Dec 03, 2018

Discovering Permeable Pavement

Replacing permeable pavers reduces flooding in Houston park

The Nature Discovery Center provides a space for locals to experience and learn about nature.
The Nature Discovery Center provides a space for locals to experience and learn about nature.

Houston’s Nature Discovery Center is a 4-acre park that offers a green oasis in the vast urban expanse of Houston-the fourth most populous city in the U.S. Located in Bellaire, Texas, a city of approximately 18,000 surrounded by Houston and established in 1988, the Nature Discovery Center invites visitors to develop respect and an understanding for nature, nourished by outdoor experience and informed by education. Visitors can experience and learn about varied habitats: pocket prairie, deciduous woodland, prairie wetland and cypress pond. The park also features a pecan grove, a natural play area, an aviary and an outdoor classroom.

Working With Mother Nature

The park’s central path is situated in a low-lying area. Water drains right toward it. Like most of Houston, the park’s soil is native clay, which allows only slow natural permeation of rainwater into the ground. In addition, the path was constructed with impervious concrete pavers. With the combination of the path’s low elevation, the imperious pavers and the clay soil underneath them, every rain event left large areas of standing water that could last for several days. Houston’s heavy rainstorms often would turn the path into a stream as deep as 4-in. 

“Frequent flooding made our old paver path unsafe and unusable,” said Henry Owen, executive director of the Nature Discovery Center. “It was a nagging problem we had to solve. We needed a new permeable path.”

In 2017, a new path was one of the Nature Discovery Center improvements funded by a donation from the Robert L. Cook Charitable Fund. Building a new permeable path presented an opportunity to redesign it, changing it from a straight line through the park into more of a nature trail with curves winding between the park’s four distinct habitats. As Owen explained, the twists and turns in the new path invite and encourage exploration, giving visitors a more natural experience of the park.

Finding the Right Solution

The Nature Discovery Center enlisted Piper Whitney Construction (PWC) to design and install the new path. The center had a challenging set of requirements based on its goals for the new path and the characteristics of the site.

  • Storm Water Infiltration Performance. Given the severity of the path’s flooding problems, maximizing storm water mitigation was the top goal. The Nature Discovery Center wanted the new path to be useable immediately after a rain event.
  • Minimally Intrusive Installation. Having invested in restoring and maintaining the four habitat zones, protecting the park and its natural features from damage during construction was critical. The Nature Discovery Center had a special concern about its pecan trees. “We have 80- to 90-year-old trees to protect in the pecan grove,” Owen said. “We wanted to avoid extensive excavation that might damage the trees’ root systems.”
  • Natural Appearance. In keeping with the idea of making the path more like a nature trail, a paving material that would harmonize with the surroundings was preferred. 
  • Expressing the Curves. As noted, changing the course of the path from a straight line to winding curves was a principal design goal. Accordingly, the selected material had to make it easy to shape and express the new path’s curves. 
  • Pedestrian Accessibility & Safety. Usability and safety for visitors are always concerns for a public park. The surface of the new path had to be safe, especially for families with children in strollers and people using walkers or wheelchairs. A non-slip paving product was preferred to reduce the risk of slips, trips and falls.
  • Easy to Maintain. As a nonprofit organization, the Nature Discovery Center wanted to minimize the long-term costs of ownership and maintenance of the new path.

Because it was constructed with impervious concrete pavers in a low-lying area with clay soil, the path flooded after every rain event.
Because it was constructed with impervious concrete pavers in a low-lying area with clay soil, the path flooded after every rain event.

Assessing Alternatives 

According to Owen, one option was a shredded rubber material with the appearance of mulch. 

“We quickly ruled it out,” he said. “It would not have given us a smooth, level surface that would be safe for all visitors, and loose mulch is difficult to keep in place.”

Another alternative was a geogrid paving system with intersecting, rigid confinement cells installed on a base of crushed aggregate. Installers fill the cells with loose stones, but keeping the stones in place was a maintenance concern. This alternative was ruled out because it was not optimal for accessibility. 

Concrete permeable pavers were the third option considered. With permeable pavers, the void space is the only place where water can drain through. In most installations, that typically is less than 10% of the total paved area. Permeable pavers would not have provided sufficient storm water infiltration. In addition, pavers typically are installed with a No. 2 stone subbase that can vary from about 4 to 8 in. in depth, an additional 4-in. open-graded base of No. 57 stone, and then a 1.5- to 2-in. bedding course of No. 8 aggregate, which is necessary to ensure a level paver surface. Excavating down 10 in. or more would have been intrusive and could have harmed the roots of the pecan trees. Finally, pavers have a more finished, formal appearance than the Nature Discovery Center desired for the new path. 

“For the Nature Discovery Center’s new path, Porous Pave XL was the permeable paving material that best met all the requirements of the project,” said Michael Bratton, who co-founded Piper Whitney Construction with his wife Kryshon Bratton. 

Porous Pave XL consists of 50% recycled rubber chips and 50% crushed granite aggregate, which are mixed on site with a liquid binder. The entire surface of Porous Pave is pervious with a total of 27% void space. Water passes through its pervious surface at a rate of 5,800 gal per hour per sq ft, allowing storm water to quickly move down through the base below to maximize storm water infiltration. Installing the material required 3 in. of excavation. Because it is a pour-in-place material, it could be shaped to conform to the new path’s curves. Its rubber content makes the paved surface non-slip and ADA-compliant. Bratton was able to install the material in a tan color that fits with the natural landscape. 

Impervious pavement also has been installed in other Houston locations, including the University of Houston (pictured).
Permeable pavement also has been installed in other Houston locations, including the University of Houston (pictured). 

Installation & Results

The new path is 600 ft long and 5 ft wide. It is elevated 1 to 2 in. above the surrounding ground and consists of 1.5 in. of Porous Pave XL batch mixed on-site and poured in place atop a 3-in. base of compacted No. 57 granite aggregate. A total of 2,500 sq ft of the material was used for the project. 

The Piper Whitney crew first excavated and removed the old pavers and regraded the site to raise its elevation. They also constructed two drainage swales to direct water away from the course of the new path. They then installed and compacted the 3-in. base. 

Next, GeoSolutions Inc., a regional dealer, delivered the permeable pavement to the site, where it was staged in a parking lot. The Piper Whitney crew used a portable mortar mixer to mix each 50-lb bag of crushed granite aggregate with a 50-lb bag of recycled rubber chips and 5 quarts of binder. Each batch yields about 16 sq ft of material at a 1.5-in. depth. The crew took the mixed material down the path in wheelbarrows. 

According to Bratton, installers have approximately 30 minutes after mixing the product before it starts to set up. 

“By the time the crew bull floated and troweled one wheelbarrow of the product down, they were ready to get another batch from the members of the crew who handled the mixing,” he said.

At the Nature Discovery Center, the installation was challenging because the site limited where the installers could move and position the mortar mixer along the course of the path without damaging plants growing along the sides. Nonetheless, Piper Whitney was able complete the project
in two days.

The new path was installed in March 2017. Five months later, the path faced a tough test when Hurricane Harvey hit Texas in August. The new permeable pavement withstood the floodwaters. When the rain stopped, Nature Discovery Center staff checked the site and found water covering the surrounding area, but the new path was dry. 

About the author

David Aquilina is a freelance writer and independent consultant. Aquilina can be reached at [email protected]