Sixty percent of bridge collapses are caused by hydraulic reasons, including scour, says Genda Chen, PhD, which is why he and a team of researchers at Missouri University of Science and Technology (Missouri S&T) created “smart” rocks to detect damage before it becomes a deadly problem.
Scour is an erosion process where water flow flushes away riverbed deposits and creates holes around a bridge’s pier or abutment. Floods intensify the scour effects to quickly make a bridge unstable.
The team’s smart rock technology provides an accurate, cost-conscious method to monitor a bridge foundation in real time. The rocks are positioned at the base of a bridge’s foundations, rolling to the deepest point of a scour hole and relaying the depth.
“It’s a simple, but very useful, concept,” says Chen, who is principle investigator and professor of civil, architectural, and environmental engineering at Missouri S&T, Rolla, Mo. “The rock follows the trail of the scour hole’s progression—as it goes deeper, the rock will also sink deeper. One reason we call it ‘smart’ is because the rock can represent the maximum depth of the hole.”
By embedding below the surface, smart rocks save the time and risk of sending divers down to look at the foundation. And whereas bridge engineers can suffer information overload from similar devices, the rocks are designed to give the right data when it’s needed.
The researchers are examining three different varieties of smart-rock technology: passive, active, and semiactive. Each passive rock has a magnet embedded that can be read by a remote magnetometer. The active smart rocks have embedded electronics (pressure sensor, gyroscope, timer, battery indicator, and individual identification) that wirelessly transmit data. Semi-active smart rocks include a free-to-rotate magnet controlled with electronic circuitry.
Strong enough to withstand floods
“There is a lot of technology that works very well in the lab, but it cannot be applied in field conditions because of damage from strong currents,” Chen says. Researchers, however, are seeing favorable results with rocks positioned at Missouri’s Gasconade River Bridge and Roubidoux Creek Bridge two years ago. The rocks should last indefinitely, with batteries lasting up to 10 years.
The smart rocks also can be used to gauge performance of riprap, or large rocks placed around a foundation to prevent soil erosion.
“No one really knows how effective riprap is during flood season; with the muddy water, you don’t know what’s going on,” Chen says. “If we mix a few smart rocks with the natural rocks around the foundation, the movement of the smart rocks will indicate the performance of the riprap system, and will also tell the engineer if there’s a problem with scour.”
The project is receiving support from the Research and Innovative Technology Administration of the U.S. DOT, as well as the Missouri DOT. The research team is looking to partner with other DOTs to engage in further studies.
Jenni Spinner is a Chicago-based freelancer and former associate editor of Public Works.