Wednesday, November 3, 2010

Permeable Pavement Systems – Worthless Without Proper Maintenance

The concept is simple enough; create a pavement surface that will allow rainwater to simply pass downward through it and into a gravel reservoir where it has a chance to percolate back into the soil. Not only will these systems greatly reduce or eliminate storm water runoff volume from a paved area, but also greatly improve the quality of the water that enters the groundwater or storm drain system. With the inception of storm water quality design constraints for many sites, more projects are deploying permeable pavement systems, in the form of pervious concrete, pervious asphalt, gravel-filled plastic reinforcing grid systems, and the ever-popular permeable interlocking concrete pavers (PICPs).

If Not Maintained, Permeable Pavement Becomes Impermeable

When pervious pavement is first installed, its top layers can infiltrate more water than would ever be experienced in any rainstorm anywhere on the face of the earth; up to 500 inches per hour! This is simply because clean gravel, or anything highly porous, will pass a great amount of water through it vertically. Over time, surface sediments, mud, silt, debris, and organic material lodge themselves in the porous spaces. Additionally, the fill gravel used in the grout spaces of PICPs can become displaced by differential settling and high-speed surface traffic. In 2 years or less, without proper maintenance, a fully-functioning system can become completely clogged, and will function just like conventional concrete. Storm water may pond and go places never intended in the original design.

What Doesn’t Work

The following methods should not be employed to maintain permeable pavement systems:
  • Power Washing – This method displaces necessary fill gravel (in PICPs), and can drive clogging particles deeper into the porous top layer
  • Infrequent Sweeping – Displaces fill gravel (in PICPs), and is simply not adequate at removing embedded materials. Some types of frequent sweeping can offer some benefit so long as surface is monitored and lost fill gravel is replaced.
  • Pulling Large Weeds – If large weeds are growing in your pavement, then the system is being poorly maintained by definition. But pulling large weeds after they’ve grown will also exacerbate the problem, because dead root material will remain behind.
  • Waiting Too Long and/or Doing Nothing – Whatever maintenance methods are utilized, waiting too long between maintenance actions is one of the biggest contributors to failure of permeable pavement systems.

Complicating and Contributing Factors

Some site conditions exacerbate pavement clogging, and should be avoided. Where they cannot be avoided, an accelerated maintenance schedule is needed.

  • Lots of tree canopy – Overhanging trees that shed debris can quickly clog a permeable pavement system.
  • Systems that ‘accept flow’ from a nearby source – Pervious pavements should not be located next to a watershed (on a downward slope from a nearby area), or at the end of a drainage channel that will dump runoff into the edge of the pavement.
  • Windblown soil and debris – These systems will clog faster when constructed near bare or denuded areas with frequent or steady winds.
  • Landscape stockpiling – Site maintenance personnel should be trained not to stockpile any materials on top of permeable pavements, where the soil or landscape debris can directly clog the top layers of the system.
  • Unsuitable Traffic Conditions – Permeable pavement systems are not appropriate for frequent heavy vehicle traffic or high-speed traffic.

The Standard for Proper Maintenance

The best maintenance program for these pavement systems includes

  • Regular vacuuming (approximately every 6 months) and/or frequent and well-monitored sweeping
  • Spray or Flame Weed Abatement (when weeds are still very small)
  • Replacement of Fill Gravel (for PICPs), restoration of gravel back to surface (performed immediately after vacuuming)

Time is the enemy of any permeable pavement system. If regular vacuuming is not performed or the pavement is abandoned for a long time, a high-powered vacuum system should be able to restore the system. But be careful; special high-powered truck vacuums cannot reach confined spaces, and even where such a vacuum can reach and extract the clogging debris, it will also extract a good deal of joint gravel as well in PICPs (which must be immediately replaced). In pervious concrete and asphalt applications, wait too long to vacuum and the porosity can never be restored – the only solution is a complete pavement replacement.

Don’t waste the expense and efficiency of an otherwise excellent storm water device by not properly planning for regular maintenance. And make sure that a new owner or site manager understands the system they are inheriting, before they find out the hard way.

John S. Coffey, PE, PLS, is founder and President of Coffey Engineering, Inc. in San Diego. He’s contributed to over a thousand civil engineering, surveying, and planning projects in San Diego and surrounding communities over the past 15 years. 858-831-0111