Porous Asphalt is Considered for Use in the Current Driveway Project

During June's Association Meeting, a discussion ensued which was initiated by our Landscaping Director who is promoting the use of "porous asphalt" in our driveway replacement program. The benefit argued is the control of water runoff from roofs onto the driveways. Our Architectural Director responded with concerns about the use of "porous asphalt", and during the Homeowner's Forum, a unit owner, a former AD, also expressed concerns. I too had concerns but meeting time constraints and more significant issues raised during the meeting prevented a discussion. Unit owners, of course, are limited in these interchanges.  I decided to research the proposed technique and provide my findings to the board. It appeared that there was a press to move and vote on this during the meeting, but that did not happen.

On Friday June 11 I sent an email to our AD, with copies to the President and to our LD. I also provided this as requested to several others. I publish a slightly abbreviated version here, for your benefit. The following from my email dated June 11, 2010:

“The discussion [during the association meeting] about “porous asphalt” seemed to be centered on the “Benefits” or possible “Advantages” of use. There was little discussion of the disadvantages. That is not good. Any project of this type must be discussed from all positions with all benefits and disadvantages alike “on the table” and in full view. That did not happen at the association meeting last night. Only when such transparency and evaluation is made can good choices be made.

I appreciate the effort of the board to explore alternatives for controlling water on the driveways. What other methods are available and what are the costs of each, the maintenance needs and costs, and the disadvantages and advantages of each? Only when such evaluations are made, can rational decisions me made.

[Our Architectural Director's] concerns and proposal to consider this on a partial or “experimental” basis is warranted. This is an expensive driveway. It seems that porous asphalt is really intended for parking lots, and there are strict design criteria. Is this an appropriate application? I’m not a civil engineer.

I did some research and I share it here. Attached is a “fact sheet” from the University of New Hampshire.

First, to quote the National Asphalt Pavement Association, “How does it work?” Their answer is this, which points out some of the source of the problems and expense. The asphalt is more expensive and a new, extensive (and expensive) sub-base is required:

“The technology is really quite simple. The secret to success is to provide the water with a place to go, usually in the form of an underlying, open-graded stone bed. As the water drains through the porous asphalt and into the stone bed, it slowly infiltrates into the soil. The stone bed size and depth must be designed so that the water level never rises into the asphalt. This stone bed, often 18 to 36 inches in depth, provides a tremendous subbase for the asphalt paving.” The emphasis is mine. The definition of an “open-graded” bed is one which uses crushed stone with only a small percentage of sand.

That “tremendous subbase” will be expensive.

As pointed out by [our former AD, a unit owner] during the homeowner’s forum, there is a serious question about “where will the water go”, specifically, the water that permeates the driveway. Some will hit the underlying clay soil and it will flow horizontally in any direction and some will most certainly go under the garage floors. So how to direct this underground flow away from the garages, and if that is unsuccessful, what are the possibilities of “freeze thaw” and heaving action and damage to the garages? Being unwilling to discuss these serious issues could result in a flawed decision and even greater expense.

We don’t have sandy soil. This area is predominantly blue clay which forms a barrier to water.

From the UNH attachment here are some of the issues:

1. “Routine (Quarterly) Vacuum Sweeping (Vac-Assisted Dry Sweeper Only) is required” and this “long term and routine maintenance” will be at additional cost. If this is not performed, the permeability is lost and the driveway will not be porous. This is an expense that will be incurred by the association each year. How much will it cost to properly clean 84 driveways four times a year?
2. “Proper Construction Stabilization and Erosion Control are Required to Prevent Clogging.” In other words, any soil, dirt from the garages and possibly even granules and materials from the roofs and trees that washes onto the driveways will clog the driveway and inhibit or prevent permeability.
3. “Quality Control for Material Production and Installation are Essential for Success.” In other words, great care must be used in the installation. Any problems, defects, etc. will result in failure.
4. “Materials Cost is approximately 20 to 25% More Than Traditional Asphalt.” However, to do this also requires the installation of a that deep sub-base, which means there will also be increased labor expense.
5. “Vacuum Sweeping Cost May Be Off-set by Reduced Deicing Costs”. In other words, there may be less winter salt required. However, plowing and so on will still be necessary.
6. “Repairs Can be Made with Standard Asphalt Not to Exceed 10% of Surface Area.”
7. The design criteria includes “Soil Permeability is Recommended Between 0.25-3.0 Inches Per Hour.” In other words, that’s the amount of water that the soil under the sub-base must be able to handle. According to the “Civil Engineers Design Manual 1995”, a “poorly graded clean sand or sand gravel mix” will handle 2.27 inches per hour of water. However, our subsoil is not a mixture of sand and gravel. It is small soil particles, sand and clay. So how permeable is clay? Water will travel vertically through three feet of gravel in about 2 minutes. Water will travel vertically through three feet of sand in about 2 hours. It will travel the same distance through silt (soil particles of sand and clay) in 200 days. As for pure clay? It will take water 200 years to travel through 3 feet of clay! Our soil is probably closer to “silt” in size. That will not provide the drainage required.
8. The “Recommended Drainage Time of 24-48 Hours.”
9. “Sub-Drains Should be Used Where Proper Drainage May be an Issue to Minimize Frost Damage.” That’s a tile pipe or pipes to carry water from the sub-base to somewhere else, such as a sewer.
10. “Most Appropriate for use with Low-Use Roadways and Parking Lots—Without a Modified Asphalt Binder.”
11. “3 -5 Feet of Vertical Separation is Needed from Seasonal High Groundwater.” In other words, the groundwater must be at least 3 feet lower than the sub-base.
12. The UNH recommends the following construction layers of material:

• 4 inches of porous asphalt.
• 4 inches of ¾” crushed stone.
• 8 to 12 inches of open graded reservoir base.
• 4 inches of ¾”> crushed stone for frost protection.
• Soil having a permeability greater than 0.5 inches/hour.

The Federal Highway department site referenced later in this email says this about the effectiveness:

“…porous pavements are not designed to sustain a high removal rate for suspended sediment. While initial removal rates for suspended sediment are very high, the removal process causes clogging of the pavement and subsequently reduces its infiltration capacity. As the infiltration capacity decreases, so does the capture and treatment of runoff pollutants. Careful attention to maintenance is necessary to reduce the potential for clogging. In addition, all adjacent areas should be stabilized to prevent sediment from washing onto the pavement surface to prevent premature clogging.”

The Federal Highway site goes on to say this about “Siting and Design Considerations”:

“Suitable sites for porous pavements are generally limited to low-traffic areas with a minimum soil infiltration capacity of 7 mm/h (0.27 in/h) (greater than 13 mm/h (0.5 in/h) is preferred). Geotechnical testing of potential installation locations is needed to quantify the infiltration capacity. In siting porous pavement, groundwater contamination can be minimized by ensuring that the depth to the seasonally high water tables is at least 1.2 m (4 ft) below the reservoir layer and that installations are no closer to drinking water wells than 30 m (100 ft). Sites that are probable sources of high contaminant loads, such as gas stations, should be avoided.

Porous pavement installations should also be 30 m (100 ft) upgradient and 3 m (10 ft) downgradient of building foundations. More detailed guidelines for the siting of porous pavements and related design specifications can be found in Evaluation and Management of Highway Runoff Water Quality (Young et al., 1996), and A Current Assessment of Urban Best Management Practices - Techniques for Reducing Non-Point Source Pollution in the Coastal Zone (Schueler et al., 1992). Additional information on existing designs and their effectiveness is available in Stormwater Infiltration (Furgerson, 1994).

The design considerations for porous pavement should be consistent with the concepts of flexible pavement design. These requirements, summarized by Rollings and Rollings (1993), include:

• The use of sufficient pavement thickness to protect the subgrade from being overstressed.
• The use of quality base and subbase materials that can support the applied loads.
• A stable surface that serves as the wearing course for traffic.
• The compaction of all materials to provide strength and to resist densification under traffic.”

Has a civil engineer been consulted about these issues? If not, why not?

Here is a link to the UNH website and paper:    UNH Porous Asphalt Fact Sheet

Here is a link to the Federal Highway Department website and Factsheet on Porous Pavements:
FH Dept. Fact Sheet

I don’t know much about “porous asphalt” for parking lots, but the design seems to be similar to the principle of a “French drain” in which a layer of crushed stone is laid over sandy soil in a trench 5 to 6 inches wide. The drain slopes in the direction that water is to be carried. The slope is at least 1% (a drop of 1 foot for every 100 foot length). In fact, I don’t see why a “french drain”” isn’t under consideration. Why not a discussion of dark stone over a bed of crushed stone in a narrow trench to direct water downhill and away from specific buildings and to the street? How much would that and a normal driveway cost as opposed to a properly installed “porous asphalt” driveway? What are the differences in maintenance costs, etc.?

It’s appropriate to consider the “total cost of ownership” or operation when considering these projects. That includes cost of initial installation, maintenance costs over the lifespan and the number of years of life. Divide by the number of years and you have true annual costs. These can then be compared for each and every method or procedure that is being considered.

If you have any questions or comments, please advise me.”

Comments, Corrections, Omissions, References
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1. It would seem that this is intended for level surfaces such as parking lots. After conducting my preliminary research, I am skeptical.  It is possible that there may be some specific areas at BLMH that could benefit from the application of "porous asphalt". However, it does not seem that sloped driveways will derive much benefit. The water on the surface slowly infiltrates that porous surface. It would seem most "run off" will simply flow across the sloped surface and will not enter the asphalt as there is insufficient time. A level surface with standing water would derive most benefit and have sufficient time to permeate the surface. The National Highway Department's statement that "Porous pavement installations should also be ...3 m (10 ft) downgradient of building foundations." supports the concern expressed by the former AD about "Where will the water that permeates the porous asphalt go?", which in our case, would be under the garages. 
2. This is an example of why I personally believe it is important for unit owners to attend association meetings. 
3. There was no discussion of total differential costs of this approach as compared to traditional methods. How much would it cost our association to do 84 driveways using this approach? Do we have sufficient reserves both on hand and collected over the life of the project to support it? What about possible increased annual maintenance costs as required for “Routine (Quarterly) Vacuum Sweeping (Vac-Assisted Dry Sweeper Only)?"