Into the void
GPR technology helps municipalities focus on roadwork, minimize traffic tie-ups
It’s not uncommon to use ground-penetrating radar (GPR) to map underground utilities, but some municipalities are using it to find, well, nothing at all.
That’s been the case in North Vancouver, a district that’s used GPR to map subsurface voids, which can lead to pavement breaks, like potholes or even damaging sinkholes.
“In one case, we did an intersection investigation where there was an excavation nearby,” says Jeff Klochnyk, section manager, streets and fleet, within the District of North Vancouver’s engineering, parks and environment department. “The excavation yielded a utility that was unknown, quite a sizeable box culvert, and physically, the crews that were doing that work saw a void outside of the box culvert.”
“That’s what brings the investigation to us,” he adds. “How do we know what the severity of that void is? How do we know if it’s something we need to be concerned with at all? Where does it go, and how big is it?”
Traditionally, answering such questions would mean cutting open a street or intersection, along with the attendant traffic disruptions and costs. But GPR gives towns and cities another tool they can use to more precisely target such digs, or maybe even decide they’re not necessary at all.
Something out of nothing
One example of a company offering GPR is GeoScan, Inc., a nine-person B.C.- based firm founded in 2010.
Owner Will Meredith says his clients are showing more interest in using GPR as a void-locating tool: “We’ve done these jobs sporadically over the past few years, but we’ve done quite a few in the past few months, so we’re starting to focus more on them,” he says.
The company’s main pieces of equipment include ground-penetrating radar with frequencies ranging from 100 MHz to 2300 MHz. (The lower frequencies penetrate deeper into the ground but don’t return as clear of a picture.) For void detection, Geo- Scan uses a Mala 450 MHz single-frequency ground-penetrating radar antenna.
“Essentially the antenna is constantly emitting a radar signal that’s bouncing up and down through whichever material you are scanning,” says Mike Neale, a senior geophysical survey technician with GeoScan. “Now when that signal hits a change in density, whether that be a pipe in the ground, the bedrock or the water table, that signal is reflected back up. And the time it takes to get back to the antenna from that object is how it measures the depth of that object.”
When the initial survey is complete, the company uses specialized software to create amplitude maps, showing peak amplitudes and negative amplitudes over the target area. It can then plot that data and present it to the client using CAD software or Google Earth.
Voids present a different challenge than searching for utilities, however.
“With a utility, if it’s metal pipe, you’ve got a strong metal reflection, whereas a void is actually nothing,” says Meredith. “They’re harder to quantify in the sense that you know roughly the shape of a pipe, whereas a void is a much more sporadic sort of shape, so it’s harder to quantify. But the signal itself is just as clear as it would be hitting something else, because it’s such a big change in density.”
Subsurface voids aren’t the only area beyond utilities where GeoScan is applying its GPR technology. Other applications range from locating underground oil tanks to archaeology and even grave locating, in situations where tombstones have either sunk or gone missing.
These jobs can sometimes be a lot like spotting subsurface voids in a road. “If it’s a metallic coffin, then we would get a metallic signature,” says Meredith. “If it’s a wooden coffin, then you would be looking more for the void space of the grave.”
Matching the tool with the job
Klochnyk, for his part, sees GPR as a tool that can make sense for detecting subsurface voids. However, he feels it’s best applied under specific circumstances, such as when a physical inspection has turned up signs of a potentially wider issue and not as a first-strike tool on its own.
“There are firms that will incorporate this type of review into a full citywide pavement-management inspection,” he says. “We’ve questioned the validity of having an entire scan of our road network with GPR. What is the benefit of that, and how do we even manage that level of data?”
Instead, he sees the technology as best used in tandem with existing historical data, particularly in a municipality like North Vancouver, which has numerous underground water systems.
“A lot of the time, there is a lot of local knowledge about where these streams have gone in the past, so the need to detect a void may not be there if we have local knowledge of the existing underground conditions,” he says. “Where we don’t have that knowledge, [GPR] is one tool we could use to help identify what a possible cause could be.” CUI