A Perfect Storm

Winter 2014

The story of Idaho’s first large-scale permeable surface

Alicia Lasek


A Perfect Storm

Click to enlarge image and view more project photos

During a hard rain in October 2012, Jeff Ward, P.E., raced to the site of a new Whole Foods Market in downtown Boise, ID. The store anchors a 5.8-acre development that has drawn much attention for its visually pleasing design, including a herringbone-patterned, permeable interlocking concrete pavement (PICP) parking lot with sustainable landscaping beds. But aesthetics were not Ward’s focus on that rainy day. Upon arrival, he scanned the lot, looking for signs of standing water and runoff. The stormwater was infiltrating on contact and the site was completely puddle-free.

This was the first chance for Ward, a civil engineer at local design firm CSHQA, to see his permeable lot design perform before opening to the public in November 2012. That rainy day experience was just one of the many firsts associated with the project. The Boise Whole Foods Market store is the first in Idaho. Likewise, the 2.6-acre parking lot, consisting of almost an acre of PICP, is the first large-scale permeable surface project in Boise. The project also marked the first time for many involved to work with PICP.

A change in plans

The Whole Foods Market parking lot is shared with a sustainably-designed Walgreens. Together, this project completes the first phase of River Park Place, a mixed-use development at the edge of Boise’s central business district. But permeable pavement was not in the original plans, says developer Rick Duggan, director of design and construction, and partner at Schlosser Development Corp. (SDC) of Austin, TX. The lot has a high groundwater table, and the company initially explored the more traditional option of draining stormwater runoff to an offsite retention pond, about 200 yards away. But those plans were scrapped when the 2007 recession hit and the project stalled indefinitely.

By the time the project rebooted in 2012, the offsite option was off the table, says Duggan. The city’s MS4 permit was up for renewal, and the U.S. Environmental Protection Agency’s draft promised future restrictions on stormwater discharge to the Boise River. Faced with the need to manage 100 percent of stormwater onsite for all phases of the development, SDC now faced a space problem. “Full detention onsite would have chewed up the parking so badly that we needed to look at alternatives,” Duggan says. “The solution that emerged was a permeable concrete paving system on top of 42- to 48-in. [106- to 122-cm] engineered base of rock and sand.”

The system needed to handle 0.5 in. (1.27 cm) of rain per hour over its 50-year life span, a relatively light load due to Boise’s high-desert climate, but requirements also necessitated that the lot contain a 100-year storm, at a projected one inch (2.5 cm) per hour. With that in mind, project stakeholders needed reassurance about water retention capabilities, possible increased liability and the impact of frequent snowplowing on the paver surface. That’s when the forethought of a local supplier came in handy.

Let it snow

Years earlier, the supplier set the stage for showcasing what PICP could do as a system. He first approached the city’s highway department to answer questions and concerns, and then encouraged area contractors to invest in paver installation equipment and ICPI certification. The supplier also aided the city by initially installing a PICP at the Boise Watershed Environmental Education Center, which created a ready-made demonstration tool.

Duggan and his team found local contractors, ICPI-certified and equipped for PICP installation. They visited the demonstration site at the Boise Watershed Environmental Center and watched a city water truck release a torrent of 10,000 gallons onto the paver installation. “The water just disappeared; didn’t puddle, didn’t run off,” Duggan says of the experience.

Duggan’s team also visited PICP in Truckee, CA, near Lake Tahoe that had weathered five years of heavy snowstorms and snowplowing with no complications. That visit removed any concerns about snowplowing, he says. Another benefit to PICP use is that Boise does not use sand to treat icy downtown streets, which can clog PICP joints, says Ward.

This perfect storm of events and local readiness made the timing right for a large-scale permeable project in Boise, Ward explains. “Nothing of this size had been done before in Boise, and 10 years ago we probably couldn’t have convinced [stakeholders] to do this, but everyone is moving in the direction of sustainable design now.”

Installing the system

With questions and concerns answered, construction started in April 2012. The design plan specified grading to direct runoff from the larger development (currently in a phase-two stage) to the parking lot’s 39,000 sf (3,600 m2) of permeable pavers, including roof drainage to the paver surface and subsurface. A geotechnical engineer using ICPI guidelines designed for anticipated vehicular traffic.

The PICP is set back from the Whole Foods Market store by an asphalt drive lane. Landscape beds with native plants separate 16 PICP parking areas. Ward says one lesson learned was the amount of labor associated with compaction needed along the perimeter of each stand-alone paver bed. “While the current design is aesthetically pleasing, another option would be to decrease all of that perimeter work by keeping the same total area of pavers but limiting the number of [stand-alone] areas.”

The design used tan-colored pavers measuring 5.5 x 11 x 4 in. (14 x 28 x 100 mm) thick, machine installed in a herringbone pattern by local installer Northwest Hardscape Specialties. Six-inch (150-mm) wide flush containment curbs divided the pavers from the asphalt driving lanes. Raised curbs separate pavers from the landscaping beds.

The system is designed so that if flooded, water will flow away from the building foundations, says Ward, though the large PICP surface area combined with the subsurface containment makes this scenario highly unlikely. The design also allayed retailer concerns about shopping cart rattle, with much of the cart “roll” time being on the asphalt drive lanes, says Duggan.

Post-installation, the team excavated the paver system in the lot’s handicapped parking stalls to install concrete, per ADA requirements. Moving, stockpiling and replacing the paver system’s layers was a seamless process, Duggan says.

Award-winning design

The lot performed well in its first year, weathering an icy winter that included snowplowing, says Duggan, whose company also maintains the property.

In the end, he says SDC got more than its money’s worth from the paver system, which accounted for about six to eight percent of total construction costs and four percent of the $6 million in total project costs.

“We are really quite pleased with it,” says Duggan. “It turned out to be a competitive installation and it solved the problem we had to solve. We’ve had no issues with maintenance. It’s worked.” The Whole Foods Market store design, including the permeable lot, won the 2013 Boise Building Excellence Award in the Best Sustainable Projects category, as well as two Green Globes awards from the Green Building Initiative “demonstrating excellent progress in the reduction of environmental impacts and use of environmental efficiency practices,” according to design firm CSHQA. CSHQA was so impressed by the experience that it has since installed a similar permeable concrete paver system in its new, sustainably-designed Boise headquarters.