An effort to monitor the effects of vibrations on the Colosseum in Rome has been extended; the next phase of the project will rely on an innovative parallel sensor system. Wikimedia Commons/Diliff
A road that curves around a portion of Rome’s Colosseum was closed to private traffic earlier this month, providing an opportunity to extend an important monitoring project.
August 20, 2013—A year ago, headlines around the world proclaimed that Rome’s Colosseum was tilting, a product of a crack discovered in its foundation. The only problem? The initial reporting by the mainstream media, which led to analogies to the Leaning Tower of Pisa in a whirlwind of coverage, misrepresented the purpose of a dynamic monitoring program that had begun in preparation for a new underground subway line that will pass near the ancient monument. Giorgio Monti, Ph.D., a professor at Sapienza Università di Roma says the crack is not causing the structure to tilt. Monti is leading a study of the movement of the ancient structure in collaboration with Giuseppe Marano, Ph.D., a professor at Polytechnic of Bari, and Giuseppe Quaranta, Ph.D., a professor at Sapienza.
But the monitoring program Monti is leading has just been extended by Rossella Rea, the archaeologist who is the superintendent of the Colosseum, after a decision earlier this month by the newly elected mayor of Rome, Ignazio Marino, to eliminate private traffic on the Via Celio Vibenna, which curves around a portion of the Colosseum. Taxis and buses will still be able to travel along the road, but they will be held to a strict 30 km/h speed limit, according to Monti.
Rea, and her team—Barbara Nazzaro and Fabio Fumagalli, both architects with the superintendent of the Colosseum—commissioned the study because a new metro line, the third for the city, is currently under construction and within the next two years will pass approximately 6.5 m beneath, and perhaps just 10 m away from, the lower edge of one side of the Colosseum’s 13 m thick concrete foundation. “We are currently monitoring the vibrations before they start boring the tunnel, so that next year we will be able to compare the results with the vibrations induced by the tunnel-boring machine,” Monti says. Now that a year’s worth of monitoring of the Colosseum has been conducted with the Via Celio Vibenna open to both public and private traffic, the superintendent has extended the monitoring study so that data can be gathered when the road is closed to the public.
The monitoring system uses accelerometers that are capable of measuring vibrations imposed on the structure by such external forces as traffic and metro trains, according to Monti. “The [sensitivity] is amazing because they can detect accelerations that cannot be felt by the human body,” he says. “The lowest detectable [force] is 1/10,000 of the gravity acceleration.”
Over the past 12 months, measurements have been collected with sensors connected by wires to one another and to a main data acquisition system that kept the sensors synchronized. After gathering the information, the acquisition system performed such basic tasks as filtering out noise to facilitate interpretation of the data, Monti says.
As a result of a request from the superintendent to devise a monitoring system without wires in order to prevent visitors from interfering with the sensors and make the system more acceptable aesthetically, the team is testing a new type of wireless sensor. “The big problem was that wireless sensors work at certain frequencies that cannot pass through the huge walls of the Colosseum because they are bounced back,” Monti says. The most obvious solution would have been to position the sensors in such a way that each would be in the “line of sight” of the next so that the signals would not have to pass through walls, but that would have been too costly, he says. “So we started planning something totally new, which is a true innovation in the field of wireless sensors: having two invisible networks working in parallel,” Monti says.
The first is a network of wireless sensors that can communicate with one another directly using Wi-Fi. A second, parallel, group, which is part of the same network but, as Monti explains, speaks a different “language,” will use low-frequency waves that can pass through the enormous travertine walls of the Colosseum. “Low-frequency waves have the good property that they pass through walls, no matter how thick they are, and in doing so they can establish a connection and synchronize,” he says. The sensors will have two types of outputs so that they can “speak” either language, he explains.
“From the accelerations measured throughout the structure, we can gather a wealth of useful information: main frequencies, modal shapes, transfer functions, damping,” Monti says. “Based on the change these quantities may have in time, we are also able to detect the development of damage in the structural parts.” Monti and his team anticipate that any damage that does occur will be caused mainly by such low-intensity vibrations as those created by steady traffic rather than by intermittent vibrations of higher intensity. The minor degradation caused by the low-level vibrations is most likely to occur at the interfaces between column parts because the dry joints there have been weakened over time by vibrations, he says.
The new, wireless sensors are currently undergoing testing, and Monti’s team expects to be able to place 14 sensors around the highest facade, which is 55 m tall, by September. That number of sensors “is more than enough to have an idea of the overall mode shape and the overall vibration of the entire facade,” he says. This portion of the facade faces the road that has been closed to traffic, so it’s the best place to monitor the effects of the traffic reduction, he says. After the sensors have gathered data for a month, they will be moved so that a step-by-step mapping of the facade’s movement can be obtained.
“We assume that the noise coming from traffic is constant in terms of frequency, content, and intensity during the year, so no matter when we record the effects on the facade, the input will always be the same,” Monti notes.
As for the story of the crack and the settlement of the monument, Monti explains that the existence of the crack has been known since the 18th century. It is a consequence of the way the Colosseum was built, he says. “Emperor Vespasian identified four different construction companies . . . and each one [undertook] the construction of a quarter of the monument,” he explains. “The crack in the foundation is simply one of the construction joints, whose opening is due to the fact that half of the foundation is on stiffer, volcanic deposits and Pleistocene sediments, and half is on softer, Holocene alluvium soil.”
The settlement, the subject of extensive media coverage last year, amounts to 40 cm. Relative to the 156 m diameter of the Colosseum, this represents a mere 0.5 percent slope from the center, Monti notes.
“You have to think of these ancient buildings as [if] they were living organisms,” Monti says. “They have been around for so many centuries that they have developed a sort of self-adaptation to the external environment: weather, smog, settlements, earthquakes, vibrations, et cetera.” The slope is a result of a series of adaptations that the Colosseum has made since its completion, in 82 AD, to accommodate the changes in the boundary conditions, he says. “So there are some cracks, like we have some wrinkles.”
The current restoration work inside the Colosseum is cosmetic, not structural, according to Monti, and is not related to the monitoring study.
The sensor system that Monti and his team are developing will enable them not only to explore the capabilities offered by monitoring carried out by parallel networks of sensors but also to establish limits for the external actions that the Colosseum will be able to bear in what he refers to as “its expectedly long future.”