2) Don’t forget about ongoing maintenance costs. One reason you’re considering pervious pavement is that it can defer or eliminate the cost of building of new stormwater infrastructure. But unlike conventional paving, pervious asphalt absolutely requires regular cleaning to prevent clogging; if leaf litter or other debris fills the voids, the roadway will clog and shed water at about the same rate as conventional paving. And then you have a bad situation: excess stormwater and no infrastructure to handle it.
So, plan your maintenance system ahead of time. For example, if your area has a lot of deciduous trees, you’ll need to sweep streets a couple of times a week during leaf dropping season, and you may also need to vacuum occasionally.
The good news is that clogged streets can usually be restored. Often, high-power vacuuming and pressure washing restore permeability. And incidentally, if your permeable pavement does become permanently clogged, you’re actually better off with asphalt as opposed to concrete. Due to asphalt’s greater flexibility, freezing and thawing of water in the top inch or so of the clogged roadway is less likely to cause raveling and aesthetic issues.
Here’s some more good news: Pervious pavements don’t seem to get as cold as conventional roadways, maybe because subsurface heat is more able to work up into the pavement. If true, and with the higher designed air void structure, you may not need to worry as much about thermal cracking.
3) Do spend money on soil analysis. Conventionally paved roadways may not need soil boring and other analysis during design phases because there aren’t many soil types that adversely affect traditional impervious paving. But soils can strongly affect the performance of pervious pavements, and even rule out their use.
For example, if soils are relatively impervious, such as heavy clays, there will be nowhere for stormwater to go after it penetrates pervious asphalt and sublayers. This can be disastrous. To see why, try putting a sponge on a countertop and pouring a glass of water over it. Not only will the pavement shed water, the increased flow of water through the asphalt strips aggregate more quickly, shortening lifespan.
On the other hand, soils that are sandy or otherwise highly permeable may require less subsurface gravel than some standards specify.
In either case, soil borings and other soil work is worthwhile; you’ll either avoid a mistake or save money.