Largest Installation of Permeable Interlocking Concrete Pavement Frames Idaho Development
By Sheryl S. Jackson
When completed at 900,000 sf, PICP streets throughout BridgeTower West development aims will be the largest PICP installation in the U.S.
When completed with almost 900,000 sf of permeable interlocking concrete pavement (PICP), the master planned community of BridgeTower West, a 500-acre residential development in Meridian, Idaho, will be the largest installation in North America. Although residential neighborhood roads are typically asphalt in Meridian, BridgeTower West’s developer wanted PICP’s upscale appearance. While some developers choose pavers only for sidewalks, crosswalks or driveways, BridgeTower West’s developer placed them in the street. That decision emerged from confident use of durable concrete pavers for communities in California.
With 14 development phases, the first two phases enabled engineers and contractors an opportunity to tweak the PICP design after evaluating paver performance. “The first phases installed by a different contractor were designed with some features that did not allow the pavers to perform appropriately,” says Blaine Bergin, president of Northwest Hardscape Specialties, who installed the pavers after the initial two phases. The initial phases followed more traditional road design with curbs and gutters that concentrated rainwater and sediment runoff into one area versus disbursing them evenly across the pavement. The result was some surface clogging.
Recognizing lessons learned, the design was slightly reworked to improve drainage into the subgrade. These changes consisted of new road profiles, a custom curb design and isolated infiltration areas to increase overall system performance, explains Pete Wilson, construction manager for Idaho Materials and Construction, the general contractor.
Machines accelerate installation of concrete pavers by placing about a square yard on screeded No. stone. Upon installation, the joints are filled with permeable aggregate and the surface is compacted. Right photo courtesy of Blain Bergin
The 3 1/8 in. thick concrete pavers were supplied by an ICPI member and layers machine-placed over 2 in. of No. 8 stone on top of 4 in. of No. 57 stone. The subbase reservoir ranged between 12 to 16 in. of No. 2 stone separated from the soil subgrade with geotextile that also covered the sides of the excavation. This provided sufficient storage for the average 11 in. of annual rainfall delivered mostly during the winter and spring.
The thick subbase layer stores and slowly infiltrates water into soil subgrade, as well as providing structural support for traffic. Because the area has silty sand soil with a high water table, drain columns that added infiltration were strategically placed every 250 feet under the sidewalk to collect and direct water into the reservoir, says Mr. Bergin. There were no local stormwater regulations requiring PICP use. However, the project was designed to handle stormwater runoff to ensure a long-lasting permeable pavement system.
Use of PICP to handle stormwater eliminated the need for catch basins, piping and surface ponds,” says Matt Munger, P.E., Director of Development Services for WHPacific, who served as design engineer for the project. “Runoff from driveways and sidewalks are accounted for in the calculations because they directly connect to the paver system,” he says. Although roof runoff may get to the system, it flows across the landscaped areas first, which reduces the amount of water from roofs, he adds. “The local highway district has jurisdiction on stormwater and prior to this project had used or allowed pavers in their right of way,” says Mr. Munger. “Since this project they now allow the use and even specify them on select internal projects.”
Utility work was completed before Mr. Bergin’s crew began placing pavers on the road. When his crew approached the concrete collars around utility covers, “We would lay the pavers by hand at these points, but there was always a height difference,” he explains. “We solved the problem by creating a square metal frame around the manholes and pave up to the frame, and then the contractor would pour concrete.” This simple change made it easier on everyone, he adds.
Improperly installed concrete pavers in some city crosswalks in the 1990s created a negative impression among local officials. This impression required convincing PICP use to Ada County transportation department officials responsible for maintenance of the completed roads. To alleviate concerns, the contractor and developer posted bonds well over the standard amount and warranty period to guarantee the roadway, says Mr. Munger. Although the warranty for asphalt roads is normally two years, the general contractor and developer provided a five-year warranty to the county.
The concrete pavers help slow drivers further contributing to pedestrian safety.
Traffic Calming Effects
“Personally, I love the product and one of the things that I like the most is an unintended benefit,” says Mr. Munger. “The pattern and the joint spacing in the road has a different feel and gives more feedback through the steering wheel than on a standard road surface making it uncomfortable to drive fast.” Since the product was used on local streets lined with homes, reduced speeds make the streets safer for the residents, he adds.
Even though county officials were initially wary of PICP roads, opinions have changed as some of the oldest streets reach five years of age, says Mr. Wilson. “The county has taken ownership of the first three phases and there has been little need for repair,” he says. “The first couple of phases were a learning experience, but even those streets are in good shape and handling stormwater well.”
“At first, inspectors and other highway department personnel were hesitant about how well permeable pavers would perform on a large surface, but now they understand more about how they work and their benefits,” says Mr. Bergin. “Permeable pavers are now required on a lot of jobs in the area, including a park-and-ride lot where permeable pavers eliminated the need for retention ponds, giving them use of more land for parking.”
Rainfall immediately infiltrates into the joints and open-graded base and subbase reservoir, thereby eliminating detention ponds. Photo courtesy of Blain Bergin
Project Credits
Owner: Cottonwood Development LLC, Viola, ID
Designer: Matt Munger, P.E., WHPacific, Boise, ID
Contractors:Northwest Hardscape Specialties, Meridian, IDIdaho Materials and Construction, Nampa, ID