When one thinks about engineering in Ancient Rome, they usually think of famous successes such as the aqueducts, the Colosseum, and the invention of concrete. Like any civil engineer, I too found great interest in these subjects during my time studying in Rome; however, the main item that caught my interest involved the influence of soil on these feats of engineering. Centuries of volcanic activity and alluvial deposits from floods influenced the geotechnical properties of the soil making it susceptible to consolidation. Over the course of this article, I will explain these geotechnical properties, the techniques utilized by Ancient Romans to combat the issues that arose from these properties, and how modern-day Rome preserves ancient structures that have been affected over the centuries by geotechnical issues.
The Impact of Floods in Rome & How They Were Addressed:
One cannot discuss the early development of Rome without mentioning the influence of the Tiber River. Frequent flooding of this river greatly changed the landscape and soil properties by distributing alluvial deposits throughout the area. As the city developed, Romans perfected their methods of combatting these floods, reducing the impact of the Tiber on the cityscape.
As displayed in the map below in Figure 1, modern day Rome developed around the Tiber, and the river ran adjacent to the Seven Hills of Ancient Rome, shown in Figure 2.
Figure 1: Map of current day Rome & the Tiber River .
Figure 2: Map of Ancient Rome, the Seven Hills, and the Tiber River .
The proximity of the marsh lands that the city is located on to the Tiber has led to great impact of the soil composition in the surrounding area. To have a greater understanding of the geotechnical influence of these floods though, one must understand the historical magnitude of the Tiber River’s floods, and what measures were taken to avoid more flooding over time.
The earliest recorded Tiber River flood occurred in BC 414 and there are primary sources documenting 33 individual instances of flooding over an 812-year period, with the last one occurring in AD 398 . However, geological evidence indicates that flooding might have been more frequent with episodes occurring every few years . In the case of the Tiber River, the repeated flooding of Rome was caused by heavy rainfall that would swell beyond its normal levels past the existing embankment during the winter or spring. Floods such as these are cyclical, hence why these instances were repeated potentially every few years. The intensity of the floods during and prior to the Middle Ages is difficult to quantify. However, there are reports from these periods of floods making the streets of Rome able to navigate by boat, drowning cattle, and sweeping people off their feet, but since there are no measurements recorded, the exact severity of these floods is unknown .
Modern floods are measured by depth and magnitude of water being deposited. Following the Middle Ages, Romans started to record these in relation to the Tiber’s floods. There are currently 38 floods recorded with known or estimated levels above 15 MASL (meters above sea level). The average MASL is 6.7, but the greatest was recorded at 19.56 MASL . Likewise, the average discharge recorded is 232.49 m^3/s and the highest value was recorded in 1900 at 3,300 m^3/s . Evidence confirms that these averages would be on par for what was experienced in flooding prior to the Middle Ages as well, confirming the previous statements that flooding at these flow rates would be highly destructive to Ancient Rome’s people and more primitive infrastructure.
Since these large-scale floods were so devastating to their city, Ancient Roman engineers searched for ways to make the city drier year-round. One of the earliest techniques to mitigate these issues was the Cloaca Maxima, a sewage system constructed during the 6th century BC . A clarification is necessary because although it was eventually used as a sewage system, its original intent was to drain accumulated water flowing down into the valley from the hills into the Tiber River. The original intent of this project was to keep Rome drier throughout the year rather than to prevent flooding though. Unfortunately, the drawback of this system was when the river did flood. Rather than draining into the Tiber, the system reversed, and the Cloaca Maxima led to water overflowing into the streets of Rome through the drains . However, the drainage system also proved useful at the end of a flood for draining water out of the city center in a timelier manner.
The oldest and most common strategy used to combat floods are earthen levees. In Rome, the first of these were created from alluvial deposits . By the end of the second century AD, concrete embankments replaced these sloped deposits along most of the Tiber River that ran through the city . When successful, these embankments let to complete protection against flooding. However, when the water levels rose to be too high and overtook the walls, they were rendered useless and often damaged in the process. Likewise, these levees often worsened the effects of flooding by constricting flow which can in turn increase it in other sections of the river. Over time, engineers improved the design of these embankments, making them more effective.
In Rome, construction of improved embankments such as these began in 1876 after a flood in 1870 devastated much of the city . These embankments were completed in 1926 and still stand today . These walls are now continuous throughout the city and 5-6 meters higher than their previous designs, providing a more thorough protection to the citizens of Rome .
Figure 3: The Tiber River & the embankments constructed in 1876.
Even though the embankments described protect modern day Rome from the major effects of flooding, the physical evidence of centuries of flooding can still be found across the city. One of these forms of historical evidence are flood lines that were marked on plaques throughout the city. Examples of these plaques are shown in Figures 3 and 4 below.
Figure 4: Various flood plaques located on a wall at the San Maria Sopra Minerva Basilica
Figure 5: A flood plaque written in Latin with the finger denoting the height of the Tiber River on the specific date shown.
These plaques help provide insight into the massive scale of these historic floods. Additionally, a way of tracking these floods is through examining bore samples of dirt. With each flood, soil would be deposited throughout the city, causing a layered effect in the soil. Figure 6 below depicts an example of a soil bore sample with visible layers of different colors taken during the construction of the San Giovanni Linea C metro station. The result of this repeated flooding and layering of soils is that many of the city’s buildings now sit upon alluvial deposits.
Figure 6: Soil sample at measured depths from the San Giovanni Metro Station
Roman Soil Geotechnical Properties:
As previously discussed, centuries of the Tiber River flooding the marsh lands of Rome, resulted in major alluvial deposits throughout the city, giving the soil properties that made it susceptible to geotechnical issues that will be further explained in this following section. However, let us first cover some background information on basic geotechnical engineering concepts used in modern times, such as properties of different soil types and consolidation.
To classify a soil, civil engineers use a classification system to test a soil and determine its type and potential properties. Soils are generally either defined as coarse-grained, fine-grained, and/or organic through simple tests. Once established that a soil is fine-grained, its exact type is determined based off a chart such as the one shown below in Figure 7.
Figure 7: Casagrande’s Plasticity Chart used to classify fine-grain soils .
Coarse-grain soils are sands or gravels whereas fine-grain soils are silts or clays. One of the major differences in the properties between coarse-grain soils and fine-grains soils is how they interact with water. Since the particle size of coarse-grain soils is by definition larger than that of fine-grain soils, these soils do not have the same cohesive properties when saturated . An important distinction between granular soils and clay soils is what dominates their geomechanical properties. The engineering behavior of granular soils is mostly influenced by particle size, whereas for clay soils, it is mostly controlled by the presence of water. Silts are an in between of these two, being affected by water with little to no plasticity but, like sands, their strength is independent of the presence of water . It is worth noting that clays have strong electrochemical interactions with water and that even a small percentage of a clay in a soil can greatly affect the engineering properties of the soil . Likewise, clay’s strong affinity for water makes it highly susceptible to consolidation, the removal of water from the pores of a soil when placed under loads for extended periods of time . In relation to structures, as consolidation occurs, cracks often occur and grow on walls, ceilings, and foundations as the settlement continues. We will analyze later in this paper how these cracks correlate to worsening of the structural integrity of a building.
The alluvial deposits that resulted from Tiber floods are defined as fine-grained and are therefore affected by long term settlement. However, the soil types that are found on the right bank of the Tiber differ from those on the left, and thus are affected by consolidation to differing degrees. On average, the soil deposits on the left bank of the Tiber are characterized by cohesive silty-clayey sediments, high in volcanic and organic matter . The deposits of the right-bank, however, are mainly composed of coarse-grain soils, such as sands, gravels, and silts, lacking organic matter almost completely . For this reason, consolidated deposits from the right-bank show a medium level of deformations whereas the organics and clays present in the left-bank have a direct correlation to higher deformability .
The Pantheon: How Ancient Romans Addressed Geotechnical Issues:
Evidence of large amounts of the soil settlement described can be found in numerous locations throughout Rome. A prime example of this can be found at the Pantheon. Additionally, the Pantheon serves as a historical lesson of how Ancient Romans combatted consolidation through construction techniques.
As was the case throughout Rome, flooding affected the level of the ground relative to the Pantheon. When first constructed in 25 BC, the original ground level was roughly 5 meters lower than it is today. There were a series of steps that led up to the Pantheon. However, the floods erased these steps, and the front of the Pantheon is now level with the current day street, seen below in Figure 8 . Additionally, the South end of the Pantheon is now below street level with a wall wrapping around the back of the building to preserve it, as shown in Figure 10. This difference in the current day street level and that of the time when the Pantheon was constructed is a direct result of the combination of soil being deposited from flooding around the building and the consolidation of the soil that the foundation was built upon over thousands of years.
Figure 8: The Pantheon front located at ground level
Analysis of the main rotunda structure provides insight into the fundamental understanding that Ancient Roman engineers had. Rather than relying on equations and structural models like modern day engineers, Ancient Roman engineers gained their knowledge through experience and apprenticeships, providing them with a vaguer understanding of how to build structures. This is evidenced in the rotunda of the Pantheon through the inclusion of three levels of relieving arches incorporated into the walls . An example of these type of arches is shown in Figure 9 below. The purpose of relieving arches was to distribute loads to stronger, thicker portions of the wall to avoid excessive settlement in any one area of the structure . Although they were unable to predict amounts of consolidation nor the exact strength of load bearing structural elements, the use of relieving arches to reduce settlement provides evidence that these engineers had a base knowledge of the subject.
Figure 9: Example of a relieving arch above archway opening at the Baths of Caracalla.
Unfortunately, the inclusion of these relieving arches was not enough to mitigate the effects of consolidation. Modern analysis of the structure suggests that construction of the abutment on the South end of the building, also known as the “grottoni”, was not originally included in the design of the Pantheon . Rather, this portion of the building is hypothesized to have been added to the project once the engineers noticed vertical cracks forming in the rotunda . The soil that the Pantheon is located on is clay, and as previously discussed, clays are often subject to large amounts of consolidation. Knowing this, one can deduce that the vertical cracks that were created during the initial construction process were a result of settlement and consolidation. The addition of the grottoni addressed this issue by distributing the load of the concrete rotunda over a larger area, mitigating the affects of the consolidation taking place. Like the relieving arches, this addition to the Pantheon demonstrates how Ancient Roman engineers had a basic understanding of how distributing forces across a foundation affects the level of settlement that takes place.
Figure 10: Basilica & “grottoni” located on the south end of the Pantheon compared to modern-day street level.
The Colosseum: How Modern Romans Address Geotechnical Issues:
Whereas Ancient Romans were able to address structural integrity issues that arose from consolidation during the construction of the Pantheon, the problems that led to the partial collapse of the Colosseum have been addressed retroactively using more modern engineering practices.
Recent studies show that the foundation of the Colosseum sits on Holocene alluvial deposits that are not homogeneously distributed . As a result of not being located on an even soil profile, the foundation experienced differing amounts of settlement in different locations, which in turn affected relative stresses on these certain portions of the structure. Settlement in addition to centuries of neglect after the last of the gladiator games in 523 until the Middle Ages led to large cracks forming throughout the structure, specifically the southern section . This weakening of the structural integrity led to the partial collapse of the southern section of the monument during the earthquake of 1349 . Despite this collapse, not much work was completed to address the structural integrity of the Colosseum until the 19th century when two buttresses were added to support the exterior .
The retrofits that were added to the Colosseum from the 19th and 20th centuries are worth noting because the additions made did not necessarily aid the monument. For example, in the retrofit of 1978/1979, post-tensioned iron cables were added to provide additional support to the façade and address cracks that had formed as a combined result of settlement and earthquakes . However, these cables were an incompatible material with the travertine columns that they were placed on and caused corrosion that then had to be later addressed in a following 2015/2016 project . Figure 11 below shows examples of the stainless-steel post tensioned cables that replaced the ones from the 1978/1979 retrofit.
Figure 11: the Colosseum exterior and examples of stainless steel post-tensioned cables
In order to avoid causing more harm than good, the philosophy around retrofit projects has adapted in more recent years. The first step in this modern retrofit process is determining if the monument is structurally sound or not. Indicators that it may have a compromised structural integrity include cracks in the walls and foundation. However, further analysis is necessary to determine if these cracks are flexural or shear because flexural cracks are usually benign, whereas shear cracks are indicative of structural failure . An example of a crack indicating that structural failure may occur is shown below in Figure 12. After determining that there are structural issues that need to be addressed, the best solution to issues that arise with structural integrity is one that is reversible, meaning that the issue can be addressed and then the added materials can be removed later without damaging the existing structure if the need arises . Additionally, a goal of modern retrofits is to preserve as much of the original architecture as possible .
Figure 12: Example of shear crack caused by consolidation in a cave passage
In the case of the Colosseum, more extensive retrofits have had to take place in recent years because of the previously mentioned issues that arose in the structural integrity of the monument due to settlement and earthquakes. The stainless-steel cables in Figure 11 are hard to notice at first but provide visual insight into how the retrofit philosophy is enacted in ancient structures. They provide additional support to the exterior façade, while not compromising the initial design intent of the structure. However, since the damage from the earthquakes was so extensive, more significant work had to be completed as well. Figure 13 below shows where the collapse of the southern portion of the Colosseum took place, and the brick abutment that was added in order to preserve the remains of the exterior wall . Where possible, the original structure was preserved to keep the architectural design intact, but in this case, large portions of brick had to be added to prevent further collapse.
Figure 13: An example of more extensive retrofit work done to the Colosseum
Consolidation and flooding have proven time and time again to be major issues for the people of Rome since historic times in their efforts to create a strong city. The alluvial deposits and volcanic material present in Rome’s soil from centuries of flooding and volcanic activity make this area even more susceptible to consolidation and threaten structures both ancient and new as a result. In ancient times, these issues were addressed through the means of strong foundations and minimizing flooding using drainage systems and levees. Throughout history, engineering has improved as have the ways in which we address these issues. To mitigate flooding, higher, stronger embankments have been installed. Likewise, a case-by-case analysis of ancient buildings is used to combat structural integrity issues that have resulted over the centuries from consolidation.
On a more personal note, the experience of witnessing these feats of ancient engineering in-person is hard to put into words. Researching the Pantheon and the Colosseum for this article provided me with background information that no tour guide would have told us. As a civil engineer, this knowledge gave me a much deeper appreciation for the work that went into these buildings and the work that goes into making sure that they stay standing for many more generations to experience.
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