Like many older cities, Kentucky’s largest is enjoying a renaissance. People who had moved to the suburbs are coming back to downtown Louisville. Abandoned buildings are repurposed into condos, shops, and hotels; sidewalks widened so restaurants can seat patrons outside. Drawn by amenities that improve quality of life, college graduates are also making downtown their home.
Streetscaping is important to this transformation, and trees are important to streetscaping. They also help control stormwater.
According to American Forests, stormwater runoff volume falls 2% for every 5% of tree cover. In addition to sucking up 15 gallons of water an hour on a hot day, a tree can also lift 100 gallons out of the ground and discharge it into the air.
But like all infrastructure assets, street trees require maintenance. Beginning with a six-tree pilot project in 2013, Louisville Metro Public Works found a way to minimize maintenance and improve public safety.
The problem with pavers
The city’s tree wells had been covered with a layer of cypress bark mulch that was difficult to keep clean. Over time, pedestrians kicked chunks of mulch out of the wells. Rain washed it away. Footsteps had compressed the material and compacted the soil beneath.
In addition to endangering tree health, soil compaction throws tree wells out of alignment. They were no longer level with the sidewalk. People getting out of cars were stepping down into the wells, which was a potential tripping hazard.
In conjunction with the Louisville Downtown Partnership, public works self-installed new tree surrounds with pavement that’s more porous than pavers.
Made by Porous Pave Inc. of Grant, Mich., from 50% recycled rubber chips and 50% stone aggregate with a moisture-cured, liquid binder, Porous Pave XL is mixed onsite, poured in place, and finished like concrete. The result is a slip-resistant, seamless tree surround that drains water through its entire surface instead of gaps that collect cigarette butts and other debris.
“This was our first time working with this material,” says Louisville Metro Public Works Roads Division Labor Supervisor Kevin Alexander. “It was pretty easy.”
Two-day installation process
Alexander and the workers watched an installation video and followed the instructions provided with the product’s bags of rubber chips and aggregate. Once the aggregate and chips are mixed with the liquid binder in a mortar mixer, installation is similar to pouring concrete.
“Mixing a trial batch before the crews got started with the actual test installation proved to be a good idea,” says Alexander. “It gave us a chance to work with material.”
Installation took two days.
The first day was devoted to preparation. Crews dug up the old mulch, being careful to finish with hand shovels to aerate the soil without damaging tree roots. They then added and compacted two inches of 3/8-inch to 3/4-inch aggregate to establish the base for the permeable pavement.
On the second day, crews mixed the material, shoveled it onto the aggregate base, and finished the surface with hand trowels. Two workers mixed the material in small batches in a mortar mixer, two manned wheelbarrows to move the material to the trees, and two spread and finished the material in the tree wells.
To leave space around the tree trunks, six-inch-diameter cardboard tubes were cut into six- to eight-inch lengths and used as sleeves around the tree’s base.
“We cut the tube pieces lengthwise, like a hot dog bun, and wrapped them around the trees before installing the material,” says Alexander. “We came back the next day after the material cured and removed the cardboard, leaving nice, smooth, round openings around the trunks, giving the trees room to grow.”
The manufacturer’s instructions clearly state that all equipment and tools should be coated and recoated with vegetable oil before and during installation. It helps put the best finish on the surface and makes cleaning up easier. After use, tools and equipment should be cleaned with bio-diesel fuel applied with a stiff-bristle brush.
“Our guys are supposed to end their day on time to avoid overtime,” says Alexander. “To save time, they didn’t thoroughly clean all the shovels and trowels. The material hardened overnight, so it took more time and effort to scrape it off the next day.
Expanding the project
Encouraged by the results, the Louisville Downtown Partnership and public works installed the material in the wells of 50 three-to-four-year-old trees.
“Permeable pavement surrounds are safer and easier to maintain than metal tree grates,” says Ken Herndon, operations director for the Louisville Downtown Partnership, which was formed in 2013 to promote redevelopment by providing a more enjoyable environment. “And, at about $400 per tree, they're more cost-effective than grates that cost $800 to $1,200 each.”
After those 50 were installed in October 2014, the partnership and public works conducted a public demonstration. A hose from a water tank poured water on the surround at the rate of 10 gallons per minute for 15 minutes.
“All the water drained directly through the surface, permeated down into the 2-inch compacted aggregate base on which it was installed, and then filtered into the soil below,” says Herndon.
“I’m downtown a couple of times a week and keep an eye on the surrounds,” says Alexander. “After a year and half, they’ve held up well and the trees are doing fine.”