permaculture

[EFMonaco <EFMonaco@compuserve.com>]

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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