Getting to the Root of the Matter

Technician performing GPR test for SDOT on a downtown sidewalk

How can Ground Penetrating Radar (GPR) save trees in  Seattle?  Read on!  GPR, historically used by SDOT to locate underground structures, tested  positive by SDOT Urban Forestry for its potential to find tree roots!   The idea, from a student in a class on tree protection , inspired SDOT landscape architect Shane DeWald to explore.  Motivated to meet canopy cover goals in Seattle’s Urban Forestry Management Plan and accessibility goals in the SDOT Pedestrian Master Plan, she took the idea to heart and contacted GPR experts at GeoRadar to ask “Can GPR see tree roots down deep …through concrete?” and the answer was “Yes!”

Check out these pictures of a test for SDOT.  With a trained eye, a GeoRadar expert sees GPR images like a radiologist reads an X-ray, and can spot roots even though they don’t look like roots at all! The GPR technicians explained, “Tree roots show up all the time when we use GPR to locate underground structures so we knew it would work!”

Tree roots as they appear on GPR screen

A mature street tree is a public resource that commonly exceeds $5,000 and is well worth saving, and an “Exceptional” tree, based on size, condition, and species, must be protected as a condition for site development.  Using GPR to inform design professionals early in the development process can expand design options and prevent costly changes during construction. 

For SDOT, GPR has unique potential and is a new tool for the trade.  When GPR reveals substantial roots well below the ground, an urban forester has more latitude to allow pruning of surface roots.  Used in combination with an air spade to carefully expose roots, GPR can help prevent damage and increase the future life of trees and sidewalks, an important  win-win!

For Seattle citizens, trees are important.  Social, economic, and environmental benefits increase as a tree remains healthy and grows canopy over paved surfaces, intercepts rain, reduces runoff, reduces air pollution and cools both air and water.  Thanks to street trees, we enjoy increased property values, lively shopping districts and healthy, attractive neighborhoods.  

The City of Seattle is committed to tree preservation as an important aspect of “Building Green” and to utilizing technology for efficiency and economic benefits.  GPR is clearly part of our green future.