Nuts & Bolts: When It Rains, It's Porous
(Page 3 of 5)
March/April 2008
By Dan Orzech
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A solid foundation
For permeable pavers—asphalt or concrete—to work correctly, they need the right foundation. They can’t be laid on compacted soil; lay down fabric if they’re on clay or silt. That lets water through but prevents the soil from moving upward and clogging the paving.
The key is a bed of gravel, usually at least 6 inches thick, beneath the pavement to store the water temporarily while it percolates into the soil. The gravel should be evenly sized, to create voids, and deep enough so the water never rises into the asphalt. It should also be rinsed to remove potentially clogging dust.
Going green
You could also create a driveway that’s literally green—by planting grass. A structural lattice lets healthy grass or other plants grow while providing support to keep cars out of muddy ruts.
Inch-thick plastic grids are installed with a mix of sand and fertilizer on top of a gravel and sand bed. (Look for grids made of recycled plastic.) These systems allow even heavy rain to percolate through the grass roots and into the stone bed below, where suspended pollutants and moderate amounts of engine oils are consumed by active soil bacteria. --NH
| | Permeable Pavers | Previous Concrete | Plantable Paver Grids | Porous Asphalt |
| COST | $10 to $12 per squre foot (based on 2,500-square foot area) | $8 to $10 per square foot* (based on 2,500- square-foot area) | $4.50 to $6.50 per square foot (installed, with sand and sod) | $4 to $5 per square
foot* (based on a
2,500-square-foot area) |
| PROS | ■ Most conventional appearance
■ Can be laid in winter
■ Snow melts more quickly because water can drain | ■ National standards for installers set by National Ready Mix Concrete Association
■ Faster to install than conventional concrete | ■ Looks like lawn
■ Can absorb large amounts of water
■ Grass and soil absorb pollutants
■ Absorbs heat and CO2 | ■ Looks like conventional asphalt
driveways
■ Can last 20 years
or more |
| CONS | ■ Fine gravel between pavers can pose a tripping hazard | ■ Requires seven days of above-freezing weather to cure
■ Vacuuming and power-washing needed (especially where sand is applied to streets) | ■ Grass must be watered and mowed like a lawn
■ Could pose a tripping hazard | ■ National standards not well established
■ Vacuuming and power-washing needed (especially where sand is applied to streets)
■ Additives to prevent clogging can be
tricky to install |
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