Engineering Rome

The Roman Streets

Introduction

The streets of Rome are arguably one of the most iconic parts of the city. Anyone who has come to Rome has noticed that these streets are very unusual. The stones are locally nicknamed “sampietrini” or “little Saint Peters” because of their initial use in the Piazza San Pietro. Not only are the sampietrini noticeably difficult to walk on, but rolling suitcases becomes an unexpected challenge as the wheels get caught in the large gaps between them. After the realization that they probably brought the wrong pair of shoes, travelers will notice several different things as they take their time walking along these stones. First, they notice their dark, charcoal coloring. Then, they’ll see that these stones are placed in a very aesthetically pleasing pattern. some of which are particularly mesmerizing. Finally, if they’re really paying attention, they’ll notice the asphalt sitting between them. The stones did not always look this way, however. And, as a great Italian once said, “there is a reason for everything”. There is a long story about how the stones appear as they do today, and this story starts all the way back in Ancient Rome.

The history of Roman road building is an interesting one. As the city’s transportation changed from horse drawn carriages to motor vehicles, and their population changed from mostly citizens to mostly tourists, their roads changed as well. The aesthetic patterns and asphalt that travelers notice is not accidental. These changes were made to solve problems, but some solutions have consequently caused other issues. This paper dives into the reasons behind these changes and explores the effects of them. Topics including effective load distributing and drainage are discussed following a detailed history of how Romans built their first roads, which is where this story begins.

This analysis and photos in this paper come from an area in Rome called Campo de’ Fiori. It is located just east of the Tiber River and southeast of Vatican City. A large scale and small scale map of the area is shown below for reference (Figure 1).

Figure 1: Map of Campo de’ Fiori

The First Roman Roads – A Detailed Summary

Although the Romans did not invent paved roads, they were the first civilization to use them extensively and to develop a long-term plan. The over-arching concept for their road system and organization was for no major point in the empire to be isolated (Cosentino). Their long, straight roads made for easy transportation of soldiers and goods. This road system was well developed and complex. It included several definitions to distinguish different types of roads, sophisticated surveying and construction techniques, as well as an employment system with specific job titles for the workers.

When defining different road types, the first two major categories are viae and strata (Nibby). Viae were roads that started in Rome and went outside the city to other regions and provinces, while strata were roads that started and ended within the city. In addition, a viae was defined as a viae if and only two carriages could pass each other moving in opposite directions. The standard for this was about 5 meters wide (Nibby). Within these two major categories are actus, iter, semita, and callis. Actus roads were small roads that could only be crossed by foot or horseback; not wagons or carriages. Iter roads were to be crossed strictly on foot. The word “semita” is a combination of “semi” and “iter”. This combination, thus, means “a smaller iter” (Nibby). Finally, callis roads were roads that lead into mountains or hills. The Romans also had a name for private roads and public roads called praetorian and agrarie, respectively (Nibby).

As it does today, construction of ancient roads began with surveying. Since the Romans were building before the invention of compasses, they used a sundial for navigation in order to set the road in the desired direction. Another commonly used surveying tool was a device called a groma. The groma was composed of two perpendicular sticks of equal length (the stelletta) perched on a vertical staff (the ferramento). Each end of the stelletta had a plumb line hanging from it. The stelletta and ferramento were connected by the rosto, which was exactly 1 foot in length and allowed the stelletta to rotate freely (Cornelius). A visual of this description is shown in Figure 2.

Figure 2: Diagram of a typical groma. Source: Edilio Boccalieri

The groma was used for surveying straight and orthogonal lines (Cornelius). With the help of the plumb lines, a groma was set vertically at each station and the stellettas could be aligned, ensuring a straight line. Orthogonal roads could also be laid out simultaneously with this system.

Ancient Roman roads were built in three layers. The roadbed was dug 30-60cm deep and was filled in part way with small stones and crushed brick, and was compacted with lime and pozzolana (Cartwright). This first layer was called the rudus. The second layer, the nucleus, consisted of finer gravel or sand. Finally, the third layer, called the agger or pavimentum, was a finished surface of stone rocks or slabs made of silica or limestone (Cartwright). The type of material used depended on what was readily available. A visual of this layering is shown in Figure 3.

Figure 3: Layers of a typical ancient Roman road. Source: Cartwright

In Figure 4, the surface of the ancient Appia Antica is seen as a weathered, seemingly random arrangement of stones. The stones have been moved and misplaced, allowing for gaps large enough for weeds and grass to grow between them. However, when these roads were built each stones was cut and shaped carefully to fit the ones surrounding it. This surface was once smooth, with gaps between stones smaller than the width of a knife (Cartwright).

Figure 4: The ancient Appia Antica Road. Source: Kellie Jaenicke

The Romans even integrated a drainage system in the design of their roads. Stagnant water along these roads was prevented by setting the stones in the middle of the road slightly higher than the rest (Cartwright). The rainwater would then flow down the sides of the road and drain into trenches that were dug along the side and were filled with small rocks and sand.

Most of the work building roads was performed by soldiers (Cornelius). The empire knew that the soldiers would build these roads well since they had an incentive to do so. Some more specialized workers were given job titles. Gromatici were the land surveyors who used the groma and sundials to set the road locations and paths. The agrimensori were the engineers who spotted the precise points where the road needed to pass through, and marked these points using poles. The Libratores were responsible for digging where the roads were to be placed, usually about 60cm. Finally, there was a curator who was responsible for maintaining the roads within their specified area. The curators work mostly consisted of replacing loose stones (Cornelius).

The Cobblestones 1585 – Switching to the Sampietrini

The sampietrini were first used in 1585 but were not popularized until 1736. When the stones were becoming popular, the Cooperative of the Selciaroli di Alfrenda took advantage of this opportunity and became the leading group of skilled workers to place these stones around Rome (Cinelli). The Cooperative had several different types of workers, including the miner, the ripper, the sbozzatore, the porter, and the paver. The miner prepared the mines and gathered material, the ripper created smaller stones out of larger ones taken from the mines, the sbozzatore created even smaller stones, the porter transported the finished stones to the paver, and the paver placed the stones in the roads (Cinelli). They did this by sitting on the ground holding a mallet, and methodically placed the stones into the sand base. The pavers had a great deal of knowledge about how to place these stones. They had different tools and techniques that were used depending on the situation. Figure 6 shows a statue of a paver placing a stone exactly how they would place these stones back then, as well as today.

Figure 5: Statue of stone placer. Source: http://www.qualcheriga.it

The stones, though often thought of being made of basalt, are made from leucitite that is found in Capo di Bove lava flow. This flow formed around 280,000 years ago by the Alban Hills, which are what remain of the ancient Latium Volcano (Selsi). They cannot be considered basalt because their silica content is very undersaturated, and is they are instead called leucitite (Robustini). These stones are characterized by having white spots scattered throughout the material. The stones are shaped into truncated pyramids (Figure 5) and placed in a bed of sand.


Figure 6: Typical stone from Campo de’ Fiori. Source: Kellie Jaenicke

Sampietrini come in a couple of standard sizes. The smallest stones are 6cm by 6cm while the largest ones are 12cm by 12cm. Larger stones have a typical depth of 18cm while smaller stones have a depth of 6cm (Zoccali). a range of stone sizes is shown in Figure 7.

Figure 7: Display of different stone sizes. Source: Kellie Jaenicke

These stones were implemented due to their superior strength compared to the ancient stones whose foundations were composed of crushed brick. Typical basalt and limestone have a compressive strength of 100-300MPa and 30-250MPa, respectively, while the sampietrini stones have a compressive strength of 241-320MPa (Zoccali). This increased strength was desired since the population of Rome was increasing, and the roads needed to be able to accommodate the influx of carriage traffic. In addition, the sampietrini provided an improved drainage system, as water could drain between the stones and into the earth rather than travel towards trenches.

Patterns and Shapes

With increased pedestrian traffic and heavier vehicles, such as those in Figure 8, it became helpful for the pavers to place the stones in the streets in certain shapes (Cosentino). The most common shapes include the straight vertical, herringbone spines, tiered rainbows, and peacock tails. Different patterns are chosen based on the type and amount of traffic, as these patterns are efficient load distributors.

Figure 8: Example of heavy traffic on stones. Source: Kellie Jaenicke

Different patterns are placed all over the city. Figure 9 shows one of these examples; a straight vertical pattern meeting a diagonal pattern. This image was taken at Piazza Trilussa, an area with high amounts of walking traffic. This change in pattern is not entirely clear, but one reason for it could have been to show the direction in which people are walking.

Figure 9: Example of different stone arrangement. Source: Kellie Jaenicke

The tiered rainbow consists of adjacent columns of stacked arc patterns that form a diagonal stone pattern at the intersection. An example is shown in Figure 10.

Figure 10: Display of diagonal pattern between arc patterns. Source: Kellie Jaenicke

This arrangement for the stones seems to make sense for heavy traffic areas. Arches are used widely in Roman construction because of their ability to distribute and control load patterns and distributions. The arc patterns for the stones are arranged so that the car reaches the keystone first. This can be seen in Figure 10. This makes sense because the horizontal load from the forward moving car will be put in the correct direction for the arch to distribute. Figure 11 shows a sketch of a load pattern of an arch design compared to a vertical stacked design. The stones are shown in blue, the external load is shown in green, and the load distribution in red. In the arch design, the load from the vehicle can be set both vertically down to the next stone or horizontally to an adjacent stone. In this way, the load is being distributed among more stones in more directions, reducing the amount of load on each individual stone. In addition, the excess load, when it reaches the end of the arch, is concentrated and moved down vertically in between the set of arches due to the diagonally stacked stones. In the vertical stacked example in Figure 10, the load from the car is distributed straight down the same few columns of stones. The load is unable to move horizontally since the stones are stacked directly on top of each other. Because of this, more load is put on fewer stones compared to the arch design. Large loads on a few stones is likely to cause larger displacements. Minimizing the amount of displacement is important for maintaining the stone pavements. Therefore, the arch design appears to be a superior choice in high traffic areas.

Figure 11: Stone patterns with arch distribution (bottom) and vertical distribution (Top)
Source: Kellie Jaenicke

Adding Asphalt

As traffic becomes a larger issue, more changes needed to be made to try and sustain these streets. Another example of this was adding asphalt in the gaps between the stones. As stated before, the stones have a very high compressive strength; much more than required to support a car. Therefore, the reason for including asphalt along with the stones was not for vertical support, but most likely horizontal support. The stones are placed in sand, so the lateral forces on the stones caused by the forward and backward motion of a vehicle were most likely causing the stones to displace in large amounts. One of the advantages for using the stones in the first place was that they could move and accommodate the motion of the earth beneath them. However, this adaptability became more of a hinderance when heavy vehicle traffic started to travel about them. Asphalt has been a common solution to this problem but has caused several other issues. The most evident of these is drainage.

To reiterate, one of the great advantages for using the sampietrini stones was that their drainage was very effective as water could simply seep through the gaps between them and into the earth. The olive tree located near Campo di Fiori square has experienced this first-hand. As Tom Rankin explained, the tree, before the asphalt was placed, used to receive its water from the rain that drained through stones. However, the asphalt placed there made this area impermeable and the tree was no longer to receive adequate amounts of water. The tree died and was then replaced by another tree of the same type. With no other changes made the tree died again. Once the reason for why the tree was dying became clear, the city input a seemingly extensive network of drains surrounding the tree for the water to reach it (Figure 12). The tree has not since died after these drains were placed.

Figure 12: Olive tree near Campo de’ Fiori. Source: Kellie Jaenicke

Another very apparent consequence of placing asphalt between the stones occurs when large amounts of rainfall hit the city. Since the ground is not able to absorb the rainwater, it quickly accumulates on the surface where it stays until it evaporates or runs into a nearby drain. However, the very irregular surface characterized by small mounds and valleys prevents the water from flowing in several spots (Figure 13).

Figure 13: Flooding during thunderstorm in Campo di Fiori. Source: Kellie Jaenicke

In addition, the drains that catch the flowing water overflow (Figure 14). This amplifies the issue by eliminating another way for the water to reach the ground beneath the stones. One reason for this drain overflow could be that these drains were not originally designed to intake these quantities of water. Pavements and their respective sewer systems are typically built off experience and what amounts of water they are expected to receive. It is likely that these sewer systems were placed before asphalt was widely used. More water was able to permeate through the stones rather than the sewers. When asphalt is placed more water runs into the sewers rather than directly into the ground. As more asphalt is placed around the stones this cycle continues until the sewers can no longer keep up with the incoming amounts of water.

Figure 14: Sewer unable to drain water after heavy rain. Source: Kellie Jaenicke

Another possibility for the inadequate drain capacity could come from another source. The sampietrini streets have many people walking along them all day long. Campo di Fiori is especially known for this because of its daily market. Each day, vendors set out their food products and clothing items and this creates a large amount of trash. This trash, in combination with the trash produced by common people throwing their cigarette buds and other small amounts of garbage of the ground, gets swept away by the flowing rainwater and taken to the sewers where it collects (Figure 15).

Figure 15: Clogged sewer drain. Source: Kellie Jaenicke

Conclusion

The Roman streets have come a long way from when they were first placed about 2000 years ago. Changes to the pavements around Rome have been justified to support the increasing population, but their consequences are very apparent, as well. The story of the Roman streets is not yet over, as the city removes more and more sampietrini stones and replaces these streets entirely with asphalt. These changes are also justified and will have other effects on the people and the city over time, either beneficial or harmful, just as the changes listed in this paper did.

Bibliography

Cartwright, Mark. “Roman Roads.” Ancient History Encyclopedia, Ancient History Encyclopedia, 24 Sept. 2019, www.ancient.eu/article/758/roman-roads/.

Cinelli, Valentina. “Selciatori.” Sampietrino, Sampietrino Cultural Association, 2019, www.sampietrino.it/selci/selciatori/.

Cornelius, Titus. “How to Use the Roman Groma.” How to Use the Roman Groma | De Legione Romana, Drupal, 13 Aug. 2012, legioneromana.altervista.org/content/how-use-roman-groma?language=en.

Cosentino, Gianluca. “Stone Road Pavements.” Stone Road Pavements, Sapienza Universita di Roma, Uniroma, 2017, it.wikisource.org/wiki/Delle_vie_degli_antichi.

Nibby, Antonio. “Delle Vie Degli Antichi.” Wikisource, Wikisource, 14 Sept. 2015, it.wikisource.org/wiki/Delle_vie_degli_antichi.

Robustini, Pasquale. “The Geology of Rome’s Sampietrini.” Pasqualerobustinicom, Pasqualerobustini.com, 9 Aug. 2018, www.pasqualerobustini.com/en/la-geologia-dei-sampietrini/.

“Selci.” Sampietrino, 2019, www.sampietrino.it/selci/.

Zoccali, Pablo, et al. “Sampietrini Stone Pavements: Distress Analysis Using Pavement Condition Index Method .” Applied Sciences, Applied Sciences, 29 June 2017, file:///C:/Users/kelli/Downloads/applsci-07-00669%20(1).pdf.

Kellie Jaenicke

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