A.   Applications and Limitations

         1.  Applications

WSDOT is taking a conservative approach towards applying permeable surface systems within projects.  Pervious concrete and asphalt pavements are a replacement for conventional concrete and asphalt pavement or other hard paving surfaces.

1.1 Possible areas for use of these permeable surface Materials include: 
 

• Public and municipal parking lots, including perimeter and overflow parking areas.
• Vehicle access areas, including emergency stopping lanes, maintenance/enforcement areas on divided highways, facility maintenance access roads, and outside portions of roadway shoulders.
• Sidewalks, bicycle trails, community trail/pedestrian path systems, or any pedestrian-accessible paved areas such as traffic islands.

1.2 Permeable surface systems function as a stormwater infiltration area and temporary stormwater retention that can accommodate pedestrians, light-to medium-load parking areas.  It is applicable to both residential and commercial applications.  This combination of functions offers the following benefits:

• Captures and retains precipitation onsite;
• Mimics natural soils filtration throughout the pavement depth, underlying subbase reservoir, and native soils for improved groundwater quality;
• Eliminates surface runoff depending on existing soil conditions;
• Greatly reduces or eliminates the need for an on-site stormwater management system;
• Reduces drainage water runoff temperatures;
• Increases recharge of groundwater;
• A media filtration layer can provide water quality treatment.

1.3 Handling and placement practices for permeable surfaces are different from conventional pavement placement.  Unlike conventional pavement construction, it is important that the underlying native or subgrade soils be nominally consolidated to prevent settling and minimize the effect of intentional or inadvertent heavy compaction due to heavy equipment operation during construction.  Consolidation can be accomplished using static dual wheel small mechanical rollers or plate vibration machines.  If heavy compaction does occur, then tilling may be necessary to a depth of two feet below the material placement.  This would occur prior to subsequent application of the separation and base layers. 

Contractors should have prior experience with constructing permeable surfaces.  If a contractor does not have this experience, the contractor should be required to construct test panels prior to the placement of the main surfacing to demonstrate their application competency.

1.4 Permeable surfaces are vulnerable to clogging from sediment in runoff and the following techniques will reduce this potential:

• Surface Run-off—Permeable surfaces should not be located where turbid run-off from adjacent areas can introduce sediments onto the permeable surface.  Designs should slope impervious runoff away from permeable pavement installations to the maximum extent possible.
• Diversion—French Drains, or other diversion structures may be designed into the system to avoid unintended off-site run-off.  Permeable systems can be separated using edge drain systems, turnpikes, and 0.15-foot high tapered bumps.
• Cold Climates—Snow removal activities (plowing) and the use of salt and abrasives can increase the risk of clogging.
• Slopes—Off-site drainage slopes immediately adjacent to the permeable surface should be less than 5 percent to reduce the chance of soil loss that would cause clogging.

         2.  Limitations

2.1 Suitable grades, subsoil drainage characteristics, and groundwater table conditions require good multi-disciplinary analysis and design.  Following proper construction techniques and having diligent field inspection during the placement of permeable surfaces is essential to a successful installation.

• Works best with level adjacent slopes (1 to 2 percent) and on upland soils; Permeable surface installations are not appropriate when adjacent draining slopes are 5 percent or greater.
• An extended period of saturation of the base material underlying the surface is undesirable.  Therefore, the subsurface reservoir layer should fully drain in a period of less than 72 hours.
• The minimum depth from the bottom of the base course to bedrock and seasonally high water table should be 3 feet unless it is possible to engineer a groundwater bypass into the system.
• Sanding or frequent snow removal can lead to a reduction in a surface permeability.  Permeable surfaces should not be used in traffic areas where sanding or extensive snow removal is carried out in the winter.

2.2 Examples of situations where the use of permeable surfaces is not recommended at the time of this publication include:

• Roadway lanes.  Because of a number of considerations (e.g., dynamic loading, safety, clogging, and heavy loads), more study and experience is needed before using permeable surfaces in these situations.  Use of any type of shoulder application requires coordinated approval from Materials, roadway design, hydraulics, and maintenance support staff.  Areas where the permeable surface will be routinely exposed to heavy sediment loading.
• Areas where the risk of groundwater contamination from organic compounds is high (e.g., fueling stations, commercial truck parking areas and maintenance and storage yards).
• Within 100 feet of a drinking water well and within areas designated as sole source aquifers.
• Areas with a high water table or impermeable soil layer as defined by WSDOT’s Interim Infiltration Design Guidance.
• Within 100 feet up-gradient or 10 feet down-gradient from building foundations.  Closer up-gradient distances may be considered where the minimum seasonal depth to groundwater lies below the foundation or where it can be demonstrated that infiltrating water from the permeable surface will not affect the foundation.

B.   Layers for Permeable Surfaces

Permeable surfaces consist of a number of components: the surface pavement, an underlying base layer, a separation layer, and the native soil or subgrade soil (refer to Pervious Pavement drawing).  An overflow or underdrain system may need to be considered as part of the pavement’s overall design.

1. Surface Layer

The surface layer is the first component of a permeable system’s design that creates the ability for water to infiltrate through the surface.

      1.1    Portland Cement-Based Pervious Pavement Materials

The surface layer consists of specially formulated mixtures of Portland Cement, uniform open graded coarse aggregate, and potable water.  The depth of the surface layer may increase from a minimum of 4 inches, depending on the required bearing strength and pavement design requirements.  The gradation required to obtain a pervious concrete pavement is of the “open” graded or coarse type (AASHTO Grading No. 67: ž inch and lower).  For additional information refer to the pervious pavement specifications.  Reinforcing steel may be used but is typically not necessary.

Due to the relatively low water content of the concrete mix, an agent may be added to retard concrete setup time.  When properly handled and installed, pervious pavement has a higher percentage of void space than conventional pavement (approximately 17 to 22 percent), which allows rapid percolation of stormwater through the pavement.  The initial permeability can commonly exceed 200 inches per hour (Chollack et al., 2001; Mallick et al., 2000). 
 

1.2    Asphalt-Based Pervious Pavement Materials

The surface asphalt layer consists of an open-graded Class D asphalt mixture.  The depth of the surface layer may increase from a minimum of 4 inches, depending on the required bearing strength and pavement design requirements.

Pervious asphalt pavement consists of an open graded coarse aggregate.  The aggregate material is cemented together with asphalt oil and mineral filler.  This creates a surface layer with interconnected voids that provide a high rate of permeability.

1.3    Paving and Lattice Stone

Paving and Lattice Stones consist of a high compressive strength stone that may increase from a minimum of 4 inches, depending on the required bearing strength and pavement design requirements.  When placed together, these stones create a structural surface layer.  An open graded fine aggregate fills the voids, which create a system that provides infiltration into a permeable base layer.  This system can be used in parking lots, bike paths, or areas that receive common local traffic.

1.4    Geo-Cell (PVC Containment Cell)

A Geo-Cell surface stabilization system consists of a high strength UV resistant PVC celled panel that is 4 inches thick.  The celled panels can be filled with soil and covered with turf by installing sod.  Base gravel may also be used to fill the celled panels.  Both applications create a surface layer.

The Geo-Cell creates a structural layer that has interconnected voids that provide a high rate of permeability of water to an infiltrative base layer.   The common application for this system is on minor slopes, pedestrian/bike paths, parking areas, and low-traffic areas.

         2.  Base Layer

The underlying base material is the second component of a permeable surface’s design.  The base material, is a crushed aggregate and provides:

• A stable base for the pavement.
• A high degree of permeability to disperse water downward through the underlying layer to the separation layer.
• A temporary reservoir that slows the migration of water prior to infiltration into the underlying soil.

2.1    Permeable Base Material

The recommended base material is crushed surfacing base stone (CSBS) aggregate (1.5 inch to 2.5 inch, clean washed stone mix, such as AASHTO No. 3 and AASHTO Grading No. 57).  This base may be stabilized as defined in the January 3, 2002 FHWA Manual on Construction of Pavement Subsurface Drainage Systems - publication number FHWA-IF-01-014 HIPA-20/1-02(500).  The placement of the base material requires nominal compaction or consolidation.  To achieve proper stability, it is important that the base material meets fracture requirements (refer to WSDOT Pervious Paving Subgrade specifications).  Other base materials that may be considered include Modified CSBC, or Modified CSTC and sand. 
 

         3.  Separation Layer

The third component of a permeable system is the separation layer.  This layer consists of a non-woven geotextile fabric and possibly a treatment media base material.  A geotextile fabric layer is placed between the base material and the native soil to prevent migration of fine soil particles into the base material followed by a water quality treatment media layer if required.

• See WSDOT Standard Specification 9-33 for Geotextile.
• For Separation Base Material – See the FHWA Manual on Construction of Pavement Subsurface Drainage Systems publication number FHWA-IF-01-014 HIPA-20/1-02(500) for aggregate gradation separation base guidance.
• A treatment media layer is not required where subgrade soil is determined to have a long-term infiltration rate less than 2.4 inches per hour and a cation exchange capacity (CEC) of the subgrade soil that is at least 5 milli-equivalents CEC/ 100 grams of dry soil or greater (WSDOE, 2001).  If a treatment media layer is used, it must be distributed below the geotextile layer and above the subgrade soil.  The treatment media may consist of a sand filter layer or an engineered amended soil.  Gradations of the treatment media should follow base layer sizing.

         4.  Subgrade Soil

The underlying subgrade soil is the fourth component of permeable surfaces.  Runoff infiltrates into the soil and moves to the local interflow or groundwater layer.  Compaction of the subgrade must be kept to an absolute minimum to ensure that the soil maintains a high rate of permeability, while maintaining the structural integrity of the pavement.

C.   General Design Criteria

All projects considering the use of permeable surfaces should be further explored in coordination with the Roadway Design Office, the Materials Office, the Hydraulics Office, and the Maintenance Office.

• The minimum infiltration rate in the subgrade soil should be 0.25 inches per hour.  Compared to other infiltration systems, permeable surfaces have a low hydraulic loading rate, typically less than one inch per hour of rainfall.  Because of this low hydraulic loading rate, permeable surfaces provide good treatment.
• For initial planning purposes, permeable surface systems will work well on Hydrologic Soil Groups A and B and can be considered for Group C soils.  Standard 3 layer placement sections for Group D soils may not be applicable. 
• For projects constructed upon Group C and D soils, a minimum of three soil gradation analyses or three infiltration tests should be conducted to establish on-site soil permeability (see Design Procedure).  Otherwise, a minimum of one such test should be conducted for soil groups A and B to verify adequate permeability.  See WSDOT’s Interim Infiltration Design Guidance for additional information.
• Ideally, the base layer should be designed with sufficient depth to meet Flow Control requirements (taking into account infiltration).  If the infiltration rate and base layer’s storage does not meet Flow Control requirements, an underdrain system may be required.  The underdrain could be discharge to a bioretention area, dispersion system, or a stormwater detention facility.
• Turbid run-off to the permeable surface from off-site areas shall not be allowed.  Designs may incorporate infiltration trenches or other options to ensure long-term infiltration through the permeable surface.
• Establish any necessary boreholes to a depth of ten feet below the base of the reservoir layer; monitor the water table at least monthly for a year.
• Infiltration systems perform best on upland soils.
• On-site soils should be tested for porosity, permeability, organic content, and potential for cation exchange.  These properties should be reviewed when designing the base layer.

         1.  Design Procedure

The following chart to provides additional guidance for permeable surface applications by soil type. 
 

NUMBERS REFERENCED IN TABLE 1-1:

NOTES REFERENCED IN TABLE 1-1:

^(A)   The separation of permeable surface installations from impermeable surface runoff may be necessary by installing an edge drain or a similar system.

^(B)    A treatment media layer is not required where subgrade soil is determined to have a long-term infiltration rate less than 2.4 inches per hour and a cation exchange capacity (CEC) of the subgrade soil that is at least 5 milli-equivalents CEC/ 100 grams of dry soil or greater (WSDOE, 2001). 

^(C)    Shall meet the requirements stipulated in the January 3, 2002 FHWA Manual on Construction of Pavement Subsurface Drainage Systems publication number FHWA-IF-01-014 HIPA-20/1-02(500) for aggregate gradation, page 20 Table 2-1, with 75% minimum fracture.  Other variances, other than fracture, from standard specifications 9-03.1(4)A and 9-03.1(4)B should be reviewed and included as applicable and as coordinated with the Materials Office, Roadway Design Office and Hydraulics Office.  Blending of other aggregate gradations shall not be allowed. 

^(D)    Shall meet the requirements stipulated in the January 3, 2002 FHWA Manual on Construction of Pavement Subsurface Drainage Systems publication number FHWA-IF-01-014 HIPA-20/1-02(500) for aggregate gradation, page 25 Table 2-3, with 75% minimum fracture.  Table 2-4 can be used if the fracture requirement can be met.  The 200-sieve variance in the previously referenced FHWA publication should not be exceeded.  Other variances, other than fracture, from standard specifications 9-03.1(4)A and 9-03.1(4)B should be reviewed and included as applicable and as coordinated with the Materials Office, Roadway Design Office and Hydraulics Office.  Blending of other aggregate gradations shall not be allowed. 

^(E)    Permeable Geotextile shall be used to keep the surface layer stable and fines from migrating up through the surface and base layers.  To obtain Geotextile classification use Geotextile for Underground Drainage, see WSDOT standard specification section 9-33. 

^(F)   Shall meet the requirements stipulated in the January 3, 2002 FHWA Manual on Construction of Pavement Subsurface Drainage Systems publication number FHWA-IF-01-014 HIPA-20/1-02(500) for aggregate gradation.

^(G)   This design approach must seek approval from the Materials Office, Roadway Design Office, and the Hydraulics Office.  Proposed placement applications shall only use type1 & 2 surface layers. 
 
 

         2. Soil Infiltration Rate

Once a permeable surface site is identified, contact the Region Materials Office to request that a Geotechnical investigation be performed.  The Region Materials Office, with assistance from the Headquarters Geotechnical Branch as needed, will determine the quantity and depth of borings/test pits required and any ground water monitoring needed to characterize the soil infiltration characteristics of the site.

Design-level infiltration should be determined by using WSDOT’s “Interim Infiltration Design Guidance.” This process for determining the infiltration rate relies on the D₁₀ of the tested soils (WSDOT, 2002).  This guidance provides a correlation between the D₁₀ size of the soils below the infiltration facility, as shown in Table 1-2.  Note that this guidance applies primarily to infiltration basins and may, therefore, exclude slower-percolating soils such as loams, which are potentially suitable for permeable surfaces. 
 
 
 

Table 1-2.  Recommended Infiltration Rates based on ASTM Gradation Testing

An alternate method is to determine the soil textural classification for the project site (e.g., from soil survey or field data) and compare to the USDA Texture Triangular (Figure 1-1).  For the underlying soil, use an infiltration rate as shown in Table 1-3.  The infiltration rates derived from both of these methods are reasonably conservative.  With the low hydraulic loading experienced by a permeable surface project (i.e., drainage area to infiltration area ratio of approximately 1:1 compared to approximately 100:1 for conventional infiltration basins), the risk resulting from an inaccurate infiltration value is low.  Therefore, it is anticipated that infiltration testing may typically not be required for permeable surface projects.  If it is required, the Pilot Infiltration Test (PIT) described in Appendix V-B of the Ecology Stormwater Manual for Western Washington (WSDOE, 2001) can be used. 
 

Figure 1-1.  Percentage of sand, silt, and clay in the major soil textural classes (USDA Texture Triangular). 
 
 

Table 1-3. Estimated Infiltration Rate for Major Soil Textural Classes.

*From Saxton (2003). 
 

         3.  Sizing of Infiltration Basin

In Western Washington use MGSFlood, the Western Washington Hydrology Model (WWHM) or other acceptable continuous runoff simulation model to size an infiltration basin.  In this case, the bottom area of the “infiltration basin” will typically be the same as the area underlying the permeable surface.  Vary the depth of the “infiltration basin” so it is sufficient to meet Flow Control requirements.  This will be equivalent to the depth of water to be stored in the base material underlying the permeable surface.

A stormwater manual is currently being prepared for Eastern Washington.  Hydrologic methods in that manual, when available, should be used for sites located in Eastern Washington.  In the interim period, use an appropriate single event-based model consistent with infiltration design.

Multiply the stored water depth by a factor of 5. This will determine the depth of the gravel base underlying the permeable surface. This assumes a porosity of 0.20, a conservative assumption.  When a base material that has a different porosity will be used, that value may be substituted to determine the depth of the base.  The minimum base depth is 6-inches, which allows for adequate structural support of the permeable surface.

For a large, contiguous area of permeable surface, such as a parking lot, the area may be designed with a level surface grade and a sloped subgrade to prevent water buildup on the surface to occur, except under extreme conditions.  Rare instances of shallow ponding in a parking lot are normally acceptable.

For projects where ponding is unacceptable under any condition, the surface of the parking lot may be graded at a one percent slope leading to a shallow swale, which would function to assure emergency drainage, similar to an emergency overflow from a conventional infiltration pond.  However, the design depth of the base material must be maintained at all locations. 
 

         4.  Construction Criteria

The key to successful installation of the permeable surface system is diligent attention to construction application techniques and proper inspection. Emphasis must be placed upon strict quality control measures during site preparation and the laying of the permeable surface.  Experience has shown that most cases of failure can be traced to faulty construction.  The Contractor should have previous experience in the construction of a permeable surface.  In lieu of this experience, the Contractor should be required to construct two test panels of the permeable surface ahead of the main surface’s placement to demonstrate competency.  These panels can then be tested to assure that the proper thickness, density, and porosity have been achieved.

Turbid off-site runoff to a project site where permeable surfaces are to be placed during construction should be avoided.  Specifications should be included requiring protection of locations where permeable surfaces are to be placed.  Should contamination of the permeable surface layers occur, a cleanup process in the contract should be stipulated requiring the contractor to remove all sediments or deleterious material in the areas designated by the engineer and to the engineers satisfaction.

• The ambient temperature during a permeable surface application should be higher than 40 degrees Fahrenheit.
• Roller consolidation, with no more than three passes, is adequate.  Repeated passes and/or vibrating compactors may significantly reduce the void space and may damage the permeable surface, therefore, its permeability.
• Pervious concrete should be covered with a minimum of 6 mil of polyethylene sheeting for a minimum of 7 days after placement.

         5. Maintenance Criteria

Permeable surfaces require more maintenance than conventional pavement installations.  The primary concern in maintaining the continued effectiveness of a permeable surface system is to prevent the surface from clogging with fine sediments and debris.

• Permeable surfaces will clog if improperly maintained.  Maintenance or cleanup within the permeable surface layer should include the use of localized high pressure vacuuming.  Maintenance should include the scheduled use of vacuum street sweepers and other practices to remove or prevent leaves, needles, or other foliage from collecting on the surface layer.
• Should clogging occur, it is usually limited to a specific area.  Remedies include localized vacuuming followed by power washing.  In severe cases, the clogged area may need to be removed and reconstructed.  Maintenance levels will generally vary depending on the potential likelihood of fine sediments clogging the surface.  Sites with a higher risk of clogging due to extensive numbers of nearby trees, high traffic volumes, or high dust levels may require that the pavement surfaces be cleaned more frequently.
• Signs should be posted on the site identifying permeable surface areas to aid maintenance crews and inform the public.  This also assures that periodic resurfacing will be done in a manner that maintains the permeable surface. 
• Regular inspections following rainstorms and active preventive maintenance practices are important.
• If spills occur, they should be immediately vacuumed up followed by a pressure wash or other appropriate rinse procedure.

D.  Preliminary Drawings and Specifications

A preliminary standard drawing for pervious pavement has been prepared and is attached.  Three Special Provisions for permeable surfaces have been prepared and are attached:

• SP 2-06.2, Aggregate for Pervious Paving and Base

• SP 5-04, Asphaltic Pervious Concrete

• SP 5-05, Portland Cement Pervious Concrete

E. Permeable Surfacing References

1. Brattebo, B. and Derek Booth.  2002. Permeable Parking Lot Demonstration Project—the Six-Year Follow-Up.  In The Washington Water Resource-Vol. 10, No. 3., University of Washington, Seattle, Washington.
2. Chollack, Tracy, et al. 2001. Porous Pavement Phase 1 Evaluation Report. Seattle Public Utilities, Report, Seattle, Washington, February 7, 2001.
3. Federal Highway Administration. 2002. Construction of Pavement Subsurface Drainage Systems. Publication FHWA IF-01-014. Washington, D.C.
4. Huber, G. 2000. Performance Survey on Open-Graded Friction Course Mixes, Synthesis of Highway Practice. No. 284, National Academy Press. Washington, D.C.
5. Mallick, R.B. et al. 2000. Design, Construction and Performance of New-Generation Open-Graded Friction Courses. National Center for Asphalt Technology. Auburn university, Alabama.
6. Newman, A. P. et al. 2002. Oil Retention and Microbial Ecology in Porous Pavement Structures. Coventry University. Coventry, England.
7. North Carolina State University. 2000. Hydraulic Design for Permeable Pavement—Workshop. Department of Biological and Agricultural Engineering, Raleigh, North Carolina.
8. Paine, Jack. 1992. Portland Cement Pervious Pavement Construction, Publication #C920655, The Aberdeen Group.
9. Saxton, K.E. 2003. Soil Water Characteristics – Hydraulic Properties Calculator. Web Address: http://www.bsyse.wsu.edu/saxton/soilwater/ Washington State University-USDA Agricultural Research Service, Pullman, Washington.
10. SMRC. 2002a. Stormwater Management Fact Sheet: Porous Pavement. The Stormwater Manager’s Resource Center, http://www.stormwatercenter.net/
11. WSDOE (Washington State Department of Ecology). 2001. Stormwater Management Manual for Western Washington. Olympia, Washington.
12. WSDOT (Washington State Department of Transportation). 2002. Project Delivery Memo #02-03- Interim .