As the research organization of the U.S. Army Corps of Engineers, the Engineer Research and Development Center (ERDC) conducts research and development in support of the Soldier, military installations, and civil works projects, as well as for other federal agencies, state and municipal authorities, and U.S. industry. From infrasound monitoring to remote structural assessment to warfighter deployment planning, a range of representative ERDC research projects are focused on the development of innovative solutions for a safer, better world.

That “better way” departed from traditional monitoring of how a bridge or other structure appears, instead focusing on how it behaves. And that behavior is broadcast through infrasound, frequencies that people can’t hear.

It’s been a full decade since some dubbed her vision “Mihan’s Singing Bridges,” but Mihan McKenna, Ph.D., a research geophysicist with ERDC’s Geotechnical and Structures Laboratory, characterizes it as a decade of success for her infrasound research.

McKenna started at ERDC in 2005, joining the bridge inspection team, “where you get to walk around in the woods and poke sticks in the holes and take pictures. It’s fun. But in the process of doing that, I realized that there had to be a better way to do it.”

That “better way” departed from traditional monitoring of how a bridge or other structure appears, instead focusing on how it behaves. And that behavior is broadcast through infrasound, frequencies that people can’t hear.

“It’s a paradigm shift from visual inspection,” McKenna explained. “In fact, the first perception we have of something, which is how it appears, is really the easiest way to fool us. You say, ‘This looks good to me,’ right? And that thinking has influenced the way that we monitor and inspect a lot of our critical infrastructure.”

She offered the example of the thousands of bridges in the national bridge inventory that currently require periodic physical inspection to monitor for issues like scour around footings, which she characterized as “the No. 1 reason that bridges fall down in the United States.”

Referring to “fundamental modes of motion,” McKenna acknowledged, “The popular perception is that a bridge just sits there. It’s a physical object that is immovable in space and time.

“But it’s not,” she quickly asserted. “Bridges and large critical infrastructure are actually living, breathing entities in their own way. They move all the time. They expand. They contract. They blow in the breezes. Not in a way or a dimension that human beings are really very well adapted to perceive, but they are moving and they do have behavior to them. So the fundamental modes of motion basically can tell you how that bridge is attached to its surrounding media.

“If you think about it like a musical instrument, a giant true truss bridge is nothing but an extremely complicated harp that we can’t, as humans, hear,” she continued. “It emits low enough frequencies that the wavelengths are on the order of kilometers, which means the propagation distance is on the order of kilometers to tens of kilometers, to hundreds of kilometers, or thousands of kilometers, depending on your source strength.”

While humans may not be able to hear the “singing bridges,” McKenna said that they can record them on things like Seismic Infrasound Audible Acoustic Meteorological arrays, which can be placed several kilometers from the target structure.

One such array has been monitoring both a rail and highway bridge over the Mississippi River at Vicksburg, Mississippi, since 2010, with McKenna pointing to another ongoing project in Dallas, Texas, and another emerging effort targeting several California highway bridges.

“We’re working with them to monitor some of these structures periodically where you have a lot them all right on top of each other,” she added. “It is really important to know this sort of information in real time. And within the next five years, we should have some near real-time software that allows for assessments or critical alerts that will say, ‘Hey, your thing is not doing well, perhaps you should do something about this.’”

Moving from bridges over water to structures under water, another ERDC project focuses on the application of new tools and technologies – from Light Detection and Ranging (LiDAR) to unmanned aircraft systems to autonomous surface systems – to conduct environmental or beachhead assessments to help prepare for things like amphibious landings or logistics over-the-shore operations.

Pratt reiterated that the information was not meant as a permanent solution, but rather to identify capabilities necessary to support missions up to approximately 120 days’ duration, noting that his current group efforts are “focusing on the technologies and the integration of those technologies into a delivery and platform, so that we can facilitate these FEST teams that go out and do the assessments and collect the data.

According to Thad C. Pratt, a research physicist and head of the Field Data Collection Group in ERDC’s Coastal and Hydraulics Lab, the explorations are part of a Joint Capabilities Technology Demonstration (JCTD) program. The program is “developing assessment tools and methods to repair pier structures once a structural assessment is complete, in order to meet mission requirements.

“My portion of the Pillar and Pier JCTD is to collect data for a structural assessment and then pass that to the subject-matter expert on the Forward Engineer Support Team [FEST], so that he or she can evaluate each component of the structure, the pier or wharf, to determine if it has the capacity to handle the loads for mission requirements,” Pratt explained.

If it doesn’t, ERDC is developing expedient methods to shore up the structure for a short-term operation such as humanitarian assistance or troop movement – whatever the condition might be – to fortify the structure to handle the required loads for short periods of time.