Penetrating the Void
Is that well-trodden roadway about to open up and swallow you and your car whole?
Probably not, though dramatic pavement chasms have made news in recent years in places such as sinkhole-prone Florida and in West Virginia, where roads sit over abandoned mines. A 2002 road collapse in Bowling Green, Ky., was as large as a football field, pulling four cars into it. Amazingly no one was hurt.
Louisville, too, has had its share of scary road collapses, and civil and environmental engineering professor J.P. Mohsen in U of L’s Speed School of Engineering has seen some of them.
“As traffic volumes grow and the pavement ages, we’re going to continue to see these problems happening,” Mohsen says.
![[Image]](/advancement/pub/impact/spring2004/images/void1.jpg)
J.P. Moshen (left) and doctoral student Zhiyong Zhao "road test" radar-on-wheels to detect underground dangers.
Voids, or gaps that develop under pavement due to erosion—often around pipes and cables—worsen under traffic loads and are very costly to repair when the ground collapses. Mohsen is examining a way to find those voids before they develop into dangerous and expensive problems.
Since summer 2003, Mohsen and doctoral student Zhiyong Zhao have studied the use of ground penetrating radar (GPR) to find subsurface voids under Louisville’s streets.
GPR already is used by utilities and cities to locate pipes, but little research has been done on using the technology to find subsurface voids.
“That’s what makes our project unique,” Mohsen says.
To discover what’s going on under the surface of roads, crews traditionally have had to dig or bore through pavement and earth—an expensive, labor-intensive and time-consuming method that often means blocking and diverting traffic.
Mohsen’s radar-on-wheels looks similar to a baby carriage. As he pushes the mobile system along the street a control unit generates electromagnetic waves that are directed by an antenna as far as 14 feet down. The signal bounces back and is converted into a patterned wavy image that can be seen instantly on a display screen.
Trying to figure out what these images represent is an important part of the research. Different underground objects and materials send back different patterns. Large and small voids can look different, as can objects such as metal and plastic pipes, trolley tracks, rebar in concrete, cables and other common infrastructure features.
“Part of what we’re doing is creating a catalog of images showing what subsurface features these different GPR signals represent,” Mohsen adds.
After surveying miles of downtown streets, the researchers are processing and analyzing the data using software that removes visual noise from the images to aid in identifying the subsurface features.
Early in the study the researchers rolled the radar over streets with suspected voids, then excavated the street to verify their existence. This helped them established “control signatures” indicating what voids looked like on the display screen.
Mohsen says the data will become part of a geographic information system (GIS) operated by a group of area agencies and utilities called the Louisville/Jefferson County Information Consortium to aid in timely, cost-effective infrastructure maintenance.
Mohsen’s study is one of several under the aegis of the Center for Infrastructure Research, a multidisciplinary research group at U of L directed by Tom Rockaway.
“Hopefully our conclusions and results also will be used to apply this technique to other cities and urban areas,” Mohsen says.
