By KATIE ZEMTSEFF
Journal Staff Reporter
March 4, 2010
Ground penetrating radar has been used for years to detect things underground, from utilities to Saddam Hussein in hiding. Recently, the Seattle Department of Transportation realized it might have a new use for the technology: identifying the location and size of tree roots.
Shane Dewald, senior landscape architect for SDOT, said the idea was born when she was a guest presenter at a local class. A student asked if she had ever used ground penetrating radar before. She answered that she had not but would look into it.
That day, she learned SDOT has used it for exploration and infrastructure but never specifically for tree roots.
Dewald was referred to Sammamish-based GeoRadar Imaging, who agreed to take on a test project at no cost.
GeoRadar is a family-owned business that was founded in 2003. It uses a machine that transmits high-frequency electro-magnetic pulse energy into the ground through an antenna.
Radar measures the time it takes for a pulse to travel to and from a target, which indicates depth and location. This creates an image, allowing someone to “see” into the ground.
Robert Schall, project manager with GeoRadar Imaging, said most of the company’s work comes from scanning post-tensioned cables and rebar in concrete. It does underground storage tanks, septic tanks, utility and roadway imaging. It also recently scanned the entire sidewalk area along the Seattle waterfront, searching for possible voids.
Schall said the company has spent a lot of time dealing with tree roots when trying to find things like underground pipe but had never used the radar to search for them before. Tree roots, he said, are essentially just like a pipe in the ground. They are denser than the dirt around it so the scanner can identify them.
Often, roots have been a source of frustration for the company. But as GeoRadar dealt with them, the company learned more about how they grow and how to identify them. Now that frustration may lead to a new business venture.
“We may be able to solve some of these problems,” Schall said. “It’s kind of a new avenue. You’ve got to be diverse in this market.”
To show SDOT what radar could do, the two set up a test-run. In February, Schall and his team used radar to look at the roots of sweet gum trees that were planted in the 1970s along Fourth Avenue in front of City Hall. Different groups of city engineers came down to look at the process.
The roots did not look like roots. Viewers, Dewald said, must remind themselves they are looking at a cross section of land.
“It’s sort of like a foreign language when you see the image.” Schall said radar images slice the ground in half and let you look in, similar to how an apple looks when it is sliced down the middle.
Schall said doing a scan before doing work or buying property can often end up saving lots of money.
Major potential
Both Schall and Dewald see major potential for city use of the technology. When tree roots cause problems like buckling sidewalks, it can be hard to fix without removing the tree. But tree removals are expensive, often require closing a street or sidewalk, and take away an important community asset.
Instead of removal, Schall said the technology could save the tree by getting a more accurate picture of its roots. Tree roots, he said, come in varying shapes, sizes and directions. If a tree is deeply rooted, it may be possible to safely remove smaller roots near the surface to allow development or ease problems.
“It costs the city a lot of money to replace trees,” he said. “You’ve got all that shade and that canvas that can take you years and years to grow … and then you just hack them down. It doesn’t make a lot of sense.”
Dewald said old trees are a public asset worth between $10,000 to $20,000 each that the city wants to keep.
Even if trees were not so valuable, their removal is expensive. Dewald said tree removal can run around $1,000 while buying a new tree and caring for it in its early life can run $500. That does not include the cost of closing sidewalks or streets to remove trees, or the environmental benefits of keeping an existing tree.
Dewald said the city will use the technology in the future. It could be used to enhance public safety and preserve trees in the best way possible. It could also be used to balance the city’s interest in preserving trees with development.
Seeing where tree roots are located, she said, would let the city understand how close a building can be sited to a tree or how close construction can occur. “I think this technology can help them kind of get as close as they possibly can and yet still be able to retain the tree.”
However, the technology is pricey so it won’t be used everywhere. Dewald said a radar ground scan would cost a minimum of $500 and likely would not be used unless a $1,000 investment is considered worthwhile.
“In comparison with the tree value that we have and the value that property owners place on being able to make the highest and best use of their private land to get an analysis that helps you free up construction space and still retain a tree, it’s very much cost justifiable.”
Dewald said the city could use the technology for projects related to the city’s right of way. Immediately, she said it could be used on the city’s Mercer corridor project. Mercer Street has some valuable trees on it near Broad Street. The city, she said, will need to be very sensitive about how construction is done around trees it chooses to retain.
“I’m thinking Mercer might be a project in the near term … that deserves the extra bit of TLC to make sure that they survive.”
As the city moves toward waterfront redevelopment, trees surrounding the Alaskan Way Viaduct project or along the waterfront could also be a focus.
Years ago, Dewald said the city experimented with an earlier version of the technology but did not find it very useful. She said she thinks many jurisdictions may have done the same thing.
Now that the technology has progressed, she said more people involved in tree protection and sustaining urban forests might consider it and find it useful.
For more information, visit www.georadarimaging.com.
- Jayme Mattson
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