In 2010, the Port of Portland, with Auto Warehousing Corporation developed an expansion to the vehicle processing and distribution facility at Terminal 6. Instead of trying to manage the large volume of storm water that would be generated from surfacing the site with conventional asphalt, the design team pursued an alternative approach. The result was a porous pavement system that provided for infiltration of 100% of the expansion site’s storm water.
The 35-acre porous pavement area serves as the First Point of Rest (FPR) for imported vehicles after offloading from the ship. The vehicles are stored within the FPR from several days to a several weeks. Vehicle loadings within the FPR are relatively light as trucks are not allowed within this area. The 15-acre impervious pavement area serves as the truck loading facility for vehicles being transported to local dealerships. Vehicle loadings within the truck loading area are much greater and necessitated a heavy duty pavement section. The combination of heavier loadings and increased likelihood of oil and hydraulic fluid spills led to the decision to use impervious ac pavement for this area. A portion of the new impervious pavement area was designed to sheet drain onto the pervious pavement, with the remainder collected via catch basins and piped to a series of vegetated bio-swales that also serve as entrance landscaping to Terminal 6.
Prior to identifying a final design, a cost-benefit analysis was prepared comparing a traditional pavement to the porous pavement system. The initial construction costs, maintenance costs, and City of Portland storm water fees were all factored into the analysis. By eliminating the piping, catch basins, outfall, additional swales, and storm water quality devices that would have been necessary for the traditional pavement system, significant savings were identified. When the design team analyzed all considerations, a porous paving system emerged as the most attractive plan.
The Port of Portland’s Terminal 6 is adjacent to the Columbia River and situated east of the confluence of the Columbia and Willamette rivers. The 50 acre expansion site has been filled with between six and eight feet of fine-grained dredge material (sand) over the natural surface. The surface of the redevelopment area was previously covered with 2”-3” crushed aggregate . The permeability of the subsurface soils below the compacted surface was measured to establish a designed infiltration rate. The subsurface soils generally drained very well with the exception of several small, localized areas.
Following field investigations that identified predicted infiltration rates on the site, the system was designed to accommodate a ten-year critical storm event within 24 hours. The design section incorporated three inches of open graded porous asphalt over ten inches of uniformly graded coarse aggregate (AASHTO No. 3). A choker course of smaller aggregate (AASHTO No. 57) was spread between the pavement section and coarse aggregate.
Additionally, a non-woven geotextile fabric was placed between the aggregate base and the sub-grade. On most undeveloped sites, the area would be cut to grade and then covered with the fabric. However, in years prior to the expansion, Terminal 6 underwent several compaction efforts that threatened infiltration. Consequently the sub-grade was reconditioned by ripping and raking to produce a uniform but uncompacted surface. Once reconditioning was completed, the fabric was rolled into place in preparation for coarse aggregate.
The coarse aggregate consisted of crushed stone with approximately 40% void space. It was critical that the equipment placing the coarse aggregate was not allowed to travel across and compact the unprotected sub-grade. Once the coarse aggregate was placed, a layer of finer choker course aggregate was spread across the site. The choker course filled some of the void space and provided a working surface for paving.
The wearing surface consisted of a three-inch lift of open graded asphalt concrete pavement. The Oregon Department of Transportation (ODOT) has used open graded (porous) mixes for various applications. The design used a design similar to ODOT specifications so that the local pavement producer had familiarity with the material.
The areas of the expansion that were paved with porous asphalt did not require any additional storm water treatment; however, the impervious truck loading area led to the construction of swales to receive runoff for treatment and infiltration. The swales have the added ability to naturally filter any spills of hydrocarbons and hydraulic fluid that may leak from the trucks. These storm water swales were built along the eastern and southern perimeters of the expansion area. Because the swales form the entrance to the facility, they serve a second purpose of aesthetic landscaping. Plant material was selected to function in swale environments and provide year round site aesthetics. Special attention was paid to the planting of native species.
Porous pavement provided a cost-effective solution that allowed for vastly superior storm water treatment and cost benefits at Port of Portland’s Terminal 6. The Port realized about $250,000 of savings by implementing a porous pavement system. Similar techniques to the T6 expansion should be considered in new construction and existing developments if they meet the necessary site criteria.