Subject: Permeable Paving

From: EFMonaco <[email-masked]>
Date: Fri, 27 Aug 1999 10:16:30 -0400

Does anyone know about permeable pavement types that can support car and
light-truck traffic?  I have a client who needs a parking area for a Knox
County (Tennessee) public justice facility.  The hired civil engineers do
not know much about them, and neither do I.  The group perception of them
is that they can be used in sandy (well-drained) soils with light or
infrequent use, that they require an initially higher investment, and
require a lot more maintenance than conventional asphalt.  In the east
Tennessee area, which lies in the Appalachian Valley and Ridge
Physiographic Province, the soils are mostly clayey and silty, underlain by
shale and limestone.  Typically they don't permeate well once the topsoil
and mulch layers are saturated, unless the water finds its way into cracks
and fissures that work their way into the limestone base (where it exists)
contributing to the sink holes that can be present.

Typical parking lot construction requires excavation of soft soils to some
depth (usually a couple feet below the surface), followed by replacement
with suitable soils (free of organics) at optimum moisture content
compacted to 95% of maximum density.  The idea is to bridge over soft spots
with compacted "structural" fill.  This is then overlain by 4" to 8" of
compacted gravel, with gradations such that the fine aggregrates interfill
the voids between the coarser aggregates.  This is overlain by 2 layers of
asphalt cement concrete. This typical pavement crossection is totally
impervious under normal loads.  Under heavy loads with freeze-thaw
conditions, it can still erode away and create potholes.  Removal of the
asphalt still yields impervious an stone base; removal of the stone base
yields impervious compacted dirt, albiet slightly less.

In Florida, where well compacted sands can still drain somewhat, I've heard
of using honeycombed concrete or steel mats that, once filled with sand and
topsoil, will support vehicle traffic, as well as primary biological
communities (grass).  The grass absorbs some rainwater, and the
micro-organisms in the soil can process (eat) the oil drippage before it
runs off under gentle rains.  Cigarrette butts might even mulch.  The mats
are alegedly expensive and I have not found specific examples.  And what
about less pervious soil applications?  And other products?

Gene Monaco

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From: [email-masked]
Date: Fri, 27 Aug 1999 12:34:30 EDT

There are custom-made bricks with openings that can be laid over properly 
prepared surface.   Any road surface should  have good provision for drainage 
under it. 

There is also a "tarmac" type surface that is permeable.  I have a friend who 
has used it at his store and I'll email him.   However any road surface or 
driveway surface contractor should know about it.  s

To some extent, the most appropriate material depends on the level of traffic 
anticipated.

For Mother Earth, Dan Hemenway, Yankee Permaculture Publications (since 
1982), Elfin Permaculture workshops, lectures, Permaculture Design Courses, 
consulting and permaculture designs (since 1981), and annual correspondence 
courses via email.  Copyright, 1998, Dan & Cynthia Hemenway, P.O. Box 52, 
Sparr FL 32192 USA  Internships. [email-masked]  

We don't have time to rush.

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From: Toby Hemenway <[email-masked]>
Date: Sun, 29 Aug 1999 08:26:24 -0700

Here's a somewhat different approach to permeable paving in parking lots
that Ben Haggard uses in New Mexico; it may be less effective in non-arid
regions like Tennessee, but I'm sure it's better than straight asphalt. He
attempts to lay out parking lots on contour if the engineer in charge will
permit it (rather than laser-leveling it-though that works too, it's
unnecessary). Then they pave the lot with concentional asphalt, leaving
about 25% of the parking area unpaved. The unpaved areas are laid out as
short, shallow swales between parking spaces, and planted with appropriate
vegetation (i.e. useful shrubs and herbs: insectaries, wildlife, N-fixers,
craft supplies, you know . . . . Trees, if any, must have leaves that won't
stain or gum up auto finish and shed them slowly over a long season so that
cars don't get blanketed, which annoys many drivers). The swales serve as
infiltration areas for runoff as well as cleaning up oil-polluted
rainwater. This design eliminates the need for storm drains in the parking
lot, saving money (though Ben told me that the engineer on one wouldn't
believe it, and put a drain in anyway. Rainwater rarely makes it to the
drain). The soils there are pretty porous, but their rain can be extremely
heavy, an inch or two in an hour sometimes. You might want to fill your
swales with lighter soil if you've got clay; I'm sure a perk test would
tell you how much unpaved area would absorb a major downpour. It's an
inexpensive way to recharge groundwater, provides greenery, and helps cool
off the parking lot.

Toby

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From: EFMonaco <[email-masked]>
Date: Sun, 29 Aug 1999 16:21:42 -0400

>Then they pave the lot with concentional asphalt, leaving
vegetation<

We looked at this in a few areas, and I'm all for it; however, the drawback
as pointed out by the engineers is that it takes a larger parking lot area
because the space between cars increases.  Altogether though, it is
definitely more aesthetic.

>The swales serve as infiltration areas for runoff as well as cleaning up
oil-polluted
rainwater. This design eliminates the need for storm drains in the parking
lot, saving money (though Ben told me that the engineer on one wouldn't
believe it, and put a drain in anyway...<

I'm running into the same thought processes (stinkin' thinking), where they
don't think the swales will absorb any water.  The reason they think this,
I think, is that in doing calculations, they assign a coefficient of
representing absorbtion to each area.  Since the swale area is generally
small, and the soils underneath relatively impermeable (clay and rock), it
tends to have a negligable impact on their calculations.  The fallacy in
the thinking is that they are designing for peak storm events, not everyday
rains.  Their calculations may be correct for peak storms, but they're
missing the boat for everyday rains.  This is a case of permaculture
thinking needing to be institutionalized.  If the engineers read this stuff
in their text books, and it was on the PE exam, they'd think differently. 
Trying to shift their thinking from the fringe is difficult.

On another note, I found some products on the web
(http://www.prestogeo.com/solutions/) that deal with my question; however,
they seem to be designed for infrequent use, like a fire lane that might
only be used during emergencies.  How well they hold up for everyday use
vs. cost is something I'm still evaluating.

Gene Monaco

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From: "Rene and Lorraine van Raders" <[email-masked]>
Date: Sat, 28 Aug 1999 20:18:56 +1000

Don't know if this is much use to you, but in Australia we can get high
density polyethylene squares (Gras-Cel Turf Paving Blocks) that are about
the same cost as laying cement.
 The hexagonal sections are designed for heavy vehicles (of the type you
mention that allows turf to grow through), usually on a compacted sand base
(as for cement) .  I don't have any personal experience.. just saw them at
a conference I attended.  Contact Total Erosion and Pollution Control
(Aust) Co, 35 Parraweena Rd, Caringbah, NSW 2229, (02) 9524 0155.

Cheers Rene

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