Porous pavement revisited: Good news and bad

Approximately 25 years ago, porous pavement was laid, amidst rave reviews and high expectations, in a section of a parking lot in a new town just north of Houston. The idea was that this new technology, which was developed in the 1970s, would enable streets to be built without the ensuing problems of rapid stormwater drainage caused by conventional pavements.

In the flat Gulf Coast areas, disturbing the natural drainage, which is bad enough in its natural state, is serious business. Flooding is a constant threat. Any increase in the velocity of the water draining to the ditches or rivers was cause for alarm. Thus, when George Mitchell, developer of The Woodlands, built his new town, he investigated every technology to interfere with the drainage as little as possible. Porous pavement seemed promising.

Early experiments with porous pavement disappointing
The experiment didn't work. The pavement crumbled and a true test was never realized since the section of the parking lot where the pavement was laid was seldom used. In short, it wasn't the answer to the problem.

Much has been learned in the past quarter century and porous pavement is slowly being accepted in the U.S. as an effective way to control stormwater drainage on parking lots and other low volume roads.

In Europe, however, it is being used for a very different reason and on more heavily-traveled roads. This time it is being sought as one, according to a report in the October 1999 issue of World Highways, of "a new generation of low noise surfacings."

Porous pavement road in England already needs resurfacing
An attempt to use it on a well-traveled 12.5-km road in southern England has, however, met with disappointment. It seems the road, which was opened only 10 months ago, already needs resurfacing. According to the Highways Agency in England, the cause for the road failure is still not known but the replacement porous pavement will meet much tougher standards in terms of compaction and binders. According to the report, despite the problems, the Highways Agency is "committed to porous asphalt and a whole range of new low-noise surfacings."

Dutch government decrees porous pavement on all trunk roads
It seems the porous pavement has been a success in the Netherlands. So successful, in fact, that the government has decreed that the entire trunk road network be surfaced with the material by 2002. The reason, again, is for noise abatement.

In the U.S., stormwater drainage remains the biggest incentive for its use. For instance, a study is now being conducted by the University of Washington, largely sponsored by the Washington DOT, on the advantages and disadvantages of alternative roadway shoulder pavements. Preliminary results indicate that porous pavement presents a happy medium between shoulders of gravel, which are generally unsafe and require significant maintenance, and shoulders of conventional pavement, which increase stormwater drainoff.

Engineering firm maintains it has used it successfully for 20 years
On the other side of the country, an environmental engineering firm, Cahill Associates, out of Pennsylvania, maintains that, contrary to prevailing wisdom, porous pavement applications can be developed successfully. That firm says it has constructed more than 40 porous pavement installations in areas ranging from 3,000 to 147,000 square feet for the past 20 years. According to Thomas Cahill, "They are still being successfully used today."

According to a report from the Cahill firm: "Experience has shown that most porous pavement failures occur because of a lack of erosion/sediment control during construction. In many instances, contractors, unfamiliar with what they were doing and why they were doing it, allowed substantial quantities of sediment to erode onto the pavement surface after installation. Construction traffic also tracks heavy loads of clay particles onto the surface. Void spaces in the porous asphalt became permanently clogged, preventing stormwater from even entering the recharge bed below.

"The fine particle silts which managed to pass through the porous pavement and through the underlying rock-filled recharge beds then settled out on the recharge bed bottom, reducing the recharge bed ability to infiltrate over time. These failures have made stormwater management program regulators and administrators generally very reluctant to recommend porous pavement as a BMP, rejecting the technology as impossible to apply in the real world."

Certain precautions must be taken to assure success
When constructing porous pavement, Cahill says there are a few precautions that must be taken to assure a successful result. They include:

• Site conditions such as permeability of the soil must be verified. Field verification of a soil layer of reasonable thickness (4 feet or more) with acceptable drainage qualities (percolation rate of 0.5 inches per hour or more) is essential.
• All sediment-laden runoff must be directed away from the porous pavement/recharge bed. Total site design and stormwater drainage planning must be tailored to porous pavement/recharge bed requirements. While all runoff from impervious surfaces (roof tops, roads, parking areas, walkways, and so forth) should be directed onto the porous pavement and then into the recharge bed, pervious zones being re-landscaped after construction must be redirected away from the bed, or pretreated so as to eliminate sedimentation and resultant clogging. Strict erosion and sedimentation controls are a must.
• Special safeguards/redundancies should be included in the porous pavement/recharge bed design. Project success in part has resulted because of certain engineering features in porous surface/recharge bed design.
• Selected filter fabric is placed generously on the floor and sides of the recharge bed after excavation/bed preparation, providing an inexpensive barrier between the stone-filled recharge bed and the soil mantle interface. This filter fabric allows water to pass readily, but prevents soil fines from migrating up into the rock basin, reducing the effective storage volume of the recharge bed.
• In the event that the porous pavement were to become clogged, the edge of the porous paved area is designed to function as a linear overflow inlet around the perimeter of the parking bay. The inlet is accomplished quite simply by allowing a width of the bed around the perimeter to go unpaved, later to be topped off with a decorative river stone of some sort. Wheel stops are placed at the edge of the pavement, preventing vehicles from disturbing this emergency overflow.
• Most intense traffic is directed away from porous surfaces. Porous surfaces are limited to parking areas receiving least wear and tear. Roadways ringing the parking areas receive conventional pavement, but drain into the recharge beds.
• Communication with contractors is essential. Contractors/workers involved with the project must understand what is being done and why compliance with specifications is essential. The nature and purpose of the porous pavement/recharge bed technique must be liberally entered onto the construction drawings and included within the written specifications for the project.
• Installation must be supervised and spot-checked. Proper inspection/supervision during construction of the porous pavement/recharge bed should be budgeted into all projects. Spot-checking by the engineer early on is essential. Regulatory agencies such as the local conservation district cannot be relied upon to make sure that plans and specifications are being executed fully. Contracts, bids, and budgets must include necessary inspection by the design engineer. A written record must be maintained including review and approval at critical project junctures, such as excavation of recharge beds, placement of filter fabric, and quality control at the stone crushing plant and asphalt mix plant. In addition, site inspection and supervision must make sure that construction vehicles are not allowed to traverse excavated recharge beds or enter the completed porous pavement, and that all erosion control measures are in place.

Although porous pavement most often consists of a relatively thin course of open graded asphalt mix over a deep base of large size crushed stones, it is also being constructed with concrete lattice blocks and porous concrete mix. All are successful in allowing stormwater to percolate into the subbase or to drain laterally.

In addition to the stormwater drainage and noise abatement advantages, Cahill pointed out that the pavement is also effective in absorbing less energy (heat), which has become a problem in highly-paved areas.

Cahill also noted that the U.S. porous pavement does not necessarily have the noise abatement characteristics. And vice-versa for the European pavements that do not necessarily promote better drainage. He believes a polymer of some sort was being used in Europe, which would interfere with the drainage characteristics.

For more information, Cahill may be contacted at 610-696-4150.