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60Permeable PavementStormwater Pollution Solutions

"Think like water. Go with the flow."

-Frances Kato , Issaquah School District Student

Solutions LibrarySolution 4: Permeable Pavement

61 Stormwater Pollution Solutions Permeable Pavement

Pervious Portland cement concrete is a rigid pavement similar to conventional concrete that adds a special cementi-tious material to the mixture to improve or change some of the plastic or hardened properties of the finished concrete. The sand component is reduced or eliminated so that voids form between the aggregate in the pavement

Permeable interlocking concrete pavements (pavers) are precast, manufactured modular units. The solid pavers are impervious, high-strength Portland cement concrete manu-factured with specialized production equipment. Stormwater infiltrates down between the spaces in the pavers Grid systems made of concrete or plastic. Concrete units are precast in a manufacturing facility, packaged and shipped to the site for installation. Plastic grids typically are delivered to the site in rolls or sections. The openings in both grid types are filled with topsoil and grass or permeable aggregate. Plastic grid sections connect together and are

What is Permeable Pavement?

Porous Concrete: http://chesapeakestormwater.net/2009/11/permeable-pavement-design-specification/

Pavement for cars and pedestrians takes up about twice the space of buildings. While essential for the movement of people, goods and services, pavement generates significant levels of heavy metals and most of the hydrocarbon pollut-ants found in stormwater. Pavement also contributes to in-creased peak flow, flow durations, and habitat degradation of streams and wetlands. Effective management of stormwater runoff from paved surfaces is critical for improving fresh and marine water conditions in Puget Sound. Typical applications for permeable paving include industrial and commercial park-ing lots, sidewalks, pedestrian and bike trails, driveways, resi-dential access and collector roads. Major roads and freeways are not recommended.

Four Types of Permeable Pavement:

Porous asphalt pavement is a flexible pavement similar to standard asphalt that uses a bituminous binder to adhere aggregate together. Bitumen is a sticky, black, viscous liquid made from petroleum (like tar) that is mixed with aggregate to create asphalt parking lots and roads. The sand that’s usu-ally added to bitumen to make regular asphalt is reduced or eliminated so that voids form between the aggregate in the pavement.

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Text Excerpted from the 2012 LID Technical Guidance Manual for Puget Sound


How Permeable Pavement Works?

The most significant difference between a porous asphalt or concrete system and a permeable paver system is the surface layer. Both systems allow water to percolate through the top layer and eventually into the existing subgrade

Explanation of Diagram

Porous Asphalt / Concrete System

Permeable Paver System

Diagram Excerpted from the 2012 LID Technical Guidance Manual for Puget Sound

Diagram Excerpted from the 2012 LID Technical Guidance Manual for Puget Sound


• Conduct a site analysis of the area you wish to infiltrate using permeable pavement.

• Conduct a soil test to see how well it percolates.

This is How To Build a Porous Pavement System

Step 1: Plan

• Proper subgrade preparation is crucial to balance the needs for structural support and distributing vehicle load while maintaining infiltration capacity. Uniformity of subgrade conditions is necessary to prevent differential settling.

• Subgrade depths of 6-36 inches are common depending on pavement type,

structural design, and storage needs. Increasing the aggregate base depth in permeable pavement systems provides the added benefit of increasing stormwater storage capacity, which can be particularly beneficial on sub-grades with low permeability.

• Immediately before placing base aggregate, remove any accumulation of fine material from erosion and scarify soil to a minimum depth of 6 inches to prevent sealing of the subgrade surface. Do not compact the soil with heavy equipment during construction and avoid subgrade preparation during wet periods because water greatly increases compaction.

• Add large aggregate (1.5-2.5 inches) as a base layer. Typical void space in base layers range from 20-40 percent. The aggregate base layer provides: 1) a stable foundation that distributes vehicular loads from the pavement to the subgrade; 2) a highly permeable layer to disperse water downward and laterally to the underlying soil; and 3) a temporary underground reservoir during peak stormwater events.

• Smaller stone can be used as a choker course between the base course and the pavement to help level the surface in preparation for the wearing surface.

• One or more under-drains may be installed at the bottom of a permeable pavement system if the infiltration capacity of the subgrade soil is not ad-equate to protect the pavement from prolonged saturation or freeze-thaw events that may lead to structural weakness. Under-drains can empty into an adjacent bioretention facility such as a rain garden.

Step 2: Sub Grade Construction

• If installed correctly, permeable top courses have very high initial infiltration rates.

• Asphalt and concrete should be installed by a contractor who specializes in permeable pavements.

• You can install pavers on your own or hire a contractor. • All permeable pavement surfaces should have a minimum slope of 1-2 per-

cent to allow for surface overflow in extreme rainfall.

Step 3: Add the surface layer or “wearing course”

Step 4: Watch the rain go right through!

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Erosion and introduction of sediment from surrounding land uses should be strictly controlled after construction by amending exposed soil with compost and mulch, planting exposed areas as soon as possible, and armor-ing outfall areas. Surrounding landscaped areas should be inspected regularly and possible sediment sources controlled immediately. Installations can be monitored for adequate or designed minimum infiltration rates

Seasonal Management Outline of Duties

• Up to 10% blockage caused by organic matter, sediment, debris or trash

• Drainage functions properly, no ponding

• 100% of water drains though pavement within 20-30 seconds or less


• Drainage functions properly, some ponding

• 100% of water drains though pavement with 30-60 seconds or less


• Drainage doesn’t function prop-erly, significant ponding

• Water drains though pavement slowly or not at all

Best Effort - Grade A Medium Effort - Grade B, C Poor Effort - Grade D, F

Freeze-thaw considerations: Properly designed permeable paving installations have performed well in the Midwest and Northeast U.S. where freeze-thaw cycles are severe. Research shows that bases under pervious concrete, porous asphalt and permeable interlocking concrete pavement do not heave during the winter and do not require thickening of the aggregate base.

Ice buildup on permeable pavement is reduced and the surface becomes free and clear more rapidly compared to conventional pavement. For western Washington, deicing and sand application may be reduced or eliminated and the permeable pavement installation should be assessed during winter months and the winter traction program developed from those observations. Vacuum and sweeping frequency will likely be required more often if sand is applied.

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Source: Rain Garden Handbook for Western Washington Homeowners, Curtis Hinman, WSU

by observing drainage immediately after heavier rain-storms for standing water. Clean permeable pavement surfaces to maintain infiltration capacity at least once or twice annually following recommendations below. Clean surfaces using suction or high-pressure wash. Sweeping alone is minimally effective. Hand held pres-sure washers are effective for cleaning void spaces and appropriate for smaller areas such as sidewalks.


Sample Cost

Project: Tahoma Porous Plaza

During any kind of hardscape construc-tion, concrete needs to be either deliv-ered to the site in a truck or mixed on site. If the area being paved is large, a truck is typically required.

Installation of porous concrete requires a professional contractor because the process is more sensitive than installing traditional concrete. You don’t want to overwork the concrete or the void spac-es will fill in.

Labor costs for installation of porous concrete tend to be higher than tradi-tional concrete because the process is more specialized.

A significant portion of the cost will be the materials. Typi-cally concrete is measured in cubic yards. The bigger the area being paved, the more cubic yardage of concrete of concrete will be needed (and therefore the higher the cost will be). If the area being paved is to be driven on by ve-hicles, the concrete pad will need to be thicker and will re-quire additional cubic yardage. There is also a crushed rock base below most porous concrete systems which will add

Description of Costto the overall cost. Delivery of the concrete to the job site will be another cost. The final cost is typically labor. Hiring a pro-fessional crew to install your porous concrete is required to ensure that the concrete gets mixed and poured properly. If the concrete is mixed or poured improperly, your porous con-crete system won’t drain water and will need to be ripped out and reinstalled.

Spreadsheet of Costs Quantity unit price COST

Asphalt demolition & removal (per sq ft) 512 4.00 2,048.00

Concrete demolition & removal (per sq ft) 40 4.00 160.00

16” curb/wheelstop installed (per lin ft) 32 50.00 1,600.00

Soil excavate and dispose (per cu yd) 40 50.00 2,000.00

Subbase prep (per cu yd) 10 50.00 500.00

Base prep (per cu yd) 10 50.00 500.00

Top prep (per cu yd) 10 50.00 500.00

Permeable Pavers (per sq ft) 512 10.00 5,120.00

Pipe, installed (per lin ft) 10 50.00 500.00

total 12,928.00


Cool Applications Everywhere


The Best of the Best - Porous Pavement

To accommodate growth at Joint Base Lewis McChord (JBLM), the base is in the process of implementing a renova-tion of 1.5 miles of roadway that extends within and outside of the base. Cascade Design Collaborative (CDC) worked closely with the base officials and engineers to transform the main commercial area into a walkable and liveable town center that satisfied security, military access, and federal con-tracting standards. The new “multi-way” boulevard features more than a mile of bioretention cells and 55,000 square feet of porous pavers for both vehicular and pedestrian areas,

Pendleton Boulevard - Joint Base Lewis McChord - Fort Lewis, Washington

Images Sourced From: Cascade Design Collaborative

and provides the needed traffic volumes, improved pedes-trian environment, and parking. CDC worked closely with the civil engineers as stormwater modeling was fine-tuned and the bioretention and permeable paver details and specifica-tions were finalized. The team was able to demonstrate the importance of construction observation and so have contin-ued to be involved. As the boulevard comes on line and the base personnel take over maintenance, CDC is assisting with weeding instructions and maintenance guidelines.

Text Source: Cascade Design Collaborative

69 Stormwater Pollution Solutions

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