Pulling Together the Pieces of the stormwater Puzzle
Porous pavement and other techniques in Portland
With an average of 37 inches per year of precipitation, Portland, OR, earns its reputation as a rainy city. Locals enjoy the city’s lush green environment as a reasonable tradeoff for its many rainy days. Over the last few years, Portland also has developed a reputation for another kind of “green” as the city has become one of the national centers of creative, innovative approaches to green development practices, including green buildings, green streets, and green parking lots.
Like other major cities in the United States, Portland’s development over the last four decades reflects the dominance of automobiles and trucks in industry, the region’s economy, and personal lifestyles. Development in the automobile age has resulted in vast areas of impervious surface for roadways, parking lots, and driveways. Engineers designed very efficient stormwater collection and conveyance systems that quickly removed stormwater from the developed area. Stormwater runoff from the constructed impervious surface was collected and routed into nearby rivers, streams, and other natural drainages. Waterways became regularly inundated with surges of stormwater, which negatively has affected both water quality (pollution runoff) and water quantity (rate of runoff.)
The result of this development approach has devastated the natural environment with flooding, pollution, degradation of riparian zones and habitat, lowering of groundwater tables, and many other problems. Agencies, municipalities, and design and construction professionals have been working to correct the situation by adding retrofits to old systems and designing new development that meets higher standards.
Development in Portland is entering a new generation of stormwater management practices by using tools like porous pavement, seepage trenches, bioswales, infiltration planters, and other innovative techniques not only to treat and retain or detain stormwater but also to attempt to more closely mimic the predevelopment hydrologic cycle for a site. Engineers and landscape architects currently are working with both public agencies and private developers in Portland to assess sites and develop realistic strategies to manage the stormwater runoff in a manner that improves water quality through treatment, infiltrates a portion of the runoff consistent with the infiltration capacity of the onsite soils, and slows the flow to reduce storm surge.
Stormwater problems are complex puzzles, as every parcel of land in each watershed is connected to the next watershed by the nearest stream or river—which transmits and perpetuates the impact of every occurrence downstream. Conversely, each new development project represents an opportunity to make one piece of the puzzle a contributor to the whole solution.
The Port of Portland Adds a Piece to the Puzzle
While a large, industrial auto storage yard might not immediately come to mind as the best candidate for environmentally friendly, green design solutions, the Port of Portland’s Terminal 6 Expansion proves that rethinking old materials and approaches can move us forward in solving the stormwater puzzle.
In 2005, the Port of Portland embarked upon a plan to improve the Terminal 6 (T6) auto storage facility, which it leases to the Auto Warehousing Corporation (AWC) for offloading and processing of import cars arriving from overseas ports. 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.
Due to increases in volume, the T6 facility was in need of expansion to resurface approximately 50 acres for auto storage. The T6 project, an industrial use located adjacent to the Columbia River, presented an excellent opportunity to address a major source of urban watershed problems, impervious pavement.
The port hired Century West Engineering, a leading Northwest 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-based landscape architecture firm with a sustainable design focus, and Cahill Associates, a nationally recognized stormwater management expert, to design the project.
The design team considered all options to manage the large volume of stormwater that would be generated from surfacing the site. In addition to meeting the functional requirements of the AWC, the team explored the potential to reduce effective impervious area impacts and mitigate runoff impacts where permeable paving was not practical.
The preferred alternative that was constructed in the summer of 2006 resulted in 35.7 acres of porous pavement and 15.4 acres of impervious pavement for a total new developed area of 51.1 acres. The combination of a porous pavement system coupled with vegetated swales provides for infiltration of 100% of the stormwater onsite.
 |
| A parking lot was retrofitted with a biofiltration swale cut into existing nonporous asphalt. |
Design Issues: Considering the Alternatives
The port had experience with large developments along the river and already had implemented many designs that utilized vegetated swales to improve water quality and, in some cases, to 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 led 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 a new stormwater outfall to the adjacent Columbia River was not an option.
Available 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 onsite. 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 portion could be infiltrated through vegetated swales if the site was surfaced with standard pavement. The swales would be located outside the paved leased area, utilizing all available space given the topography and zoning restrictions on the site. Stormwater not treated via swales would have to be routed through an alternative stormwater-quality treatment system prior to disposal, such as an underground vault system or ponds.
The porous pavement option allowed 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, runoff from this area was collected and routed to vegetated swales that were included to treat and infiltrate the stormwater.
Site Screening for Porous Pavement Compatibility
Terminal 6 at the Port of Portland is adjacent to the Columbia River just east and upstream of the confluence of the Columbia and Willamette rivers. The site has been filled over time with approximately 6 to 8 feet of sandy fine-grained dredge material over the natural surface in the old Columbia River floodplain. 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- to 3-inch-minus crushed aggregate with fines. Due to the size of the material and the difficulty keeping the larger aggregate in place, the tenant uses 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 to 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.
Cost Benefit Analysis
The two options analyzed were:
- A traditional pavement with catch basin collection system, pretreatment, and disposal into the Columbia River and to the City of Portland stormwater system
- A porous pavement and swale system to infiltrate 100% of the stormwater onsite
A cost benefit analysis comparing a traditional pavement approach to the porous pavement concept was conducted. The initial construction costs, long-term maintenance, and City of Portland stormwater fees (which are calculated based on the area of impervious surface on the site) were all factored into the analysis.
The cost of the asphalt and thickened base added to the cost of the porous pavement section, but costs were saved by eliminating the piping, catch basins, outfall, additional swales, and other 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, based on avoiding the need for US Army Corps of Engineers or Oregon Division of State Lands permits for an outfall to the Columbia River. This approach enabled construction during 2006, allowing the port to meet the needs of the tenant.
The port and the design team concluded that construction time savings, a similar construction cost, and significantly lower ongoing maintenance costs and reduced stormwater fees made porous pavement the preferred alternative.
 |
| Rolling out filter fabric at Terminal 6 |
Design Concept
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.
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 |
 |
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Porous Pavement Section
The design of the porous pavement system was performed by Cahill Associates, who performed the field investigation. Based on the infiltration rates encountered, Cahill Associates 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 nonwoven geotextile fabric between the stone base and the subgrade.
 |
| Detail of porous pavement layers |
For porous pavement applications, geotextile fabric is placed over uncompacted subgrade. On many undeveloped sites, this would likely mean cutting to grade and 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.
 |
| Close-up of porous pavement over choker course/coarse aggregate infiltration reservoir |
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 3-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 asphalt concrete (AC) pavement in order to use a mix that the local AC pavement providers were familiar with using.
Integrating Landscape
Working collaboratively with the project’s engineers, GreenWorks landscape architects designed the T6 landscape to meet the following five goals:
- Respect land use and natural environment context.
- Meet City of Portland code requirements.
- Protect and enhance the Columbia River natural resource zone along northern perimeter of project area.
- Provide landscaped stormwater swales to treat runoff from impervious pavement areas.
- Accommodate capabilities of port maintenance staff.
 |
| The entrance rain garden is heavily planted with mostly native plants. |
Although the T6 site is industrial, a large park and a densely forested buffer abuts the western boundary, and the Columbia River with a riparian forest buffer abuts the northern perimeter. Landscape improvements were designed to focus on preserving the natural environmental context while maintaining and enhancing the existing industrial landscape.
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 such as Sitka willow and red-osier dogwood (Salix sitchensis and Cornus sericea) were specified to provide a permanent cover for long-term erosion control adjacent to the existing riparian buffer.
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 important criteria. Plant material was selected to function in swale environments and provide year-round site aesthetics.
Temporary irrigation systems were installed to ensure success of ground layer and shrub/tree plantings for the riparian buffer plantings along the Columbia River. Irrigation systems were installed to establish the stormwater swale plantings. River rock was substituted for bark mulch in all stormwater swales to prevent bark mulch from floating off and clogging overflow pipes. Native plants were primarily specified to reduce long-term maintenance requirements. Native plantings included red alder (Alnus rubra), western red cedar (Thuja plicata), creeping mahonia (Mahonia repens), red meidiland rose (Rosa ‘Meidiland Red’), yellow twig dogwood (Cornus stolonifera ‘Flaviramea’), California gray rush (Juncus patens), Kelsey dogwood (Cornus stolonifera ‘Kelseyi’), and kinnikinnick (Arctostaphylos uva-ursi).
 |
Construction Phase
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 siltier 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.
Once the subgrade preparation was completed, the reservoir course and choker course were placed. The quality assurance/quality control program ensured that the crushed aggregate arriving onsite 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.
 |
| Gravel mulch, rather than floatable bark
mulch, provided temporary erosion control. |
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’s foreman for the project from Coffman Excavation suggested that Lakeside Industries, the paving contractor, use an offset transfer device 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 overcompact and break down 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.
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Results
When the T6 Expansion was completed, it was given a final test—running a stream of water from a hose on the surface of the lot. Portland’s largest application of porous pavement performed well as the “thirsty asphalt” swallowed water at a rate demonstrating it could easily absorb a rainfall of 20 inches per hour.
The Port of Portland’s Terminal 6 Expansion demonstrates one successful approach to piecing together the stormwater puzzle. Under the right set of conditions, porous pavement systems provide an economical and environmentally friendly stormwater management solution. By choosing not to pave the 51-acre industrial site with asphalt known for contributing to ongoing stormwater problems, the Port of Portland implemented a cost-effective and vastly superior stormwater treatment that mimics the predevelopment condition on the site while reducing the impact on the storm surge that would have been created with a standard outfall.
September 2007
Pulling Together the Pieces of the stormwater Puzzle
Porous pavement and other techniques in Portland
With an average of 37 inches per year of precipitation, Portland, OR, earns its reputation as a rainy city. Locals enjoy the city’s lush green environment as a reasonable tradeoff for its many rainy days. Over the last few years, Portland also has developed a reputation for another kind of “green” as the city has become one of the national centers of creative, innovative approaches to green development practices, including green buildings, green streets, and green parking lots.
Like other major cities in the United States, Portland’s development over the last four decades reflects the dominance of automobiles and trucks in industry, the region’s economy, and personal lifestyles. Development in the automobile age has resulted in vast areas of impervious surface for roadways, parking lots, and driveways. Engineers designed very efficient stormwater collection and conveyance systems that quickly removed stormwater from the developed area. Stormwater runoff from the constructed impervious surface was collected and routed into nearby rivers, streams, and other natural drainages. Waterways became regularly inundated with surges of stormwater, which negatively has affected both water quality (pollution runoff) and water quantity (rate of runoff.)
The result of this development approach has devastated the natural environment with flooding, pollution, degradation of riparian zones and habitat, lowering of groundwater tables, and many other problems. Agencies, municipalities, and design and construction professionals have been working to correct the situation by adding retrofits to old systems and designing new development that meets higher standards.
Development in Portland is entering a new generation of stormwater management practices by using tools like porous pavement, seepage trenches, bioswales, infiltration planters, and other innovative techniques not only to treat and retain or detain stormwater but also to attempt to more closely mimic the predevelopment hydrologic cycle for a site. Engineers and landscape architects currently are working with both public agencies and private developers in Portland to assess sites and develop realistic strategies to manage the stormwater runoff in a manner that improves water quality through treatment, infiltrates a portion of the runoff consistent with the infiltration capacity of the onsite soils, and slows the flow to reduce storm surge.
Stormwater problems are complex puzzles, as every parcel of land in each watershed is connected to the next watershed by the nearest stream or river—which transmits and perpetuates the impact of every occurrence downstream. Conversely, each new development project represents an opportunity to make one piece of the puzzle a contributor to the whole solution.
The Port of Portland Adds a Piece to the Puzzle
While a large, industrial auto storage yard might not immediately come to mind as the best candidate for environmentally friendly, green design solutions, the Port of Portland’s Terminal 6 Expansion proves that rethinking old materials and approaches can move us forward in solving the stormwater puzzle.
In 2005, the Port of Portland embarked upon a plan to improve the Terminal 6 (T6) auto storage facility, which it leases to the Auto Warehousing Corporation (AWC) for offloading and processing of import cars arriving from overseas ports. 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.
Due to increases in volume, the T6 facility was in need of expansion to resurface approximately 50 acres for auto storage. The T6 project, an industrial use located adjacent to the Columbia River, presented an excellent opportunity to address a major source of urban watershed problems, impervious pavement.
The port hired Century West Engineering, a leading Northwest 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-based landscape architecture firm with a sustainable design focus, and Cahill Associates, a nationally recognized stormwater management expert, to design the project.
The design team considered all options to manage the large volume of stormwater that would be generated from surfacing the site. In addition to meeting the functional requirements of the AWC, the team explored the potential to reduce effective impervious area impacts and mitigate runoff impacts where permeable paving was not practical.
The preferred alternative that was constructed in the summer of 2006 resulted in 35.7 acres of porous pavement and 15.4 acres of impervious pavement for a total new developed area of 51.1 acres. The combination of a porous pavement system coupled with vegetated swales provides for infiltration of 100% of the stormwater onsite.
|
| A parking lot was retrofitted with a biofiltration swale cut into existing nonporous asphalt. |
Design Issues: Considering the Alternatives
The port had experience with large developments along the river and already had implemented many designs that utilized vegetated swales to improve water quality and, in some cases, to 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 led 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 a new stormwater outfall to the adjacent Columbia River was not an option.
Available 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 onsite. 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 portion could be infiltrated through vegetated swales if the site was surfaced with standard pavement. The swales would be located outside the paved leased area, utilizing all available space given the topography and zoning restrictions on the site. Stormwater not treated via swales would have to be routed through an alternative stormwater-quality treatment system prior to disposal, such as an underground vault system or ponds.
The porous pavement option allowed 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, runoff from this area was collected and routed to vegetated swales that were included to treat and infiltrate the stormwater.
Site Screening for Porous Pavement Compatibility
Terminal 6 at the Port of Portland is adjacent to the Columbia River just east and upstream of the confluence of the Columbia and Willamette rivers. The site has been filled over time with approximately 6 to 8 feet of sandy fine-grained dredge material over the natural surface in the old Columbia River floodplain. 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- to 3-inch-minus crushed aggregate with fines. Due to the size of the material and the difficulty keeping the larger aggregate in place, the tenant uses 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 to 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.
Cost Benefit Analysis
The two options analyzed were:
- A traditional pavement with catch basin collection system, pretreatment, and disposal into the Columbia River and to the City of Portland stormwater system
- A porous pavement and swale system to infiltrate 100% of the stormwater onsite
A cost benefit analysis comparing a traditional pavement approach to the porous pavement concept was conducted. The initial construction costs, long-term maintenance, and City of Portland stormwater fees (which are calculated based on the area of impervious surface on the site) were all factored into the analysis.
The cost of the asphalt and thickened base added to the cost of the porous pavement section, but costs were saved by eliminating the piping, catch basins, outfall, additional swales, and other 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, based on avoiding the need for US Army Corps of Engineers or Oregon Division of State Lands permits for an outfall to the Columbia River. This approach enabled construction during 2006, allowing the port to meet the needs of the tenant.
The port and the design team concluded that construction time savings, a similar construction cost, and significantly lower ongoing maintenance costs and reduced stormwater fees made porous pavement the preferred alternative.
|
| Rolling out filter fabric at Terminal 6 |
Design Concept
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 Section
The design of the porous pavement system was performed by Cahill Associates, who performed the field investigation. Based on the infiltration rates encountered, Cahill Associates 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 nonwoven geotextile fabric between the stone base and the subgrade.
|
| Detail of porous pavement layers |
For porous pavement applications, geotextile fabric is placed over uncompacted subgrade. On many undeveloped sites, this would likely mean cutting to grade and 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.
|
| Close-up of porous pavement over choker course/coarse aggregate infiltration reservoir |
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 3-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 asphalt concrete (AC) pavement in order to use a mix that the local AC pavement providers were familiar with using.
Integrating Landscape
Working collaboratively with the project’s engineers, GreenWorks landscape architects designed the T6 landscape to meet the following five goals:
- Respect land use and natural environment context.
- Meet City of Portland code requirements.
- Protect and enhance the Columbia River natural resource zone along northern perimeter of project area.
- Provide landscaped stormwater swales to treat runoff from impervious pavement areas.
- Accommodate capabilities of port maintenance staff.
|
| The entrance rain garden is heavily planted with mostly native plants. |
Although the T6 site is industrial, a large park and a densely forested buffer abuts the western boundary, and the Columbia River with a riparian forest buffer abuts the northern perimeter. Landscape improvements were designed to focus on preserving the natural environmental context while maintaining and enhancing the existing industrial landscape.
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 such as Sitka willow and red-osier dogwood (Salix sitchensis and Cornus sericea) were specified to provide a permanent cover for long-term erosion control adjacent to the existing riparian buffer.
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 important criteria. Plant material was selected to function in swale environments and provide year-round site aesthetics.
Temporary irrigation systems were installed to ensure success of ground layer and shrub/tree plantings for the riparian buffer plantings along the Columbia River. Irrigation systems were installed to establish the stormwater swale plantings. River rock was substituted for bark mulch in all stormwater swales to prevent bark mulch from floating off and clogging overflow pipes. Native plants were primarily specified to reduce long-term maintenance requirements. Native plantings included red alder (Alnus rubra), western red cedar (Thuja plicata), creeping mahonia (Mahonia repens), red meidiland rose (Rosa ‘Meidiland Red’), yellow twig dogwood (Cornus stolonifera ‘Flaviramea’), California gray rush (Juncus patens), Kelsey dogwood (Cornus stolonifera ‘Kelseyi’), and kinnikinnick (Arctostaphylos uva-ursi).
|
Construction Phase
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 siltier 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.
Once the subgrade preparation was completed, the reservoir course and choker course were placed. The quality assurance/quality control program ensured that the crushed aggregate arriving onsite 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.
|
| Gravel mulch, rather than floatable bark
mulch, provided temporary erosion control. |
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’s foreman for the project from Coffman Excavation suggested that Lakeside Industries, the paving contractor, use an offset transfer device 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 overcompact and break down 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.
Results
When the T6 Expansion was completed, it was given a final test—running a stream of water from a hose on the surface of the lot. Portland’s largest application of porous pavement performed well as the “thirsty asphalt” swallowed water at a rate demonstrating it could easily absorb a rainfall of 20 inches per hour.
The Port of Portland’s Terminal 6 Expansion demonstrates one successful approach to piecing together the stormwater puzzle. Under the right set of conditions, porous pavement systems provide an economical and environmentally friendly stormwater management solution. By choosing not to pave the 51-acre industrial site with asphalt known for contributing to ongoing stormwater problems, the Port of Portland implemented a cost-effective and vastly superior stormwater treatment that mimics the predevelopment condition on the site while reducing the impact on the storm surge that would have been created with a standard outfall.