09 Sep Bellevue Park Case Study
The Root of the Problem
In September 2016, the general contractor responsible for redeveloping Downtown Bellevue Park in Bellevue, Washington, contacted Root Cause LLC to help protect its trees. The project involved overhauling the park’s land, completing a half-mile long circular walkway around the park and building several children’s playground structures.
Lying at the edge of downtown Bellevue, Bellevue Downtown Park is an approximately 20-acre park that welcomes several hundred—and at times, as many as 1,000—people who visit to enjoy the weather and walk the track. The walking track, though not yet a complete circle, was ringed by approximately 200 middle-aged London plane trees planted within the track. Thus, pedestrians are constantly trampling on the critical tree root zones, and the ground surface is free of organic matter and heavily compacted. This environment is far from optimal for tree survival, and the mediocre soil conditions limit their size. Still, the trees are a valuable part of the park experience.
By the time we were contacted, the developer had already removed numerous trees on one side of the park. They had damaged many of the remaining trees while excavating land for sidewalks, so these trees also needed to be removed. The public had repeatedly contacted City of Bellevue officials to express concern over the park’s remaining trees.
The planned completion of the track involved placing in-ground uplights through the interior of the track; in many instances, close to the mature trees. Before the general contractor had contacted us, the electrical contractor had planned to dig the trenches for the uplights using an excavator.
A Solution for a Cause
To supply power to the uplights, we dug a central trench for the mainline and then dozens of side trenches to power the individual lights. The inside edge of the track is delineated by the curved concrete wall of a canal and is outside of the drip line of the trees. Pothole testing confirmed only minimal root growth in this area, so we decided to dig the central trench using a mechanical trencher as a cost-saving measure. All incursions into the drip line would then be done using compressed air and a Supersonic Air Knife to soften the soil, and soil would be manually removed using trenching shovels.
Our goal from the beginning was to lay as soft a footprint in the root zone as possible. As the central trench required a 30-inch depth, we had to try out two smaller machines before settling on the Ditch Witch RT45 ride-on trencher to create the central line. We dug branches from that central line into the root zones, using an Air Knife and a 185 CFM towable compressor, capable of 125 PSI. These trenches required only a 20-inch depth. Dust was not an issue on this site, because we did all of the work in December and January (dust control would have been needed at other times of the year).
The park and as much of the track as possible needed to remain open during the project, so we enclosed 100- to 200-yard areas at a time. As sections of the trench were open, the electricians followed and installed conduit. We then backfilled, covered the surface with gravel and compacted it as needed to allow the sections of the track to be reopened.
Traditional trenching, using an excavator into the root zones would have mangled the roots of half or more of these trees and led to hazard conditions and eventual decline. By digging the uplight trenches with air, we created tunnels under large roots, which allowed placement of conduit without damage to roots―many as large as 8 inches in diameter. This project saved the lives of countless trees and helped preserve the urban oasis nature of the park.