The City of DuPont (pop. ~8,200) is a planned community in Pierce County, WA, that was built out between 1994 and 2008. In the course of development, roughly 7,000 trees were installed in 5-foot wide planting strips located between the street and sidewalk, all within the public right of way.
In recent years, sidewalks in older neighborhoods began buckling or ‘tenting’ due to tree roots, and the problem was expected to continue as the city’s younger street trees grow larger.
The following tells the story of the city’s effort to address this issue:
In summer, 2015, the City conducted its first pilot study with wholesale replacement of sidewalks and street trees on a single block. This project replaced 720 feet of sidewalk and replaced 43 street trees. The total project cost was $100,800 and included construction, design, arborist services, construction management, trees, and materials.
The estimated cost of repairing sidewalks citywide using traditional methods was over $15.5 million—far too costly for the city to take on.
After a search for other, less expensive remedies, the city experimented with an alternative method using a piece of vacuum lift equipment called the Big Sidewalk Sucker.
The Sidewalk Sucker is used to lift out whole panels of concrete sidewalk. Once lifted, the exposed tree roots can be pruned, and once pruned, the sidewalk panel can be lowered back into place and leveled.
The city enlisted a consulting arborist to advise them on root pruning techniques and identify trees that would not survive pruning and, therefore, should be removed.
To cut costs on replacement trees, the city, with volunteer support from the city’s Tree Advisory Board, started their own municipal tree nursery. Bare root stock was purchased at $2 to $3 a tree and planted into 15 gallon fabric root bags with topsoil. They installed an above-ground drip irrigation system and a 6-foot fence to keep out local deer.
In September 2016, city staff lifted 400 feet of sidewalk adjacent to 13 street trees which were then root-pruned. Root barriers were installed as a measure to deter new roots from repeating the problem in the future.
The vacuum lift and root pruning approach was estimated to cost just $2.5 million over 5 years, including sidewalk repair, removal of trees that will not survive root pruning, root pruning for trees that could be saved, replanting of replacement trees, and installation of root barriers.
This approach is saving substantial amounts of money and time, and preserves many trees that might have otherwise been removed using more traditional methods.
For more information about this project, contact: Gus Lim, Public Works Director, City of DuPont, 253-912-5380 or firstname.lastname@example.org
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