PERMEABLE PAVEMENT

Table 1: Summary of Stormwater Functions Provided by Permeable Pavement
Stormwater Function	Level 1 Design	Level 2 Design
Annual Runoff Reduction	45%	75%
Total Phosphorus Removal 1	25%	25%
Total Nitrogen Removal 1	25%	25%
Channel Protection	● Use RRM spreadsheet to calculate Curve Number Adjustment;     OR ● Design extra storage (optional; as needed) in the stone/ underdrain layer to accommodate larger storm volumes, and use NRCS TR-55 Runoff Equations2 to compute CN Adjustment.
Flood Mitigation	Partial. May be able to design additional storage into the reservoir layer by adding perforated storage pipe or chambers.
1 Change in event mean concentration (EMC) through the practice. Actual nutrient mass load removed is the product of the removal rate and the runoff reduction rate. 2 NRCS TR-55 Runoff Equations 2-1 thru 2-5 and Figure 2-1 can be used to compute a curve number adjustment for larger storm events based on the retention storage provided by the practice(s). Sources: CWP and CSN (2008) and CWP (2007).

Table 2:  Comparative Properties of the Three Major Permeable Pavement Types
Design Factor	Porous Concrete (PC)	Porous Asphalt  (PA)	Interlocking Pavers (IP)
Scale of Application	Small and Large Scale Paving Applications	Small and Large Scale Paving Applications	Micro, Small & Large Paving Applications
Pavement Thickness 1	5 to 8 inches	3 to 4 inches	3 1/8 inches
Bedding Layer 1, 8	None	2 inches No. 57 stone	2 inches of No. 8 stone
Reservoir Layer 2, 8	No. 57 stone	No. 2 stone	No. 2 stone 3-4 inches of No.57 stone
Construction Properties 3	Cast in place, seven day cure, must be covered	Cast in place, 24 hour cure	No cure period; Manual or mechanical installation of pre-manufactured units, over 5000 sf/day per machine
Design Permeability 4	10 feet/day	6 feet/day	2 feet/day
Construction Cost 5	$ 2.00 to $6.50/sf	$ 0.50 to $1.00/sf	$ 5.00 to $ 10.00/sf
Min Batch Size	~ 500 sf		NA
Longevity  6	20 to 30 years	15 to 20 years	20 to 30 years
Overflow	Drop Inlet or Overflow edge	Drop Inlet or Overflow Edge	Surface, drop inlet or overflow edge
Temperature Reduction	Cooling in the Reservoir Layer	Cooling in Reservoir Layer	Cooling at the pavement surface & reservoir layer
Colors/Texture	Limited Range of Colors and Textures	Black or Dark Grey Color	Wide Range of Colors, Textures, and Patterns
Traffic Bearing Capacity 7	Can handle all traffic loads, with appropriate bedding layer design.
Surface Clogging	Replace paved areas or install drop inlet	Replace paved areas or install drop inlet	Replace permeable jointing stone materials
Other Issues		Avoid Seal Coating	Snowplow Damage
Design Reference	American Concrete Institute # 522.1.08	Jackson (2007) NAPA	Smith (2006) ICPI
1 Individual designs may depart from these typical cross-sections, due to site, traffic and design conditions. 2 Reservoir storage may be augmented by corrugated metal pipes, plastic arch pipe, or plastic lattice blocks 3 ICPI (2008) 4 NVRA (2008)  5 WERF 2005 as updated by  NVRA (2008) 6 based on pavement being maintained properly, Resurfacing or rehabilitation may be needed after the indicated   period 7 depends primarily on on-site geotechnical considerations and structural design computations 8 Stone sizes correspond to ASTM D 448 Standard Classification for Sizes of Aggregate for Road and Bridge   Construction

Table 3:  Permeable Pavement Design Criteria
Level 1 Design (RR:45; TP:25; TN:25)	Level 2 Design (RR: 75 TP:25; TN:25)
Tv = (1)(Rv)(A) / 12 – volume reduced by upstream BMP1	Tv = (1.1)(Rv) (A) / 12
Soil infiltration less than 0.5”/hr	Soil infiltration rate exceeds 0.5”/hr
Underdrain required	Underdrain not required; or If underdrain is utilized, a 12” stone sump must be provided below underdrain invert; or Tv has at least 48 hour drain time as regulated by a control structure.
CDA = The permeable pavement area, and upgradient parking as long as the ratio does not exceed 2:1	CDA = The permeable pavement area
1 Contributing drainage area to permeable pavements should be limited to paved surfaces to avoid sediment wash-on, and sediment source controls and/or a pretreatment strip or sump should be used. When pervious areas are conveyed to permeable pavement, pretreatment must be provided, and may qualify for a runoff reduction credit.

• Available Space:  A prime advantage of permeable pavement is that it does not normally require additional space at a new or redevelopment site, which can be important for tight sites or areas where land prices are high.

• Soils:  Soil conditions do not constrain the use of permeable pavement, although they do determine whether an underdrain is needed.  Impermeable soils in Hydrologic Soil Group (HSG) C or D usually require an underdrain, whereas HSG A and B soils often do not.  In addition, permeable pavementshould never be situated above fill soils unless designed with an impermeable liner and underdrain.

• External Drainage Area: The external drainage area contributing to permeable pavement should generally not exceed twice the surface area of the pavement, and should be as close to 100% impervious as possible.  Some field experience has shown that an upgradient drainage area (even if it is impervious) can contribute particulates to the permeable pavement and lead to clogging (Hirschman, et al., 2009).  Therefore, careful sediment source control and/or a pre-treatment strip or sump (e.g., stone or gravel) should be used to control sediment run-on to the permeable pavement section.

 

• Pavement Slope:  Steep slopes can reduce the stormwater storage capability of permeable pavement and may cause shifting of the pavement surface and base materials.  Designers should consider using a terraced design for permeable pavement in sloped areas, especially when local slope are several percent or greater.

• Minimum Hydraulic Head: The elevation difference needed for permeable pavement is generally nominal, although two to four feet of head may be needed to drive flows through underdrains. Flat terrain may affect proper drainage of Level 1 permeable pavement designs, so underdrains should have a minimum 0.5% slope.

• Minimum Depth to Water Table: A high groundwater table may cause runoff to pond at the bottom of the permeable pavement system.  Therefore, a minimum vertical distance of 2 feet shall be provided between the bottom of the permeable pavement installation (i.e., the bottom invert of the reservoir layer) and the seasonal high water table.
• Setbacks: Permeable pavement should not be hydraulically connected to structure foundations in order to avoid harmful seepage. Setbacks to structures and roads vary based on the scale of permeable pavement (see Table 3).  At a minimum, small and large scale pavement applications should be located a minimum horizontal distance of 100 feet from any water supply well, and 50 feet from septic systems, and at least 5 feet down-gradient from dry or wet utility lines. Setbacks can be reduced at the discretion of the local program authority for designs that use underdrains and/or liners.

• Informed Owner:  The property owner should clearly understand the unique maintenance responsibilities inherent with permeable pavement, particularly for parking lot applications. The owner should be capable of performing routine and long term actions to maintain its hydrologic function (such as vacuum sweeping) , and avoid future practices, such as winter sanding, seal coating or repaving that diminish or eliminate them.
• High Loading Situations: Permeable pavement is not intended to treat sites with high sediment or trash/debris loads, as they will cause the practice to clog and fail.

• Groundwater Protection: Section 4 of this specification presents a list of potential stormwater hotspots that pose a risk of groundwater contamination. Infiltration of runoff from designated hotspots is highly restricted or prohibited.
•  Limitations: Permeable pavement can be used as an alternative to most types of conventional pavement at residential, commercial and institutional developments, with two exceptions:

• Permeable pavements should not been used for high speed roads, although it has been successfully applied for low speed residential streets, parking lanes and roadway shoulders; and
• Permeable pavements should not be used to treat runoff from stormwater hotspots. Refer to Specification No. 8: Infiltration (Section 9.1).

1. Design Scales: There are three scales by which permeable pavement can be installed:

1. The smallest scale is termed Micro-Scale Pavements that are applied to convert impervious surfaces to permeable ones on small lots and redevelopment projects, and may range from 250 to 1000 square feet (sf) in total area. Where re-development or retrofitting of existing impervious areas include a larger foot-print of permeable pavers (on the scale of Small- or Large- as noted below), the designer should implement the Load Bearing, Observation Well, Underdrain, Soil Test, and Building Setback criteria of the appropriate scale.
2. Small-scale pavement applications treat portions of a site between 1000 and 10,000 sf in area, and include areas that only occasionally receive heavy vehicular traffic. 
3. Large scale pavement applications exceed 10,000 square feet and typically are installed within portions of a parking lot. 

Table 4: The Three Design Scales for Permeable Pavement
Design Factor	Micro-Scale  Pavement	Small-Scale Pavement	Large-Scale Pavement
Impervious Area Treated	250 to 1000 sq. ft.	1000 to 10,000 sq. ft.	More than 10,000 sf
Typical Applications	Driveways Walkways Court Yards Plazas Individual Sidewalk	Sidewalk Network Fire Lanes Road Shoulders Spill-Over Parking Plazas	Parking Lots with more than 40 spaces Low Speed Residential Streets
Most Suitable Pavement	IP	PA, PC, and IP	PA, PC and IP
Load Bearing Capacity	Foot traffic Light vehicles	Light vehicles	Heavy vehicles (moving & parked)
Reservoir Size	Infiltrate or detain some or all of the Tv	Infiltrate or detain full Tv, and as much of CPv and design storms as possible
External Drainage Area?	No	Yes, Impervious cover up to twice the permeable pave area may be accepted as long as sediment source controls and/or pretreatment is used
Observation Well	No	No	Yes
Underdrain?	Rare	Depending on soils	Back up underdrain
Required Soil Tests	One per practice	Two per practice	One per 5000 sq. ft of proposed practice
Building Setbacks	5 feet down-gradient 25 feet up-gradient	10 feet down-gradient 50 feet up-gradient	25 feet down-gradient 100 feet up-gradient

1. Total traffic;
2. In-situ soil strength;
3. Environmental elements; and
4. Bedding and Reservoir layer design.

1. VDOT Pavement Design Guide for Subdivision and Secondary Roads in Virginia, 2000; or latest edition;
2. AASHTO Guide for Design of Pavement Structures, 1993; and,
3. AASHTO Supplement to the Guide for Design of Pavement Structures, 1998.

1.               

• Contributing drainage area (Ac) should not contain pervious areas.
• Native soil infiltration rate (i) for design purposes should be the field-tested soil infiltration rate divided by a factor of safety of two. The minimum acceptable native soil infiltration rate is 0.5”/hr
• Void ratio (Vr) for No. 57 stone = 0.4
• Maximum drain time for the reservoir layer should not be less than 24 hours or greater than 48 hours

1. 

1.            

1.            

• Elevated Underdrain.To promote greater runoff reduction for permeable pavement located on marginal soils, an elevated underdrain should be installed with a stone jacket that creates a 12 to 18 inch deep storage layer below the underdrain invert. The void storage in this layer can help qualify a site to achieve Level 2 design.

• Rapid Drawdown: When possible, permeable pavement should be designed so that the target runoff reduction volume stays in the reservoir layer for at least 36 hours before being discharged through an underdrain.
• Conservative Infiltration Rates. Designers should always decrease the measured infiltration rate by a factor of 2 during design to approximate long term infiltration rates.
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• Place a perforated pipe horizontally near the top of the reservoir layer to pass excess flows after the base has been filled. The placement and/or design should be such that the incoming runoff is not captured, such as placing the perforations on the underside only.
• Increase the thickness of the top of the reservoir layer by as much as six inches (i.e., create freeboard).  Design computations to size the reservoir layer often assume that no freeboard is present
• Create underground detention within the reservoir layer of the permeable pavement system. Reservoir storage may be augmented by corrugated metal pipes, plastic or concrete arch structures, etc.
•    Route excess flows to another detention or conveyance system that is designed for the management of extreme event flows.
• Set the storm drain inlets flush with the elevation of the permeable pavement surface to effectively conveying excess stormwater runoff past the system (typically in remote areas). The design should also make allowances for relief of unacceptable ponding depths during larger rainfall events.

• The reservoir below the permeable pavement surface should be composed of clean, washed stone aggregate and be sized for both the storm event to be treated and the structural requirements of the expected traffic loading.
• The storage layer may consist of clean washed No. 57 stone, although No. 2 stone provides additional storage and structural stability.
• The bottom of the reservoir layer should be completely flat so that runoff will be able to infiltrate evenly through the entire surface.

Underdrains

Underdrains should be placed within the reservoir and encased in 8-12 inches of clean, washed stone. 

The underdrain outlets can be fitted with a flow-reduction orifice as a means of regulating the stormwater detention time.  The minimum diameter of any orifice should be 0.5 inch.

Underdrains can also be installed and capped at a downstream structure as an option for future use if maintenance observations indicate a reduction in the soil permeability.

Periodic Vacuum Sweeping.The pavement surface is the first line of defense in trapping sediment that may otherwise enter the base and soil subgrade. The rate of sediment deposition should be monitored and vacuum sweeping done once or twice a year, and adjusted according to the intensity of use and deposition rate on the permeable pavement. At least one sweeping pass should occur at the end of winter.

Protecting the Bottom of the Reservoir Layer. There are two options to protect the bottom of the reservoir layer from intrusion by underlying soils. The first method involves covering the bottom with nonwoven, polypropylene geotextile that is permeable, although some practitioners recommend avoiding the use of filter fabric since it may become a future plane of clogging within the system. Permeable filter fabric is still recommended to protect the excavated sides of the reservoir layer to prevent soil piping. The second method is to form a barrier of choker stone and sand. In this case, underlying native soils should be separated from the reservoir base/subgrade layer by a thin, 2-4 inch layer of clean, washed, choker stone (ASTM D 448 No. 8 stone) below 6-8 inches of course sand.

Observation Well. An observation well, consisting of a well-anchored, perforated 4-6 inch PVC pipe that extends vertically to the bottom of the reservoir layer, should be installed at the downstream end of all large scale permeable pavement systems. The observation well should be fitted with a lockable cap installed flush with the ground surface (or under the pavers) to facilitate periodic inspection and maintenance. The observation well is used to observe the rate of drawdown within the reservoir layer following a storm event.

Overhead Landscaping.Most local communities require from 5 to 10 % landscaping around the parking lots, so large scale permeable paving applications should carefully plan on how to integrate this landscaping to maximize runoff treatment and minimize the chances that grass clippings, leaves, nuts, and fruits will inadvertently clog the paving surface.

Table 5:  Material Specifications for Underneath the Pavement Surface
Material	Specification	Notes
Bedding Layer	PC: None PA: 2 in depth No. 8 stone. IP:  2 in depth, No. 8 stone over 3-4 inch of No. 57	ASTM D448 size No. 8 stone (e.g. 3/8-3/16 inch in size). Should be double-washed and clean and free of all fines
Reservoir Layer	PC: No. 57 stone  PA: No. 2 stone.  IP:  No. 57  stone	ASTM D448 size No. 57 stone (e.g. 1 1/2  to 1/2 inch in size); No. 2 Stone (e.g. 3 inch to 3/4 inch in size). Depth based on the pavement structural and hydraulic requirements. Should be double-washed and clean and free of all fines
Underdrain	4-6 inch perforated PVC (AASHTO M 252) pipe, with 3/8-inch perforations at 6 inches on center; each underdrain on a minimum 0.5% slope located 20 feet or less from the next pipe (or equivalent corrugated HDPE for smaller load-bearing applications). Perforated pipe for the full length of the permeable pavement cell, and non-perforated pipe, as needed, to connect with the storm drain system. T’s and Y’s as needed, depending on the underdrain configuration. Extend cleanout pipes to the surface with vented caps at the Ts and Ys.
Filter Layer	The underlying native soils should be separated from the stone reservoir by a thin, 2-4 inch layer of choker stone (e.g. No. 8) and a 6-8 inch layer of coarse sand (e.g. ASTM C 33, 0.02-0.04 inch)	The sand should be placed between the stone reservoir and the choker stone, which should be placed on top of the underlying native soils.
Filter Fabric (optional)	Needled, non-woven, polypropylene geotextile with grab tensile strength greater or equal to 120 lbs (ASTM D4632), Mullen Burst strength greater or equal to 225 lbs/sq in (ASTM D3786), Flow rate greater than 125 gpm/sf (ASTM D4491) and Apparent Opening Size US # 70 or # 80 sieve (ASTM D4751). Geotextile AOS selection is based on the percent passing the No. 200 sieve in “A” Soil subgrade, using FHWA or AASHTO selection criteria
Impermeable Liner	Minimum thirty mil PVC Geomembrane liner covered by 8 to 12 oz/yd2 non-woven geotextile. USED ONLY FOR KARST REGIONS
Observation Well	Perforated 4-6 inch vertical PVC pipe (AASHTO M 252), with lockable cap installed flush with the surface with surface cap.

 

SECTION 6:  REGIONAL AND SPECIAL CASE DESIGN ADAPTATIONS

The design adaptations described below permit permeable pavement to be used on a wider range of sites; however, it is important not to force this practice onto marginal sites.  Other runoff reduction practices are often preferred alternatives for difficult sites.

6.1. Karst Terrain

Active karst regions are found in much of the Ridge and Valley physiographic regions of Virginia, and complicate both development and stormwater design. A detailed geotechnical investigation may be required for any kind of stormwater design in karst terrain (see CSN Technical Bulletin No. 1; and the VA SWM Handbook)

 

6.2. Coastal Plain

Experience in North Carolina has shown that properly designed and installed permeable pavement systems can work effectively in the demanding conditions of the coastal plain.

6.3. Piedmont/Clay Soils

In areas where the underlying soils are not suitable for complete infiltration, permeable pavement systems with underdrains can still function effectively to reduce runoff and nutrients.

6.4. Winter Performance

In winter conditions, freeze-thaw cycles may affect the structural durability of the permeable pavement system. In these situations, the following design adaptations may be helpful:

SECTION 7:  PERMEABLE PAVEMENT CONSTRUCTION SEQUENCE AND INSPECTION

Experience has show that proper installation is absolutely critical to the effective operation of a permeable pavement system,

7.1. Construction Sequence

Construction Stage ESC controls.

7.2. Permeable Pavement Installation

The following is a typical construction sequence to properly install permeable pavement, which may need to be modified to depending on whether PA, PC or IP designs are employed.

Step 1. Construction shall only begin after the entire contributing drainage area has been stabilized.  The proposed site should be checked for existing utilities prior to any excavation.  Do not install the system in rain or snow, and do not install frozen bedding materials.

Step 2. Temporary erosion and sediment (E&S) controls are needed during installation to divert stormwater away from the permeable pavement area until it is completed.  Special protection measures such as erosion control fabrics may be needed to protect vulnerable side slopes from erosion during the excavation process. The proposed permeable pavement area must be kept free from sediment during the entire job.  Construction materials that are contaminated by sediments must be removed and replaced with clean materials.

Step 3.  Where possible, excavators or backhoes should work from the sides to excavate the reservoir layer to its appropriate design depth and dimensions. For micro and small-scale pavement applications, excavating equipment should have arms with adequate reach so they do not have to work inside the footprint of the permeable pavement area. Contractors can utilize a cell construction approach, whereby the proposed permeable pavement area is split into 500 to 1000 sf temporary cells with a 10-15 foot earth bridge in between, so that cells can be excavated from the side. Excavated material should be placed away from the open excavation so as to not jeopardize the stability of the side walls.

Step 4. The native soils along the bottom and sides of the permeable pavement system should be scarified or tilled to a depth of 3-4 inches prior to the placement of the filter layer or filter fabric. In large scale paving applications with weak soils, the soil subgrade may need to compacted to 95% standard Proctor to achieved desired load bearing capacity (Note: this effectively eliminates the infiltration function of the installation, and must be addressed during hydrologic design.

Step 5:  Filter fabric should be installed on the bottom and the sides of the reservoir layer. In some cases, an alternative filter layer, as described in Section 7.6 may be warranted. Filter fabric strips should overlap down-slope by a minimum of two feet, and be secured a minimum of four feet beyond the edge of the excavation. Where the filter layer extends beyond the edge of the pavement (to convey runoff to the reservoir layer), install an additional layer of filter fabric one foot below the surface to prevent sediments from entering into the reservoir layer.  Excess filter fabric should not be trimmed until the site is fully stabilized.

Step 6. Provide a minimum of two inches of aggregate above and below the pipe.  The underdrains should slope down towards the outlet at a grade of 0.5 percent or steeper.  The up-gradient end of underdrains in the reservoir layer should be capped.  Underdrain pipe connected to structures shall be non-perforated within one foot of the structure.  Ensure that clean-outs and observation wells are non-perforated within one foot of the surface.

Step 7.  Moisten and spread 6 inch lifts of the appropriate clean, washed stone aggregate (usually No. 2 or 57 stone).  Place additional aggregate to at least four inches above the underdrain, and then compact using a vibratory roller in static mode until there is no visible movement of the aggregate.  Do not crush the aggregate with the roller.

Step 8.  Install the desired depth of the bedding layer, depending on the type of pavement as follows:

Step 9.  The installation sequence depends on the type of pavement, and should be done in accordance with manufacturer or industry specifications.

9a Installation of Pervious Asphalt (PA).  The following has been excerpted from various documents, most notably Jackson (2007).

9b Installation of Pervious Concrete (PC). The basic installation sequence for pervious concrete is outlined in American Concrete Institute (2008). It is strongly recommended that concrete installers successfully complete a recognized pervious concrete installers training program, such as the Pervious Concrete Contractor Certification Program offered by NRMCA.    The basic procedure is as follows:

9c  Installation of Interlocking Pavers (IP). The basic installation process is described in greater detail by Smith (2006).

Permeable interlocking concrete pavement (IP) systems require edge restraints to prevent vehicle loads from moving the paver blocks.  Edge restraints may be standard VDOT curbs or gutter pans, or precast or cast in place reinforced concrete borders a minimum of six inches wide and 18 inches deep, constructed with Class A3 concrete.  Edge restraints along the traffic side of a permeable pavement block system are recommended. 

7.3. Construction Inspection

Inspections before, during and after construction are needed to ensure that permeable pavement is built in accordance with these specifications.   Use detailed inspection checklists that require sign-offs by qualified individuals at critical stages of construction, to ensure that the contractor’s interpretation of the plan is consistent with the designer’s intent.

Some common pitfalls can be avoided by careful construction supervision that focuses on the following key aspects of permeable pavement installation:

It may be advisable to divert the runoff from the first few runoff-producing storms away from larger permeable pavement applications, particularly when up-gradient conventional asphalt areas drain to the permeable pavement.  This can help reduce the input of fine particles that are often produced shortly after conventional asphalt is laid down.

SECTION 8. PERMEABLE PAVEMENT MAINTENANCE

8.1. Maintenance Agreements

Section 4VAC 50-60-124 of the regulations specifies a maintenance agreement to be executed between the owner and the local program.  The section requires a schedule of inspections, compliance procedures if maintenance is neglected, notification of the local program upon transfer of ownership, and right-of-entry for local program personnel.
In addition, the maintenance agreements should also note which conventional parking lot maintenance tasks must be avoided (e.g., sanding, re-sealing, re-surfacing, power-washing) Signs should be posted on larger parking lots to indicate their stormwater function and special maintenance requirements.

When micro or small-scale permeable pavement are installed on private residential lots, homeowners will need to be educated on their routine maintenance needs, understand the long-term maintenance plan, and be subject to a deed restriction or other mechanism enforceable by the qualifying local program to help ensure that the pervious paver system are maintained and functioning. The mechanism should, if possible, grant authority for local agencies to access the property for inspection or corrective action.

8.2. Maintenance Tasks

It is difficult to prescribe the specific types or frequency of maintenance tasks that are needed to maintain the hydrologic function of permeable pavement systems over time. Most installations work reasonably well year after year with little or no maintenance, whereas some have problems right from the start.

The one preventative maintenance task for large-scale applications may involve vacuum sweeping on a frequency consistent with the use and loadings encountered in the parking lot. Many consider an annual, dry weather sweeping in the spring months to be important. The contract for sweeping should specify that a vacuum sweeper be used that does not use water spray, since these may lead to subsurface clogging. Vacuum settings for large-scale IP applications should be calibrated so they do pick up the stones between pavement blocks.

8.3. Maintenance Inspections

It is highly recommended that a spring maintenance inspection and cleanup be conducted at each permeable pavement site, particularly at large-scale applications.

Maintenance of permeable pavement is driven by annual inspections that evaluate the condition and performance of the practice (see Table 7). Based on inspection results, specific maintenance tasks will be triggered. Based on the inspection results, a series of maintenance tasks are scheduled to keep the facility in operating condition.

 

SECTION 9:  COMMUNITY AND ENVIRONMENTAL CONCERNS

WERF. 2005. Performance and Whole-life Costs of Best Management Practices and Sustainable Urban Drainage Systems. Water Environment Research Federation