Highway Sweeping -- Pitfalls and Safeguards
An Overview on Porous Pavement Surfaces and Current Sweeping Best Practices
by Ranger Kidwell-Ross, based upon an interview with storm water consultant, Steven Trinkaus, PE
In the United States and around the world clean, unpolluted, water is increasingly being seen as the most precious resource on the planet. It is also being depleted rapidly.
One of the best practices stormwater managers have devised to address stormwater runoff pollution is to design porous, which is also called permeable, pavement. This is pavement that has been designed such that will infiltrate to a subsurface layer, utilizing the ground under the pavement as a recharge area.
If you're not familiar with the porous pavement concept, or would like a refresher, it is suggested that you start by viewing a brief porous asphalt demonstration video of the process, along with a comparison to traditional pavement, check out the 7-minute YouTube video by contractor Gray & Sons, that is embedded below.
One of the difficulties with porous pavement is ensuring that the pores do not get clogged and, thus, impede the flow of water to the subsurface. That's where sweeping comes in. Not only is it important to sweep the pavement frequently, but the sweeping must be done utilizing best practices for porous pavement.
In a nutshell, this means not using a broom sweeper and, in almost all circumstances, not even using a gutter broom when utilizing an air sweeper. Some experts specify use of a vacuum sweeper, such as an Elgin Whirlwind. Others insist that a regenerative air sweeper will produce equally robust results.
This article is based primarily upon information provided by Steven Trinkaus, PE, who has many years of experience with porous pavement and has been a consulting engineer for over 35 years. More than 15 of those years has included designing Low Impact Development (LID) systems. Today, that previous LID designation has been, in many areas, replaced with LISD: Low Impact Sustainable Development. With an increasing frequency, this means specifying building with permeable pavement, which has very stringent cleaning requirements.
Permeable pavement comes in many different formats. These include permeable asphalt, porous concrete and permeable interlocking concrete pavers, which are really paving stones with gaps between the stones through which the water infiltrates. With any of these, the surface layers can get clogged and prevent infiltration. At that point, the system basically quits working.
When it comes to the removal of runoff pollutants, which is the primary reason for installing these surfaces in the U.S., results can be excellent. The best results are achieved where the native soil have moderate to high infiltrative capacity so the water goes into the native soil rather than being discharged via an underdrain. In these situations, there is a good reduction in the runoff of hydrocarbons and metals as they get trapped by the native soil particles. Non-point source pollutants which are found on the surface are infiltrated with the runoff and thus are eliminated from the runoff stream.
In a typical installation, right below the pavement is a thin pea gravel layer; below that, there is a layer of bank-run sand and gravel, which is known as the filter course. As water infiltrates down through the pavement, rather than running off into a storm drain and, ultimately, a body of water, the water quality benefits are achieved. If pollutants are fully infiltrating into the ground then pollutant loads that previously ended up on the pavement surface are eliminated to near zero.
The advantage: Studies have shown that when pollutants such as oil and gas are allowed to go into the soil subsurface, bacteria accumulate that break them down and negate any problems. The installation of permeable pavement surfaces is now being recognized as one of the best ways to remove pollutant loads in the urban environment.
One aspect all permeable pavement installations have in common is that routine, preventive, maintenance – primarily via correct air sweeping at the correct frequency – is extremely important. In all cases, if the system is maintained properly from the start then maintenance will be very simple and relatively inexpensive. It will also be very effective. On the other hand, if the surface is not maintained correctly and the surface pores become clogged, then it can become quite difficult and potentially more expensive to restore adequate infiltration capacity.
If you are in an area where you get snow and ice, then the first rule is that you do not apply sand to any of the surfaces for snow abatement. It is important that only chloride, or other liquid deicers, are used on the porous pavement before the snow falls. Alternatively, rock salt may be applied by either hand or by machine after a snow event. Since sand is one of the primary items that will clog any of these openings on permeable surfaces, one of the key aspects of porous surface maintenance is that not using sand eliminates one of the major causes of clogging.
If you do end up with sand or other sediment on the surface, which can still occur in a variety ways, then it's important to remove it correctly. One way sediment can be introduced onto a porous surface is via track-in from off-site. Sand and other dirt may also be introduced via construction activities.
In a worst-case situation, sand can build up on the surface of the pavement and fill in the open pores of the surface, reducing the infiltrative capacity. Since this occurrence is to be avoided, sweeping should be done on a regular basis, perhaps as frequently as monthly or possibly weekly, during winter months, when weather allows, with an air-based sweeper.
A true vacuum sweeper is usually recommended by those who have done the most research on keeping permeable pavement clean. This includes the University of New Hampshire (UNH), which in 2004 created a porous pavement area in a 9-acre parking lot so they could monitor what occurs under very high-traffic conditions.
Their conclusion was that a vacuum sweeper does the best job of pulling foreign material like sand out of the permeable surface. When done on a routine basis, the infiltrated capacity of the pavement is restored to near new capability.
When it comes to interlocking concrete pavers, initially installations called for filling the gaps between the pavers with a coarse sand. However, it has been determined that the sand became very prone to clogging on the surface. As a result, current recommendations for interlocking porous pavers is to use a fine crushed stone consisting of 3/8" stone, instead.
For these surfaces, another basic recommendation is to use a leaf blower several times a year to remove any loose debris that accumulates on the surface. This includes any organics, such as leaves that might have fallen and then been degraded by tires. If this is done quarterly or, when needed given the circumstances, any type of infiltration problems whatsoever may be avoided.
For other porous pavement applications, routine air sweeping should be done, as needed, to keep infiltration through the pavement surface working well. For some applications – and especially in areas where there is high usage or significant introduction of leaves and other foreign material – this might be as often as once per month. In other situations, twice a year might do it; one sweep would be after the leaves fall, with the second sweep being done late March or April to remove any detritus from the winter season.
Those charged with maintenance of porous surfaces need to do a regular assessment of the surface. This assessment is really nothing more than an informed, commonsense, review of the pavement surface to see if significant material that might get ground into the pores of the pavement has collected. In all circumstances, it is much easier and less expensive to keep the surface clean than it is to mitigate material that has been ground down into the pores of the pavement.
One of the caveats of a porous concrete surface is that salt cannot be put onto it in the first winter after installation. That is because the chloride will eat the concrete surface away since it will not have cured sufficiently. If sand must be used for snow and ice abatement, then a sweeper must be used regularly to remove the material as soon as possible after each application. After the first year, though, the application of sand should be eliminated via a move to chloride or similar deicing products.
From a maintenance standpoint, permeable systems are actually very low maintenance; however, it is a different type of maintenance than most property managers are used to doing. Maintenance personnel should be retrained to schedule routine evaluation of the condition of any porous pavement surfaces on the properties they are responsible for.
When the University of New Hampshire initially put in its porous parking lot in 2004, in order to evaluate the results the organization intentionally did no maintenance for the first four years. Immediately after installation, the surface had infiltration rates of about of about 1000-inches/hour. Even after four years of zero maintenance, which included heavy usage by cars, the infiltration rates were still on the order of 300-400 inches/hour. That, of course, is more than any rain intensity could ever be.
On the other hand, there are certainly instances where, through heavy usage, track-out or other causes, porous surface infiltration has been reduced to near zero through lack of care and maintenance. In those instances, a high-powered vacuum sweeper will need to be utilized although even then it may not do the job.
In that case, a potential solution may be to flood the surface with a water truck in order to loosen the clogging particles and then vacuum the surface again. Another option to try would be the time-consuming task of utilizing a sweeper's remote hand hose or the one on a storm sewer cleaner, since these have a stronger vacuum than that produced by an air sweeper on its own.
Unfortunately, depending on the severity of clogging, these methods may or may not work sufficiently well. This underscores how important preventive maintenance is in contrast to trying to mitigate a severe clogging situation after it has been allowed to occur.
In any new construction, it's very important that the permeable pavement surface not be put down until all heavy construction, including construction truck traffic, has been concluded. Porous pavement surfaces must not be installed during the active construction phase because the pavement's surface will inevitably be degraded.
Steven Trinkaus related a project where the Connecticut Department of Transportation put in a porous pavement parking lot next to one of the train stations and, while they were building the train station itself, trucks and other heavy equipment drove in and out over the permeable pavement. A year after the payment was initially installed, about half of it was clogged with sediment simply because of that error.
When sweeping, the typical use of gutter rooms is not advised. If there is debris along a gutter, or elsewhere not available to the sweeper, this material should be transferred into the sweeper's path with a backpack blower. In most applications the sweeper will be able to drive along the edge of the surface and do a sufficient job. Since there is typically little vehicle traffic at the edges, in most installations that area will not receive enough traffic to allow the pores to become clogged.
It is very important that the sweeper is used on a routine basis, whatever frequency is required given the application so that clogging is not allowed to occur. If they are well-maintained, permeable surfaces could outlast traditional paved areas. That's because most paved surfaces eventually fail because water infiltrated the base.
One of the design hallmarks of permeable pavement systems is they are built on a very well-drained base so water will not pool under the surface. As a result, they are not prone to frost heaving and other typical problems that degrade paved areas.
Although the aggregate surface freezes, the void spaces in between still stay open. So, even if it's 20-degrees out water may be poured onto the surface and it will still travel quickly through the void spaces in the material. UNH found that if they have a foot of frost and get a rainfall event of 1/2 inch, the rainfall is enough to completely remove the frost.
Because the application of permeable pavement has only been around a little over a decade, all the results are not yet in. However, it appears that most of permeable surfaces, when well-maintained, will last for at least 15 or 20 years, and perhaps longer, before a section will need to be replaced.
Trinkaus agrees that it appears clear that regular street sweeping is one of the most cost-effective ways to reduce stormwater runoff pollution. When sand is being put down every other week, he says, then monthly street sweeping is not enough to offset the pollutant load that will build up. Where pollutant runoff is a problem, sweeping needs to be conducted on more like a weekly basis to make the most significant impact. When it comes to pollutant removal, being proactive is a lot better than being reactive; and, in the long run, it's much more cost effective, as well.
Although a typical porous pavement application will run 15 to 20% higher than a standard pavement, that cost is mitigated by the elimination of structural drainage, pipes, and underground galleries that might be needed for volume control and other mitigation. The net result is there tends to be a 10 to 15% savings over a conventional design. So, while the material is more expensive, when the components that did not have to be included are factored into the equation, the net result is a savings in overall expense.
The sweeping industry needs to recognize that it has a seat at this table, says Trinkaus. The care and maintenance of porous pavement surfaces should become an important addition to the standard business model for sweeping contractors, as it is becoming for professional paving contractors. The key is to understand the parameters and frequencies needed for cleaning and to become an expert on how the operation should be carried out.
If you would like to listen to an approximately 20-minute audio podcast interview conducted with Steven Trinkaus on this topic by the author, click here (opens in a new browser window).
Many thanks to Steven Trinkaus, PE, for much of the material for this story. Trinkaus is the owner of Trinkaus Engineering, LLC, located in Southbury, Connecticut. The company's website is www.trinkausengineering.com.
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