Underwater Sensors Keep Bridges Safer

Wednesday, September 1, 2010 @ 06:09 PM gHale


Underwater sensors could soon help keep bridges upright especially after a flood, which cuts down more bridges than fire, wind, earthquakes, deterioration, overloads and collisions combined.
In cases of flooding, the speed and turbulence of an overflowing stream scours away the river bottom that provides the support for a bridge foundation, causing more than 60 percent of bridge failures in the U.S. in the last 30 years.
“There is no way to determine risk during these crucial events,” said Xiong “Bill” Yu, an assistant professor of civil engineering at the Case School of Engineering.
Yu designed a smart infrastructure consisting of underwater sensors that relay real-time information about how much roiling water has stripped the river bottom away and how stable, or unstable, the supports of a bridge remain.
“We don’t fully understand how scouring takes place,” Yu said. Water passing a bridge support forms vortices, which can erode the river bottom. How and at what rate scour occurs is complex. River bottoms usually consist of sand, clay, shale or sandstone or a mix, and each material acts a little differently in a strong current, he said.
To characterize each vortex, Yu’s lab is building flow sensors based on tiny, hair-like sensors that salmon have on the sides of their bodies. Researchers found the fish determine flow direction by the direction the hairy cells move and speed by the time delay as turbulence passes different sensors. Yu’s lab built sensors comprised of micro pillars made with piezoelectric fibers mounted on flexible copper rods. The fibers produce electric signals reflecting flow direction and speed. These have proven sensitive and accurate; the lab is now developing arrays for real-time flow and turbulence sensing.
To determine the amount of sediment scoured away, Yu built sensors that constantly measure the topography where the water meets the river bottom around the bridge supports. These sensors employ a technology called time domain reflectometry, in which radar fires along waveguides installed at critical ground locations. The electromagnetic waves return at different speeds depending on the materials they strike and distance traveled. They then analyze the waves with an algorithm to reveal minute changes in the depth and density of the substrate sediments.
The sensors proved durable, sensitive and accurate when tested on bridge supports 10-20 feet below the surface. The next step is to determine the maximum depth and flow conditions under which the sensors provide accurate and immediate information.
Yu’s lab is also investigating sensors that can monitor the stability of the bridge structure itself.
The package of sensors will provide warnings not currently available and enable Yu’s lab to develop computational scouring models and effective scouring countermeasures.



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