Porous pavement is a permeable surface composed of open-pore pavers, concrete, or asphalt, stone, and glue. Permeable pavement catches rain water and surface runoff, allowing it to slowly percolate into the soil below. The most common uses of permeable pavement are parking lots, low-traffic roads, sidewalks, and driveways.

The advantage of using Porous Pavement (Porous Asphalt or Porous Concrete) is that your typical concrete will not allow rainwater to pass through the surface. This creates problems at parks and walkways on hills because the water floods into sewers and storm drain creating stoppages and flooding.

By choosing to work with this porous surface, you insert more water into the soil, trees, grass, and plants nearby.

Porous Pavement also directly benefits trees because it acts as a protection layer for their roots. The surface is not dense or rigid as asphalt and creates and the cushioned barrier between people and animals walking over the roots. The roots of a tree maintain the structural integrity of a tree and when they are protected, the tree has a better likelihood of survival.

Since water and precipitation seep through the pavement, there won’t be any ice formation on the porous pavement. Even in freezing temperatures, the pavement will remain warm and proves to be a safe surface for driving and walking.

Lastly, it can be made using recycled materials helping to create less waste in landfills and promoting cleaner practices across cities.

we were asked to install a walking path of Porous Pave in a complex crisscross patten for Seattle Housing Authority. The walkway sits at the base of some “turf mounds”, designed as a park amenity for kids to run up and over. These mounds have a concrete base underneath, which means all the rainwater that falls on them is directed to the footpath below. Processing rainwater onsite, and keeping it out of the storm drain and ultimately Puget Sound, is a more and more important goal of many landscape projects, and flexible porous paving is one of the tools we use to meet that goal. The project was particularly difficult as we had to connect the walkway to various levels of benches, city sidewalk, and the turf mounds, and installing it flat and level presented many challenges. With the ingenuity of Derek, ever creative foreman for all Root Cause operations, we found a way to make it all fit together and serve the public for years to come.

We are always impressed by the access to nature here in Seattle. Our CEO, Brian Holers, was on a project on Mercer Island and he realized how different life can be just 50-100 yards from the busy highway. It’s quiet, peaceful, calm, and beautiful. We maintain an urban forest for this exact reason. It creates an opportunity for all of us to stay connected with Mother Nature and the natural wonder of plants and trees.

City trees need a sufficient soil volume in order to grow to a certain size. A tree never stops growing, but if it does, it will die. The process of growing, even a tiny bit, is a stressful process and the amount of soil the tree is planted in makes a huge difference. This week Brian was out on Mercer Island evaluating the health of multiple trees that line a major street.

“It’s not easy being green.”

These words were made famous by Jim Henson’s iconic frog, but also ring true for our city trees. The stoic sentinels that line our streets and dot our parks are in danger, and tree advocate and arborist Brian Holers is trying to make it a little easier to be green.

Holers started the company Root Cause to promote the survival and wellbeing of city trees through arboriculture. This task is often more daunting than it might sound. “Trees and development are natural enemies,” he said. “If your job is to build a house on a small city lot and there’s a tree in the way, the first obvious step would be to cut down the tree.”

While tree preservation is not in the best interest of most developers, Eastside life simply would not be the same without these trees. And many Eastside trees would not be the same without Root Cause.

Root Cause specializes in air excavation technology, which allows arborists to dig safely around trees and improve the surrounding soil. Another technology Holers uses is porous pavement, which allows the roots to receive water and nutrients, while also protecting the tree from some of the harsh effects of city life.

“We always start with the assumption that a mature tree has value,” Holers said. “A tree has a function. It cleans the air, water; it takes up pollutants; it cleans the soil; gives us shade … we start with the assumption that, with care, city trees can be preserved.”

Root Cause’s jobs range widely, from creating porous pavement walkways between Microsoft’s office treehouses to protecting the roots of an iconic elm tree at the University of Washington.

However, arboriculture was not always Holers’ plan. After graduating with a degree in psychology and religious studies from Louisiana State University, Holers took a while to figure out his professional calling. He traveled to East Africa, Southeast Asia, and Jerusalem with his wife and their child. He even tried writing a novel.

Holers now balances his creativity and pragmatism through his work with Root Cause.

“Try to imagine what it would look like if there were no trees. Trees are awesome,” Holers said. “But city trees can only grow with the nutrients we give them. What lies beneath is always a mystery, and that’s why I like my job so much.”

Continue reading see what this tree-loving titan does on a typical day.

Geek of the Week profiles the characters of Pacific Northwest tech, science, games, innovation, and more.

“When he was growing up, Brian Holers had a love for the outdoors — fishing, playing baseball, running through cornfields and climbing trees. That last passion eventually led him to where he is today… As an arborist certified by the International Society of Arboriculture, Holers now runs Root Cause from his home base in Mercer Island, Wash. He provides services using modern technology aimed at preserving trees and tree roots, and has even installed “porous pavement” on Microsoft’s Redmond campus.”

Alison Morrow of King5-TV met up with Brian in Belltown while installing new Porous Pavement around trees on 2nd Ave. Check out their video below or visit their website here:


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.


Thriving Results

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.

Back in January, we were tasked of installing new porous pavement around a pair of trees just two blocks away from Pike Place Market in Downtown Seattle. The struggles with working downtown include narrow sidewalks, a busy lunch rush and of course, dealing with the cold, rainy weather.

This pavement installation spanned the course of a few days from when we first removed the top soil from the original tree pit. After removing a few inches off the top, we filled in a new layer of gravel. After quickly mixing together our standard porous pavement solution, we needed to quickly lay it down. With the cold and wet weather, we need to cover the surface for a few days to avoid any crowds walking over the roots.