proposed bmp standard/specification … dcr7… · web viewguidelines for 1993 aashto pavement...

DRAFT VA DCR STORMWATER DESIGN SPECIFICATIONS No .7: PERMEABLE PAVEMENT

DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 7

PERMEABLE PAVEMENTVERSION 1.0

Note to Reviewers of the Stormwater Design Specifications

The Virginia Department of Conservation and Recreation (DCR) has developed an updated set of non-proprietary BMP standards and specifications for use in complying with the Virginia Stormwater Management Law and Regulations. These standards and specifications were developed with assistance from the Chesapeake Stormwater Network (CSN), Center for Watershed Protection (CWP), Northern Virginia Regional Commission (NVRC), and the Engineers and Surveyors Institute (ESI) of Northern Virginia. These standards and specifications are based on both the traditional BMPs and Low Impact Development (LID) practices. The advancements in these standards and specifications are a result of extensive reviews of BMP research studies incorporated into the CWP's National Pollution Removal Performance Database (NPRPD). In addition, we have borrowed from BMP standards and specifications from other states and research universities in the region. Table 1 describes the overall organization and status of the proposed design specifications under development by DCR.

Table 1: Organization and Status of Proposed DCR Stormwater Design Specifications:

Status as of 9/24/2008# Practice Notes Status 1

1 Rooftop Disconnection Includes front-yard bioretention 22 Filter Strips Includes grass and conservation filter strips 23 Grass Channels 24 Soil Compost

Amendments3

5 Green Roofs 16 Rain Tanks Includes cisterns 27 Permeable Pavement 18 Infiltration Includes micro- small scale and

conventional infiltration techniques 2

9 Bioretention Includes urban bioretention 310 Dry Swales 211 OPEN12 Filtering Practices 213 Constructed Wetlands Includes wet swales 214 Wet Ponds 215 ED Ponds 21 Codes: 1= partial draft of design spec; 2 = complete draft of design spec; 3 = Design specification has undergone one round of external peer review as of 9/24/08

Reviewers should be aware that these draft standards and specifications are just the beginning of the process. Over the coming months, they will be extensively peer-reviewed to develop standards and specifications that can boost performance, increase longevity, reduce the maintenance burden, create attractive amenities, and drive down the unit cost of the treatment provided.

Permeable Pavement of 15 9/18/081

Timeline for review and adoption of specifications and Role of the Virginia’s Stormwater BMP Clearinghouse Committee:

The CSN will be soliciting input and comment on each standard and specification until the end of 2008 from the research, design and plan review community. This feedback will ensure that these design standards strike the right balance between prescription and flexibility, and that they work effectively in each physiographic region. The collective feedback will be presented to the BMP Clearinghouse Committee to help complement their review efforts. The Virginia Stormwater BMP Clearinghouse Committee will consider the feedback and recommend final versions of these BMP standards and specifications for approval by DCR.

The revisions to the Virginia Stormwater Management Regulations are not expected to become finalized until late 2009. The DCR intends that these stormwater BMP standards and specifications will be finalized by the time the regulations become final.

The Virginia Stormwater BMP Clearinghouse Committee will consider the feedback and recommend final versions of these BMP standards and specifications for approval by DCR, which is vested by the Code of Virginia with the authority to determine what practices are acceptable for use in Virginia to manage stormwater runoff.

As with any draft, there are several key caveats, as outlined below:

Many of the proposed design standards and specifications lack graphics. Graphics will be produced in the coming months, and some of graphics will be imported from the DCR 1999 Virginia Stormwater Management (SWM) Handbook. The design graphics shown in this current version are meant to be illustrative. Where there are differences between the schematic and the text, the text should be considered the recommended approach.

There are some inconsistencies in the material specifications for stone, pea gravel and filter fabric between ASTM, VDOT and the DCR 1999 SWM Handbook. These inconsistencies will be rectified in subsequent versions.

While the DCR 1999 SWM Handbook was used as the initial foundation for these draft standards and specifications, additional side-by-side comparison will be conducted to ensure continuity.

Other inconsistencies may exist regarding the specified setbacks from buildings, roads, septic systems, water supply wells and public infrastructure. These setbacks can be extremely important, and local plan reviewers should provide input to ensure that they strike the appropriate balance between risk aversion and practice feasibility.

These practice specifications will be posted in Wikipedia fashion for comment on the Chesapeake Stormwater Network’s web site at http://www.chesapeakestormwater.net, with instructions regarding how to submit comments, answers to remaining questions about the practice, useful graphics, etc. DCR requests that you provide an email copy of your comments, etc., to Scott Crafton ([email protected]). The final version will provide appropriate credit and attribution on the sources from which photos, schematics, figures, and text were derived.

Thank you for your help in producing the best stormwater design specifications for the Commonwealth.

Permeable Pavement of15 9/18/082

mailto:[email protected]

http://www.chesapeakestormwater.net/

Figure 1: A Permeable Pavement System Can Be Designed With or

Without an Underdrain

Source: Hunt and Collins, 2008

Figure 1: A Permeable Pavement System Can Be Designed With or

Without an Underdrain

Source: Hunt and Collins, 2008


DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 7

PERMEABLE PAVEMENTVERSION 1.0

SECTION 1: DESCRIPTION OF PRACTICE

Permeable pavements are alternative paving surfaces that allow stormwater runoff to filter through voids in the pavement surface into an underlying stone reservoir where it is temporarily stored. Often, the filtered runoff is collected in an underdrain and returned to the storm drain system. If infiltration rates in native soils permit, permeable pavement practices can be designed without an underdrain for full infiltration. A combination of these methods can be used to infiltrate a portion of the filtered runoff.

There are a variety of permeable pavement surfaces available in the commercial marketplace, including pervious concrete, porous asphalt, permeable interlocking concrete pavers, concrete grid pavers, and plastic grid pavers. While the specific design configuration may vary according to each product, nearly all permeable pavement types have the same general structure, consisting of a surface layer, aggregate base, and sub-base.The aggregate base layer serves to retain stormwater and also supports the design traffic loads.


Permeable pavements are typically designed to treat rainfall on the pavement surface area, but can also be used to treat run-on from small adjacent impervious areas, such as impermeable driving lanes or rooftops. Permeable pavements promote runoff reduction and provide high pollutant removal. Permeable pavement can also be used to reduce the impervious cover of a development site.

SECTION 2: PERFORMANCE CRITERIA

The overall stormwater functions of permeable pavement are summarized in Table 1.

Table 1: Summary of Stormwater Functions Provided by Permeable PavementsStormwater 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 Moderate.

Reduced Curve Numbers and Time of Concentration (?)Flood Mitigation Partial.

Reduced Curve Numbers and Time of Concentration1 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 and will be higher than these percentages, as calculated using the Runoff Reduction Spreadsheet Methodology.Sources: CWP and CSN (2008) and CWP (2007).

SECTION 3: PRACTICE APPLICATIONS AND FEASIBILITY

Available Space: Permeable pavement systems do not require additional space. The thickness of the underlying aggregate base layer is determined by both a structural and hydrologic design analysis.

Site Topography: Permeable pavement systems may be installed on development sites with slopes of up to 5%, although a maximum 2% slope is recommended.

Available Hydraulic Head:. No restrictions.

Depth to Water Table:. The water table should be no less than 2 feet below the invert of the pavement system (bottom of the aggregate base).

Soils: Soil conditions do not constrain the use of permeable pavements, although they 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. Designers should verify that underlying soil permeability exceeds 1 in/hr for non-underdrain designs. Shrink–swell? If underdrains are required, a stone sump layer below the underdrain can still be used to boost runoff reduction and promote partial infiltration.



Contributing Drainage Area: Permeable pavements generally manage the pavement surface rainfall only. However, they can be used to manage stormwater runoff from other impervious parts of a development site, as long as the ratio of the surface area of the contributing drainage area (CDA) to that of the permeable pavement system does not exceed 2:1.

Hotspot Land Uses: Permeable pavements should not be used to treat hotspot runoff.

Setbacks: To prevent damage to building foundations and contamination of groundwater aquifers, permeable pavement systems should be located at least the following setbacks:

10 feet from building foundations

10 feet from property lines

100 feet from private water supply wells

1,200 feet from public water supply wells

100 feet from septic systems

100 feet from surface waters

400 feet from public water supply surface waters

Setbacks can be reduced at the discretion of the local program authority for designs that use underdrains and/or liners.

SECTION 4: ENVIRONMENTAL AND COMMUNITY CONSIDERATIONS

Not applicable. (NOTE: Are there any concerns that should be listed here?)

SECTION 5: DESIGN APPLICATIONS AND VARIATIONS

The most important design factor to consider when applying permeable pavement to development sites is the expected traffic load and structural requirements of the pavement system. Sites with heavy traffic loads will require a thick aggregate base, and in the case of porous asphalt and pervious concrete, may require the addition of an admixture for strength.

Permeable pavement has been used at commercial, institutional, and residential sites in spaces that are traditionally impervious. Typical locations for permeable pavement include:

Parking stalls or small parking lots

Overflow parking areas

Driveways and alleys


Fire lanes

Pedestrian plazas

Walkways, sidewalks, and bike paths

Retrofitting

A number of options are available to retrofit permeable pavement in the urban landscape, as described in Profile Sheet RR-7 of Schueler et al (2007).

SECTION 6: SIZING AND TESTING GUIDANCE

6.1: Overall Sizing

The thickness of the permeable pavement aggregate base layer must be sized to support structural loads and to temporarily store the design storm volume (water quality Treatment Volume, channel protection, and/or flood control). On most development and redevelopment sites, the structural support requirements will determine the depth of the underlying stone reservoir.

Structural Design: The structural design of permeable pavements considers four main site elements:

Total Traffic

In-Situ Soil Strength

Environmental Elements

Layer design

The structural design requires careful planning, and designers should consult the following references for detailed structural design guidance:

Virginia Department of Transportation (VDOT) (2003). Guidelines for 1993 AASHTO Pavement Design. Virginia Department of Transportation, Materials Division. Pavement Design and Evaluation Section, available at:

http://www.virginiadot.org/business/resources/bu-mat-pde-AASHTOForConsultants0503.pdf

Hydraulic Design: Permeable pavement systems are typically sized to store the water quality design storm in the aggregate base layer. The following calculation is used to design the base layer:




where:dp = depth of aggregate base layer (m)Qc = Depth of runoff from contributing drainage area, not including permeable

paving surface (m)R = Ac/Ap = Ratio of contributing drainage area (Ac) to permeable paving area (Ap)P = Rainfall depth (m)i = Infiltration rate for native soils (m/day)T = Time to fill aggregate base (typically 2 hr)Vr = Void ratio for aggregate base (typically 0.4)

Note that the contributing drainage area (Ac) should not contain pervious areas.

The maximum allowable depth of the filter bed is calculated using the following equation:

where:dmax = Maximum depth of aggregate base layer (m)i = Infiltration rate for native soils (m/hr)Vr = Void ratio for aggregate base (typically 0.4)ts = Time to drain aggregate base (typically 24 hours; 72 hours max) (hr)

The permeable pavement system can also be designed to address, in whole or in part, the detention storage needed for channel protection and/or flood control. The designer can model and provide routings for designs by factoring in storage within the stone aggregate layer, expected infiltration (multiplied by 0.5 to provide a safety factor), and any outlet structures used as part of the design. Note: spreadsheet models may be available to assist with this.

6.2: Soil Infiltration Rate Testing

The second key sizing decision is based on measuring the infiltration rate of subsoils below the permeable pavement area to determine if an underdrain will be needed. The infiltration rate of subsoils must exceed 1 inch per hour to design without an underdrain. On-site soil infiltration rate testing procedures are outlined in Appendix A of the Infiltration Design Specification (No. 8). A minimum of one test shall be taken per 2500 sq. ft. of planned permeable pavement surface area. At least one soil boring must be taken to confirm the underlying soil properties at the depth where infiltration is designed to occur (depth to water table, depth to bedrock. active kartst, etc.) Since clay lenses or any other restrictive layers located below the bottom of the permeable pavement system will reduce soil infiltration rates, soil infiltration testing should be conducted within any confining layers that are found within 4 feet of the bottom of a proposed permeable pavement system.


SECTION 7: DESIGN CRITERIA

7.1: Level 1 and 2 Permeable Pavement Design Guidelines

The major design goal for the Chesapeake Bay is to maximize nutrient removal and runoff reduction. To this end, designers may choose to go with the baseline permeable pavement design (Level 1) or choose an enhanced Level 2 that maximizes nutrient and runoff reduction. To qualify for Level 2, the design must meet all design criteria shown in the right hand column of Table 2.

Table 2: Permeable Paver Design GuidanceLevel 1 Design (RR:45; TP:25; TN:25) Level 2 Design (RR: 75 TP:25; TN:25)

TV= (1)(Rv)(A) / 12 TV = (1.1)(Rv) (A) / 12Soil infiltration less than 1-inch/hr Soil infiltration rate exceeds 1-inch/hrUnderdrain needed Underdrain not required or stone sump

provided below underdrain invertCDA = The permeable pavement area, and upgradient parking as long as the ratio does not exceed 2:1

CDA = The permeable pavement area area

Slopes from 2-3% Slopes less than 2%Sand layer between choker stone and paver blocks, if relevant and allowed by manufacturer’s specifications (not applicable to porous asphalt, concrete)

Sand layer between choker stone and paver blocks, if relevant and allowed by manufacturer’s specifications (not applicable to porous asphalt, concrete)

An appropriate permeable pavement surface should be selected for the intended application. If it will be used in a parking lot, street or roadway, the pavement surface must be able to support the maximum traffic load. The design will vary according to the type of pavement selected, and the manufacturer’s specific recommendations should be consulted.

7.2: Pretreatment

Pretreatment for most permeable pavement applications is not necessary, since the surface acts as pretreatment to the aggregate base below. Additional pretreatment may be desired if the pavement receives run-on from an adjacent impervious area. A gravel filter strip can be used to trap coarse sediment particles before they reach the pavement surface to prevent premature clogging.

7.3: Conveyance and Overflow

Permeable pavement designs should include methods to convey larger storms (i.e. 2-yr, 10-yr) to the storm drain system. The following methods that can be used to accommodate this:

Set the storm drain inlets slightly above the elevation of the permeable pavement surface, allowing for some surface ponding, but effectively conveying excess stormwater runoff past the system.



Place a perforated pipe near the top of the aggregate base layer to pass excess flows after the base has been filled.

Provide detention on the surface of the permeable pavement area with a suitable overflow structure.

Place an underground detention system beneath or adjacent to the permeable pavement system.

7.4: Aggregate base and underdrain design

The stone reservoir directly below the permeable surface should be composed of layers of stone aggregate and be sized for both the storm event to be treated and the structural requirements of the expected traffic loading. The bottom of the reservoir should be completely flat so that runoff will be able to infiltrate evenly through the entire surface. If the system is not designed for infiltration, the bottom of the reservoir should be sloped at 1-5% toward the underdrain.

If no underdrain is required, underlying native soils should be separated from the aggregate base by a thin, 2-4 inch layer of clean, washed, choker stone (No. 8) below 6-8 inches of sand. If an underdrain is required, it should be placed beneath the aggregate base layer and encased in 8-12 inches of clean, washed No. 57 stone. The slope of the underdrain should be 0.5-2%. To promote greater runoff reduction for permeable pavement areas using underdrains, a 12-18 inch deep storage layer may be located below the invert of the underdrain. The storage layer may consist of clean, washed No. 57 stone.

An observation well, consisting of a perforated a 4-6 inch perforated PVC (AASHTO M 252) pipe that extends to the bottom of the aggregate base layer, should be installed at the downstream end of all permeable pavement systems. The observation well can be used to observe the rate of drawdown within the reservoir following a storm event.

7.5: Filter Media and Pavement Surface

These should be designed according to the product manufacturer’s recommendations

SECTION 8: REGIONAL AND CLIMATE DESIGN ADAPTATIONS

The design criteria 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 stormwater practices, such as bioretention, wet swales, ditch wetland restoration and smaller linear wetlands, are often preferred alternatives for difficult sites.

Clay Soils: Poorly drained soils (such should be designed with an underdrain (Level 1). Designers should use additional runoff volume reduction practices to supplement the stormwater management benefits provided by the Level 1 permeable pavement design.


Sandy Soils and/or Areas of Groundwater Concern (e.g., source water protection areas): Well drained soils, such as HSG A and B soils, enhance the ability of permeable pavement to reduce stormwater runoff volumes, peak flow rates, and pollutant loads, but they may allow stormwater pollutants to reach groundwater aquifers with greater ease. Designers should avoid the use of infiltration-based stormwater management practices, including non-underdrain permeable pavement systems, at stormwater hotspot facilities and in areas known to provide groundwater recharge to aquifers used for water supply. [In areas known to provide groundwater recharge to water supply aquifers, permeable pavement systems should be designed with liners and underdrains to capture and treat runoff for applications for the permeable pavement system exceeds 2,500 square feet, or a size deemed appropriate by the local authority.] (NOTE: The preceding sentence is a run-on sentence that may have been intended as two separate sentences. DCR and CWP need to correct this and clarify the meaning.)

Shallow Water Tables: A high groundwater table may cause runoff to pond at the bottom of the permeable pavement system. Designers should ensure that the distance from the bottom of the permeable pavement system to the top of the groundwater table is at least 2 feet. Stormwater wet ponds, constructed wetlands, and wet swales can be used instead of permeable pavement to manage stormwater runoff in these areas.

Flat Terrain: Flat terrain may affect proper drainage of Level 1 permeable pavement designs. The underdrain should have a minimum 0.5% slope to ensure proper drainage.

Steep Terrain: Steep slopes can reduce the stormwater storage capabilities of permeable pavements and may cause shifting of the pavement surface and base materials. Designers should consider using a terraced design for permeable walking surfaces. For slopes less than 5%, a terraced base and sub-base layer can be designed to improve the retention of water.

Karst Terrain: Active karst regions are found in much of the Ridge and Valley province of the Bay watershed, and complicate both development and stormwater design. The use of Level 2 (i.e. infiltration) permeable pavement designs in active karst regions may cause the formation of sinkholes. When the available soil column above subterranean karst features/bedrock is less than 10 feet thick and/or a detailed geotechnical survey has been conducted to the satisfaction of the local approval authority, then permeable pavements should be designed to meet karst separation distance requirements (3 feet) and possess an impermeable bottom liner and an underdrain. Level 2 permeable pavement designs are not recommended in any area with a moderate or high risk of sinkhole formation.

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

Ensure complete drainage of the permeable pavement system within 24 hours of a rainfall event.

Extend the filter bed and underdrain pipe below the frost line and/or oversize the underdrain by one pipe size to reduce the freezing potential.



Sand or other granular materials should never be applied for winter traction, since they will quickly clog the system.

When plowing most paver block and plastic reinforced grid pavements, snow plow blades should be lifted 1/2-1 inch above the pavement surface to prevent damage to paving blocks, turf, or aggregate. Porous asphalt and permeable concrete pavement applications can be plowed similar to traditional pavements.

SECTION 9: TYPICAL GRAPHIC DETAILS

To be provided. (NOTE: Recommendations?)

SECTION 10: MATERIAL SPECIFICATIONS

Permeable pavement material specifications vary according to the specific pavement product selected. Please consult the manufacture’s technical specifications for specific information. Table 3 describes general material specifications.

Table 3: Permeable Pavement Material SpecificationsMaterial Specification NotesPermeable

Interlocking Concrete Pavers

Open void content: 8-20%.Thickness: minimum 2.35 inches. Compressive strength: 55 Mpa.

Must conform to ASTM C936 specifications. Aggregate layer required to support the structural load.

Concrete Grid Pavers

Open void content: 20-50%.Thickness: 3.5 inches.Compressive strength: 35 Mpa.

Must conform to ASTM C 1319 specifications. Aggregate layer required to support the structural load.

Reinforced Grid Pavers

Void content: depends on fill material.Thickness: varies. Compressive strength: varies, depending on fill material.

Aggregate layer required to support structural load.

Pervious Concrete

Void content: 15-25 %.Thickness: typically 4-8 inches.Compressive strength: 2.8-28 Mpa.

May not require an aggregate base layer to support the structural load, but a layer can be included to increase storage or infiltration.

Porous AsphaltVoid content: 15-20 %.Thickness: typically 3-7 inches. (depending on traffic load)

Aggregate layer required to support the structural load.

Sand Bedding Layer (optional)

2 inches of clean, washed sand (e.g. ASTM C 33, 0.02-0.04 inch) should be placed over the choker stone layer for sand-filled paver applications only.

Choker stone Layer

2-4 inch layer of clean, washed choker stone (e.g. ASTM D 448 size No. 8 or No. 89)

Base Layer

ASTM D448 size No. 57 stone (e.g. 1-1/2 to 1/2 inch in size); should be double-washed and clean and free of all fines.

Depth of the stone storage layer is based on the pavement structural requirements, but usually ranges from 8-36 inches.


Table 3: Permeable Pavement Material SpecificationsMaterial Specification Notes

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.

Sub-base filter layer

If no underdrain is required, the underlying native soils should be separated from the stone reservoir by a thin, 2-4 inch layer of choker stone (e.g. ASTM D 448 size No. 8 or No. 89) and a 6-8 inch layer of 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.

SECTION 11: CONSTRUCTION SEQUENCE AND INSPECTION

11.1 Construction Sequence

Construction Stage E&S Controls: All permeable pavement areas should be fully protected by silt fence or construction fencing, particularly if the installations will incorporate infiltration (i.e., no underdrains). Permeable pavement areas should remain outside the limit of disturbance during construction to prevent soil compaction by heavy equipment. During construction, care should be taken to avoid tracking sediments onto the permeable pavement surface to avoid clogging. To help prevent soil compaction, heavy vehicular and foot traffic should be kept out of permeable pavement areas during and immediately after construction. To help accomplish this, clearly delineate permeable pavement areas on all development and grading plans. It is also advisable to divert the runoff from the first three runoff-producing storms away from the permeable pavement, especially from up-gradient asphalt areas that drain to the permeable pavement. This is because the first several storms typically generate a high load of fines from new asphalt installations.

Permeable Pavement Installation

The following is a typical construction sequence to properly install a permeable pavement practice. These steps may be modified to reflect different types of permeable pavement applications or expected site conditions:

Step 1: Permeable pavement 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 E&S controls are needed during permeable pavement 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 construction process. The area where the pavement is to be constructed must be kept free from sediment during the entire job. Sediment-contaminated materials should be removed and replaced with clean materials.

Step 3: Excavation for permeable pavement systems should be limited to the width and depth specified in the design. 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 choker stone, sand and stone reservoir.

Step 5: Moisten and spread 6 inches of washed No. 57 stone on the excavated bed and then install the perforated pipe underdrain (if applicable). Pack additional washed No. 57 stone to the specified depth above the top of underdrain. The aggregate should be moistened and compacted using a vibratory roller until there is no visible movement of the No. 57 stone. Do not crush the aggregate with the roller.

Step 6: Moisten and lay 3 inches of pea gravel (No. 8) as a layer of pavement bedding. If applicable, top off with a 2-inch layer of sand bedding. The bedding material should not be subject to any pedestrian or vehicular traffic before installation of the permeable pavement begins.

Step 7: Install the pavement material, according to the manufacturer’s construction specifications. If using modular pavers, fill the voids or joints with the specified fill material. Sweep the pavement surface to remove any excess fill material on the surface. If using turf pavers, water them during weeks of no rain for the first two months to establish vegetation.

Step 8: Conduct the final construction inspection (see Section 11.2), and log the GPS coordinates for each facility to enter into the local BMP maintenance tracking database.

(NOTE: Future versions will highlight differences for various applications.)

11.2 Construction Inspection

Inspections during construction are needed to ensure that the permeable pavement practice 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. An example construction phase inspection checklist for permeable pavement areas can be accessed at CWP website at

http://www.cwp.org/Resource_Library/Center_Docs/SW/pcguidance/Tool6.pdf .


http://www.cwp.org/Resource_Library/Center_Docs/SW/pcguidance/Tool6.pdf

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

Store materials in protected area to keep them free from mud, dirt, and other foreign materials.

The contributing drainage area should be stabilized prior to directing water to the permeable pavement area.

Check aggregate material to confirm that it meets specifications and is installed to the correct depth.

Check elevations such as the invert of the underdrain, inverts for inflow and outflow points, and the surface slope.

Make sure the permeable pavement surface is even, runoff evenly spreads across it, and the

storage bed drains within 48 hours.

Ensure that caps are placed on the upstream (but not the downstream) end of the underdrain.

Make sure the desired surface cover has been installed.

Inspect the pretreatment structures (if applicable) to make sure they are properly installed and working effectively.

SECTION 12: OPERATION AND MAINTENANCE

12.1: Maintenance Operations

Maintenance is very important for permeable pavement, particularly in terms of ensuring that they continue to provide measurable stormwater management benefits over time. Consequently, a legally binding inspection and maintenance agreement and plan should be put in place to ensure that permeable pavement systems are regularly maintained after installation. These maintenance agreements must contain recommended maintenance tasks and a copy of an annual inspection checklist. When permeable pavements are applied on private residential lots, homeowners will need to be educated on their routine maintenance needs. The existence and purpose of the permeable pavement area should be noted on the deed of record that is transferable to new owners upon sale. Table 4 provides a list of maintenance activities typically associated with permeable pavement systems.



Table 4: Typical Maintenance Activities Associated with Permeable Pavement SystemsActivity Schedule

Inspect the contributing drainage area and permeable pavement surface for sediment build-up and debris, and remove any accumulation.

Look for any bare soil or sediment sources in the contributing drainage area, and stabilize immediately.

Check the pavement for excessive ponding, dead plants vegetative cover (if applicable), or areas of clogging, and take appropriate remedial action.

Monthly

Vacuum-sweep the permeable pavement surface to keep the surface from clogging.

Inspect the permeable pavement system following large rainfall events, to ensure proper drainage

Quarterly

Inspect the permeable pavement surface for deterioration or spalling. Replace or repair affected areas, as necessary.

Annually

12.2: Maintenance Inspections

It is highly recommended that a spring maintenance inspection and cleanup be conducted at each permeable pavement site. A detailed annual maintenance inspection checklist for infiltration practices can be accessed at the Center for Watershed Protection website at

http://www.cwp.org/Resource_Library/Center_Docs/SW/pcguidance/Tool6.pdf .

SECTION 13: REFERENCES

Hunt, W. and K. Collins. 2008. “Permeable Pavement: Research Update and Design Implications”. North Carolina Cooperative Extension Service Bulletin. Urban Waterways Series. AG-588-14. North Carolina State University. Raleigh, NC. Available Online: http://www.bae.ncsu.edu/stormwater/PublicationFiles/ PermPave2008.pdf.

Schueler, T., Hirschman, D., Novotney, M., and J. Zielinski. 2007. Urban Stormwater Retrofit Practices Version 1.0. Center for Watershed Protection. Ellicott City, MD.

US paver specification. SECTION 32 14 13.19. PERMEABLE INTERLOCKING CONCRETE PAVEMENTNeed reference info from TRS

Virginia Department of Transportation (VDOT). 2003. Guidelines for 1993 AASHTO Pavement Design. Virginia Department of Transportation, Materials Division. Pavement Design and Evaluation Section. Available:http://www.virginiadot.org/business/resources/bu-mat-pde-AASHTOForConsultants0503.pdf



http://www.bae.ncsu.edu/stormwater/PublicationFiles/%20PermPave2008.pdf

http://www.cwp.org/Resource_Library/Center_Docs/SW/pcguidance/Tool6.pdf

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