The Secret Lives of Bridges and Dams

The calm waters behind a dam belie its deadly potential. The public, with boats on the reservoir or in houses downstream, think of dams as immovable objects that have met the forces of nature and will continue to do so forever.

But forever is a long time. And let’s not underestimate the forces of nature.

Civil engineers have learned by theory and sometimes the hard way—by experience. They fear that no matter how hard they try to plan for the worst case, there might be an even worse case out there. And dams, because of their massive size and tremendous potential, can be the civil engineer’s worst nightmare.

The St. Francis Dam in Los Angeles County broke in 1928 and billions of gallons of water flooded the valley. Chief engineer, William Mulholland, who did not have an engineering degree, had dismissed the cracks and leaks in the dam. An investigation cleared Mulholland, blaming the soft rock on which the dam was anchored, but Mulholland’s reputation never recovered from the disaster.

Quite recently, two clay and earth dams collapsed after torrential rains in Derna, Libya (population 100,000). Over ten thousand people are still missing after about a third of the city washed into the Mediterranean.

But dams and bridges built in the last century can take advantage of today’s technology, including Bentley’s iTwin, to ensure the safety not only of those who dwell downstream but also of maintenance and inspection crews. Using reality capture software and drones saves the inspector who has to dangle from ropes under a bridge or be harnessed to the steep incline of a dam. With reality capture that can map the entire surface of the bridge, AI to detect the cracks and their growth, and sensors, with Internet of Things (IoT) to relay information to a digital twin, can make all the difference, alerting crews to repair, and in the worst case, safely evacuate dam workers and communities that are downstream.

Civil engineers who design bridges have their own cautionary tale, the Tacoma Narrow Bridge (aka Galloping Gertie). There were no fatalities with this failure, fortunately, but the manner in which Galloping Gertie tore itself apart, a victim of wind-induced oscillation, is branded on the brain of every civil engineer.

Bridges, which are more prone to failure than dams in general, can also be automatically monitored with sensors and modeled with reality capture. Drones can be flown under the bridge, obviating the need to have a bridge inspector dangling underneath it.

Bentley Systems, no stranger to infrastructure on a massive scale, provides software for bridge and dam inspection, monitoring and safety. Best of all, sensors combined with cameras can create an automatic, real-time evaluation of surface irregularities.

Bentley can adapt its iTwin technology for bridges and dams, using iTwin IoT and iTwin Capture to provide real-time monitoring of the assets, assess their performance as well as monitor prevailing conditions of the surroundings. Problems are spotted with AI-based evaluation of sensor data, cataloged by severity and tracked over time to show the rate of progression. Remediation of problems can also be observed and tracked.

Perhaps no software company understands the secret life of bridges and dams in terms of how they move and their need of care better than Bentley Systems. The Pennsylvania-based company exists to design, construct—and most important to our story today—maintain infrastructure.

We reach out to Bentley’s chief engineers, Dustin Parkman, Bentley’s VP of Transportation, and Gregg Herrin, PE and VP of Water Infrastructure for Bentley Systems, to dispel popular misconceptions about bridges and dams—and to find out how their technology could help.

The idea that dams don’t move and bridges need no care (and vice versa) is simply not true, says Herrin. “We can’t expect them to last forever. Dams have a finite lifetime. And a lot of dams are approaching their intended life. There are over 90 thousand dams in the U.S. alone, and their average age is well into the 50s.”

Engineers, especially civil engineers and geologists, know about movement, even movement that is too slow for most to detect. They know that everything moves. Terra firma in anything but firm. The movement may be imperceptible to humans, but we have very sensitive and precise instrumentation and sensors to register such changes.

“Temperatures cause movement,” adds Parkman. “Concrete swells and shrinks. That’s what we would call a natural and acceptable movement depending on how the bridge was structured and how long it was. The same is true for dams. There will be a set of criteria that defines acceptable movement. What we are doing is monitoring and looking for movement outside the norms.”

“We can also take into account what is happening close to the bridge or dam,” continues Parkman. “Like construction, earthworks and geological shifts. A lot of different factors can come into play and there’s a wide range of different sensors that can be used to monitor the bridge (or dam) from a structural health perspective.”

It’s the same … and different for bridges, says Parkman.

“The organizational structure is a little bit different. Bridges come under DOTs [Department of Transportation]—and there’s a lot of them. But bridges also are averaging 50-plus years old and 10 percent have a higher risk factor than they would like.”

Parkman lists the types of sensors that can be used on bridges. The sensors that resonate the most for Bentley customers are those that sense structural deflection. But there are many others.

“Besides that, there are a number of ways to measure movement. You can use stationary total stations to take periodic measurements. There are other types of sensors specific for structural movement you can equip a bridge with, and they’ll tell you whether it’s tilting, whether it’s moving. There are certain ones for bridge piers, for the decking that are very specific. Sensors can measure CO₂, the impact of heat and temperature on concrete. Geotechnical sensors can be used to understand how the Earth around the bridge is behaving. If the bridge spans water, sensors can detect scour and erosion, the impact of water, such as from floods or seasonal rains on the bridge or pier footing.

A typical installation will involve three or four stationary total stations that take periodic measurements up and down and below the bridge and then see if there is anything outside an unacceptable tolerance.”

There are similarities with dams, though the tolerances may be different, says Herrin. You can be sitting on a bridge in traffic when a tractor-trailer goes by and you feel the movement. It’s naturally what happens, but it does happen. It’s movement at a different scale. Dams, as well, have some natural amount of deflection from water levels rising or falling. Temperature shifts can certainly impact a huge concrete dam. A dam in the sun for a long time is going to naturally expand. The question is really ‘What’s outside of the acceptable tolerances?’ or ‘What’s outside of historical norms?’ that we might want to take a closer look at to understand if there’s something of concern involving the structural aspects of it. We are looking at cracks, monitoring them, but in addition, we can get into things like tracking water levels, looking at level gauges for the amount of water behind the dam, but also examining what’s happening with piezometers and pressure plates. You’re looking not only at the dam itself but the surroundings, like the water pressure in the foundation below the dam. You might have a situation where the dam itself, the structure, is fine, but there’s nothing underneath it anymore.”

“It’s true for both infrastructure types, dams and bridges, says Herrin. “You can see a crack, or in some cases, there might be a surface spall and you can see rebar underneath. That does not automatically mean that there will be catastrophic failure, that you need to grab your boots and get to higher ground. This infrastructure is designed and constructed with safety factors. There’s an expectation of some amount of maintenance required to nullify local damage. We have ways of doing major repairs in one area while bolstering the rest of the structure. It’s not necessarily game over. But it is important for the people who are doing these inspections and monitoring these systems to understand what they’re looking at. While the crack might not be a big deal, if you didn’t see that crack a month ago, or if the crack got much bigger, underwent rapid change, or started seeping water or showed mineral deposits where there weren’t any before, that might be an indication that something is changing too quickly. What seemed like it was safe a few months ago might now require further investigation.”

“Doing an inspection digitally not only allows you to do more comparisons over time; it allows you to share them with other experts who can help you assess them. You used to have to send a guy out in a van, have him hang from a rope to see the surface in question. That is not safe and it will be only one person’s expertise. You had to rely on that person’s limited memory or limited experience. Are they able to pull up records of what the surface looked like previously? But if you have, for example, a reality capture and a 3D mesh of what that dam looked like a few months ago, you can compare it to a recent mesh and see that six new cracks have opened up—a small crack starting to spider out. You might notice mineral deposits here and there; some look damp, indicating some seepage.”

To be continued…