Land and Water

Completed terminal 6 auto storage yard with porous pavement.

When faced with the challenge of designing improvements for the 51 acre development of the Auto Warehousing Corporation (AWC) auto storage facility at the Port of Portland (Port) Terminal 6 (T6) in Portland, Oregon, the design team considered all options to manage the large volume of stormwater that would be generated from surfacing the site. The preferred alternative that was constructed in the summer of 2006 was a porous pavement system with vegetated swales that provided for infiltration of 100% of the stormwater onsite.

Terminal 6 Porous Pavement Project facts:

Design Team
The Port hired Century West Engineering (Century West), a leading NW consulting firm in municipal infrastructure and sustainable design, for improvements to the auto storage area including new pavement, fencing, landscaping, and lighting. As the project developed, Century West partnered with GreenWorks, a Portland, Oregon-based landscape architecture firm with a sustainable design focus, and Cahill Associates, a nationally-recognized stormwater management expert, to design the project.

Design Considerations
The proposed development was an expansion of an existing auto storage facility at T6. Imported cars are offloaded from ships and the yard serves as the first point of rest for the cars where they can sit from a few days to a few weeks. The loading on the pavement is light auto traffic except in the areas where the cars are loaded onto large auto hauler trucks in an area called the “truckaway”. The truckaway area required a thickened pavement section to accommodate the heavier loading.

The Port had experience with large developments along the river and had already implemented many designs that utilized vegetated swales to improve water quality and in some cases, infiltrate the stormwater generated by new development. Unfortunately, the methods used at other facilities did not meet the design criteria or available land for this project.

Several factors influenced the design and ultimately lead to the use of porous pavement with vegetated swales. The primary concern was the tenant’s desire to have the facility constructed within the next construction season. This meant that the somewhat arduous and time-consuming process of permitting an outfall to the adjacent Columbia River was not an option. Permitting an outfall would mean losing a construction season, and subsequently was off the table.

Vegetated swale to manage stormwater runoff from impervious pavement.

The remaining options were to treat the stormwater and connect to the City of Portland storm system that ran adjacent to the site and/or to infiltrate the stormwater on site. The site is divided into two areas; one roughly 43 acres and the other roughly 8 acres. Preliminary calculations indicated that stormwater from approximately 10 acres of the 43-acre portion of the site and about half of the 8-acre site could be infiltrated through vegetated swales if the site was surfaced with standard pavements. The swales would be located outside the paved leased area utilizing all available space given the topography and zoning restrictions on the site. Stormwater that was not able to be treated via swales would have to be routed through a stormwater quality treatment system prior to disposal.

The porous pavement option would allow stormwater to infiltrate through the pavement section eliminating the need for an outfall or a stormwater quality treatment system and offsite disposal. Due to the need to pave the truckaway area with a thicker structural section, vegetated swales were included to treat and infiltrate the portion of the stormwater from the impervious pavement that could not be drained to the porous pavement section.

A cost benefit analysis comparing a traditional pavement to the porous pavement concept was conducted. The initial construction costs, long-term maintenance, and City of Portland Stormwater Fees were all factored into the analysis. The construction costs for the porous pavement system were similar to the traditional pavement system, but had significantly lower ongoing maintenance costs and reduced stormwater fees that made it the preferred alternative. The cost of the asphalt and thickened base added to the cost of the porous pavement section, but cost were saved by eliminating the piping, catch basins, outfall, additional swales, and stormwater quality devices that would have been necessary for the traditional pavement system. The porous pavement system also had a significantly shorter design and permitting schedule that allowed for earlier construction to meet the needs of the tenant.

Site Conditions
Terminal 6 at the Port of Portland is adjacent to the Columbia River just east of the confluence of the Columbia and Willamette rivers. The site has been filled over time with approximately six to eight feet of sandy fine-grained dredge material over the natural surface in the flood plain. Test pits excavated during the field investigation revealed fairly uniform conditions across the majority of the site with some occasional variations in the composition of the fill material. During construction, this assessment proved to be mostly correct, however one silty-clay area was encountered that required the material to be over excavated and replaced with sandy material from a nearby borrow source.

The surface of the redevelopment area was previously covered with 2-3” minus crushed aggregate with many fines. Due to the size of the material and the difficulty keeping the larger aggregate in place, the tenant used a 12 ton roller on a weekly basis to compact the material between waves of new autos being offloaded. The result was the compaction of the top 12-15 inches of subgrade material.

The permeability of the subsurface soils below the compacted surface was measured to establish an infiltration rate for design. The subsurface soils were generally very well draining with a few localized areas that were poorly draining. It was determined that the poorly draining areas could be over excavated or reconditioned to provide better drainage.

Design Concept
With confirmation from the field work that a porous pavement system was a good candidate for the site, the design was initiated. During the design process it became clear that to match existing grades on the larger site it would be necessary to fill as much as 4 feet from existing grade. To accomplish this and balance the cut and fill on the site, it was determined that the existing surfacing material could be used to fill along the existing perimeter road and surfaced with traditional impervious pavement that would be graded to drain to the porous pavement. The relatively high infiltration capacity on the site allowed for approximately 7 acres of impervious pavement to be drained to the 33 acres of porous pavement.

Another significant design issue was the treatment of the runoff from the surfacing of the truckaway area with impervious pavement. About half of the area based on grades could be drained and infiltrated in the porous pavement section adjacent to the truckaway. The remaining area was collected via catch basins and piped to a series of vegetated swales that also served as the entrance landscaping to the AWC portion of T6.

Porous pavement over coarse aggregate infiltration bed.

Porous Pavement Section
The design of the porous pavement system was performed by Cahill Associates. They performed the field investigation, and based on the infiltration rates encountered developed a pavement section to store the 10 year storm event in the pavement section and drain it within 24 hours per the requirements in the code. The design section was 3 inches of open graded asphalt cement concrete (porous pavement) over 10 inches of uniformly graded coarse aggregate (AASHTO NO 3) with a choker coarse (AASHTO NO 57) between the pavement section and coarse aggregate and a non-woven geotextile between the stone base and the subgrade.

For porous pavement applications geotextile fabric is placed over uncompacted subgrade. On many undeveloped sites this would likely mean cutting to grade and then placing the fabric over the uncompacted soil. Due to the routine compaction efforts on the Port site, it was necessary to recondition the subgrade surface by ripping the subgrade and then using a rake behind a farm tractor to condition the material at the surface. Once the material was ripped and raked close to the design grade, the fabric was rolled into place in preparation for the placement of the coarse aggregate.

The coarse aggregate is uniformly graded clean crushed aggregate and has approximately 40% void space in place. Imagine the spaces in a bowl of marbles, but with angular fractured faces that allow the rock to stack and lock together providing a stable base for the pavement surface course. The void space provides the storage capacity for the stormwater in the base.

The placement of the coarse aggregate is accomplished in a similar manner to base construction during winter conditions. The fabric is rolled out and the aggregate is placed starting from one end working the material out from the working base of aggregate. It is critical that the equipment placing the coarse aggregate is not allowed to travel across and compact the unprotected subgrade.

Once the coarse aggregate has been placed to design grade, it may be necessary to place a choker course of clean uniformly graded crushed aggregate that is smaller than the coarse aggregate base material. The choker course fills some of the void space and provides a working surface for paving.

The wearing surface of the porous pavement system for the Port project was a three inch lift of open graded asphalt concrete pavement. Open graded mixes have been used for various applications for a number of years by the Oregon Department of Transportation (ODOT). The design team only slightly modified the ODOT specifications for open graded AC pavement in order to use a mix that the local AC pavement providers were familiar with using.

Geotextile fabric and coarse aggregate placed over uncompacted subgrade.

Integrating Landscape
Landcape Goals
The goals for landscaping were fivefold:

1. Respect land use and natural environment context.
2. Meet City of Portland Code Requirements.
3. Protect and enhance Columbia River natural resource zone along northern perimeter of project area.
4. Provide landscaped stormwater swales to treat runoff from impervious pavement areas.
5. Accommodate capabilities of Port maintenance staff.

Land Use and Natural Environment Context
The site is industrial and adjoins heavy industrial property to the east (Marine Terminal 6), and light industrial to the south, across Marine Drive. Kelly Point Park with a dense forested buffer abuts the west boundary, and the Columbia River with a riparian forest buffer abuts the northern perimeter. Landscape improvements would focus on preserving the natural environmental context and maintaining / enhancing the existing industrial landscape.

City of Portland Code Requirements
Landscape requirements for storage yards are focused on perimeter screening requirements. Perimeter screening requirements were met by existing forested areas on the west (Kelly Point Park) and north perimeters (Columbia River riparian forest). Perimeter landscaping requirements were met on the eastern and southern perimeters by existing landscaping areas.

Columbia River Natural Resource Zone Protection and Enhancement
The project focused on temporary and permanent erosion control measures to offset environmental impacts from constructing the auto storage yard adjacent to the Columbia River. Hydroseeding and application of straw wattles were used to minimize erosion along the regraded north bank of the auto storage yard adjacent to Columbia River riparian forest. Cuttings of native plants (Salix sitchensis and Cornus sericea) were specified to provide a permanent cover for long-term erosion control adjacent to the existing riparian buffer.

Stormwater Treatment Swales
Those areas of the project that were paved with porous asphalt required no additional stormwater treatment, however, the areas that were paved with impervious asphalt required stormwater quality facilities per city code. The "truckaway" areas were paved with impervious pavement and were constructed with stormwater swales to receive runoff for treatment and infiltration. These stormwater swales were built along the eastern and southern perimeters of the project area. Due to the proximity of the new swales to the major site entrance and along Marine Drive (highly - visible locations) they were designed with aesthetics as a design criteria. Plant material was selected to function in swale environments and provide year round site aesthetics.

Landscape Maintenance Considerations
Temporary irrigation systems were installed to insure success of ground layer and shrub/tree plantings for the riparian buffer plantings along the Columbia River. Irrigation systems were installed for the stormwater swale plantings to insure their establishment. River rock mulch was substituted for bark mulch in all stormwater swales to prevent bark mulch from floating off and clogging overflow pipes. Native plants were prioritized to reduce long-term maintenance requirements

Construction
The assumptions made based on the field investigation were tested when the grading operation began to remove the existing crushed aggregate at the surface to cut to the top of subgrade elevation. In large part the subsurface conditions were similar to the conditions anticipated for the site. There were several localized areas where the material appeared to be considerably more silty and even a few clay areas. This and the variation in the degree of compaction encountered at the top of subgrade elevation required visual inspection and infiltration testing to verify that the design infiltration rates would be realized. This testing allowed the Port construction inspector and the contractor to tailor the amount of reworking of the subgrade to account for the varying conditions and ensure that the subgrade would provide the necessary drainage.

Preliminary site grading

Once the subgrade preparation was completed, the coarse aggregate and choker course were placed. The QA/QC program ensured that the crushed aggregate arriving on site met the gradation requirements and the quality of the aggregate supplied by the contractor was excellent. The material was graded to top of base elevation per the design and then set into place by a few static passes with a 12-ton roller.

When it came time to pave, the contractor noticed that truck traffic on the coarse aggregate surface would create ruts which would make it difficult to get a smooth finished product. The General Contractor Foreman for the project from Coffman Excavation suggested that Lakeside Industries, the paving contractor, use an offset transfer devise to deliver the hot mix to the hopper on the paver. This idea worked very well and allowed a roller to make a final pass to smooth the surface of the base just in front of the paving machine. This method was so crucial to the proper placement of the material that it will be a requirement in the specifications for future projects at the Port.

Paving with open graded mix for a porous pavement application is very similar to standard paving. The main difference is that it is easy to over compact and breakdown the mat by over rolling. The roller operators should provide a breakdown pass soon after the mat is laid down by the paving machine. After the breakdown pass, the operators must hold off until the mat has cooled into the compaction range identified in the mix design. At this point the rollers can make another pass or two, but special care has to be taken not to over roll the mat. After a few passes, the roller operators must allow the mat to cool again well below the compaction temperature range before getting back on the mat to finish roll.

Conclusions
Each new project provides challenges and opportunities to assess the needs of the end user and develop the best overall strategy to meet those needs in a manner that provides for the best solution for the client, the community, and the environment. This project was an example where all three were well served by the design. The porous pavement system provided a cost effective solution that allowed a greatly reduced schedule compared to the alternatives. It also provides vastly superior stormwater treatment mimicking the predevelopment condition on the site and reducing the impact of the project on the storm surge that would have been created with a standard outfall.

For more information, contact Port Project Manager: Dave Dittmer (503) 944-7342, Century West Project Manager: Matt Rogers (503) 419-2134, GreenWorks Principal: Mike Faha (503) 222-5612, or Cahill Associates Principal: Tom Cahill (610) 696-4150.