads, Romans did not invent aqueducts. Aqueducts or canal systems were in use from the Mesopotamian era, but many of these were more like canals than the famous arch bridges that come to mind upon hearing the word aqueduct. These structures were the Romans’ work, and their use of these structures, like with their roads, became a defining feature of their empire. By the third century AD, Rome’s population of over a million was supplied, through a system of eleven aqueducts spanning over 500 miles (800km), with over one million cubic meters of water per day. This capacity is 26% greater than the modern water supply of Bangalore, India, whose population exceeds six million.
Pont du Gard, France
The arch bridges that symbolize the Roman aqueducts only represent a fraction (approx. only 30 miles of Rome’s system) of Roman aqueducts. The Romans generally tried to keep their aqueducts running within 0.5 to 1m beneath the surface, with bridges used to cross valleys, gorges, and rivers. When faced with deep or wide valleys, an alternative was an inverted siphon. Here the channel or conduit ended with a header tank. Using pressure and gravity the water would flow down the valley through pipes and cross at a “venter” bridge which would feed it to a lower receiving tank, from which it would continue along the channel.
Inverted Siphon on Roman Aqueduct
They were also constructed with very low gradients: Vitruvius advises a gradient of at least 1:4800, while the Pont du Gard in France drops only 34 centimeters per kilometer, descending only 17m over its entire 50km length.
This dependence on aqueducts was rooted in a problem modern people can relate to. Rome’s river, the Tiber, was too polluted to drink safely. Water had to be brought in from elsewhere, and the aqueducts allowed water to be channeled from over 55 miles (90km) away. This allowed the Romans to overcome what was possibly the greatest obstacle to the growth of a city: in the past no city’s population could exceed the capacity of drinking and irrigation water provided by its nearby river. Think of the Nile in Egypt, or the Tigris and Euphrates in Mesopotamia, which enabled the civilizations around them to exist. With aqueducts to bring water from elsewhere, the Romans enjoyed public baths, fountains, and latrines. Farmers could also receive authorization to tap into aqueducts for irrigation water. All this required that the aqueducts be maintained, often by teams of slaves, laborers, and sometimes even legions to repair and clean the aqueducts regularly.
Sewers:
Cloaca Maxima opening into the Tiber (modern day)
There was even a wastewater system to deal with the runoff. Around 800BC, centuries before Rome’s rise to power, a massive drainage system termed the Cloaca Maxima (greatest sewer in Latin, before ornithologists blanch) was constructed to drain nearby marshes around Rome into the Tiber River. The Romans suffered from malaria and other diseases spread by mosquitoes, and while they did not realize the mosquitoes’ role they recognized the role of stagnant water. Vitruvius himself advised strongly that cities not be built near marshes. Over the centuries more wastewater channels were constructed leading to the Cloaca Maxima, forming the basis for a sewer system. Continuous flow from the aqueducts flushed water from baths and latrines through channels to the Cloaca and other channels, eventually flushing the wastewater into the Tiber. Few private homes were connected to the system: most people had to use the public facilities or empty their chamber-pots into roadside sewage ditches that led to the main system.
Roman Sewage Ditch
Conclusion:
The Romans’ achievements would not be seen again for centuries after the fall of their empire. There is a very good reason the centuries following the fall of the empire were termed the Dark Ages. The Romans were not only the dominant power militarily, scientifically, and culturally, but because they had infrastructure that allowed the spread and advancement of their people like no civilization had before. Roads, bridges, aqueducts, sewers…modern civilization would be impossible without them, yet two thousand years ago the Romans were building an empire on them.
See Also:
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