Permeable paving is a range of sustainable materials and techniques for
permeable pavements with a base and subbase that allow the movement of
stormwater through the surface. In addition to reducing runoff, this
effectively traps suspended solids and filters pollutants from the water.
Examples include roads, paths, lawns and lots that are subject to light vehicular
traffic, such as car/parking lots, cycle-paths, service or emergency access
lanes, road and airport shoulders, and residential sidewalks and driveways.
Although some porous paving materials appear nearly indistinguishable from
nonporous materials, their environmental effects are qualitatively different.
Whether pervious concrete, porous asphalt, paving stones or concrete or
plastic-based pavers, all these pervious materials allow stormwater to percolate
and infiltrate the surface areas, traditionally impervious to the soil
below. The goal is to control stormwater at the source, reduce runoff and improve
water quality by filtering pollutants in the substrata layers.
Description and applications Permeable solutions can be based on:
porous asphalt and concrete surfaces, concrete pavers, or polymer-based grass
pavers, grids and geocells. Porous pavements and concrete pavers enable
stormwater to drain through a stone base layer for on-site infiltration and
filtering. Polymer based grass grid or cellular paver systems provide load
bearing reinforcement for unpaved surfaces of gravel or turf.
Grass pavers, plastic turf reinforcing grids, and geocells are honeycombed 3D
grid-cellular systems, made of thin-walled HDPE plastic or other
polymer alloys. These provide grass reinforcement, ground stabilization and
gravel retention. The 3D structure reinforces infill and transfers vertical
loads from the surface, distributing them over a wider area. Selection of the
type of cellular grid depends to an extent on the surface material, traffic
and loads. The cellular grids are installed on a prepared base layer of
open-graded stone or engineered stone. The surface layer may be compacted
gravel or topsoil seeded with grass and fertilizer. In addition to load support,
the cellular grid reduces compaction of the soil to maintain permeability, while
the roots improve permeability due to their root channels.
In new suburban growth, porous pavements protect watersheds. In existing built-up
areas and towns, redevelopment and reconstruction are opportunities to
implement stormwater water management practices. Permeable paving is an
important component in Low Impact Development, a process for land
development in the United States that attempts to minimize impacts on water
quality and the similar concept of Sustainable Drainage Systems in the
United Kingdom. The infiltration capacity of the native
soil is a key design consideration for determining the depth of base rock for
stormwater storage or for whether an underdrain system is needed.
Advantages = Managing runoff=
Permeable paving surfaces have been demonstrated as effective in managing
runoff from paved surfaces. Large volumes of urban runoff causes serious
erosion and siltation in surface water bodies.
= Controlling pollutants= Permeable paving surfaces keep the
pollutants in place in the soil or other material underlying the roadway, and
allow water seepage to groundwater recharge while preventing the stream
erosion problems. They capture the heavy metals that fall on them, preventing
them from washing downstream and accumulating inadvertently in the
environment. In the void spaces, naturally occurring micro-organisms
digest car oils, leaving little but carbon dioxide and water. Rainwater
infiltration is usually less than that of an impervious pavement with a
separate stormwater management facility somewhere downstream.
= Trees= Permeable pavements may give urban trees
the rooting space they need to grow to full size. A "structural-soil" pavement
base combines structural aggregate with soil; a porous surface admits vital air
and water to the rooting zone. This integrates healthy ecology and thriving
cities, with the living tree canopy above, the city's traffic on the ground,
and living tree roots below. The benefits of permeables on urban tree
growth have not been conclusively demonstrated and many researchers have
observed tree growth is not increased if construction practices compact materials
before permeable pavements are installed.
Disadvantages = Runoff volumes=
Permeable pavements are designed to replace Effective Impervious Areas, not
to manage stormwater from other impervious surfaces on site. Use of this
technique must be part of an overall on site management system for stormwater,
and is not a replacement for other techniques.
Also, in a large storm event, the water table below the porous pavement can rise
to a higher level preventing the precipitation from being absorbed into
the ground. The additional water is stored in the open graded crushed drain
rock base and remains until the subgrade can absorb the water. For clay-based
soils, or other low to 'non'-draining soils, it is important to increase the
depth of the crushed drain rock base to allow additional capacity for the water
as it waits to be infiltrated. The best way to prevent this problem is
to understand the soil infiltration rate, and design the pavement and base
depths to meet the volume of water. Or, allow for adequate rain water run off at
the pavement design stage. = Pollutant load=
Highly contaminated runoff can be generated by some land uses where
pollutant concentrations exceed those typically found in stormwater. These
"hot spots" include commercial nurseries, recycling facilities, fueling
stations, industrial storage, marinas, some outdoor loading facilities, public
works yards, hazardous materials generators, vehicle service and
maintenance areas, and vehicle and equipment washing and steam cleaning
facilities. Since porous pavement is an infiltration practice, it should not be
applied at stormwater hot spots due to the potential for groundwater
contamination. All contaminated runoff should be prevented from entering
municipal storm drain systems by using best management practices for the
specific industry or activity. = Weight and traffic volumes=
Reference sources differ on whether low or medium traffic volumes and weights
are appropriate for porous pavements. For example, around truck loading docks
and areas of high commercial traffic, porous pavement is sometimes cited as
being inappropriate. However, given the variability of products available, the
growing number of existing installations in North America and targeted research
by both manufacturers and user agencies, the range of accepted applications seems
to be expanding. Some concrete paver companies have developed products
specifically for industrial applications. Working examples exist at
fire halls, busy retail complex parking lots, and on public and private roads,
including intersections in parts of North America with quite severe winter
conditions. = Siting=
Permeable pavements may not be appropriate when land surrounding or
draining into the pavement exceeds a 20 percent slope, where pavement is down
slope from buildings or where foundations have piped drainage at their
footers. The key is to ensure that drainage from other parts of a site is
intercepted and dealt with separately rather than being directed onto
permeable surfaces. = Climate=
Cold climates may present special challenges. Road salt contains chlorides
that could migrate through the porous pavement into groundwater. Snow plow
blades could catch block edges and damage surfaces. Sand cannot be used for
snow and ice control on perveous asphalt or concrete because it will plug the
pores and reduce permeability. Infiltrating runoff may freeze below the
pavement, causing frost heave, though design modifications can reduce this
risk. These potential problems do not mean that porous pavement cannot be used
in cold climates. Porous pavement designed to reduce frost heave has been
used successfully in Norway. Furthermore, experience suggests that
rapid drainage below porous surfaces increases the rate of snow melt above.
= Cost= Some estimates put the cost of permeable
paving at two to three times that of conventional asphalt paving. Using
permeable paving, however, can reduce the cost of providing larger or more
stormwater BMPs on site, and these savings should be factored into any cost
analysis. In addition, the off-site environmental impact costs of not
reducing on-site stormwater volumes and pollution have historically been ignored
or assigned to other groups The City of Olympia, Washington is studying the use
of pervious concrete quite closely and finding that new stormwater regulations
are making it a viable alternative to storm water.
= Longevity and maintenance= Some permeable pavements require
frequent maintenance because grit or gravel can block the open pores. This is
commonly done by industrial vacuums that suck up all the sediment. If maintenance
is not carried out on a regular basis, the porous pavements can begin to
function more like impervious surfaces. With more advanced paving systems the
levels of maintenance needed can be greatly decreased, elastomerically bound
glass pavements requires less maintenance than regular concrete paving
as the glass bound pavement has 50% more void space.
Plastic grid systems, if selected and installed correctly, are becoming more
and more popular with local government maintenance personnel owing to the
reduction in maintenance efforts: reduced gravel migration and weed
suppression in public park settings. Some permeable paving products are prone
to damage from misuse, such as drivers who tear up patches of plastic & gravel
grid systems by "joy riding" on remote parking lots at night. The damage is not
difficult to repair but can look unsightly in the meantime. Grass pavers
require supplemental watering in the first year to establish the vegetation,
otherwise they may need to be re-seeded. Regional climate also means that most
grass applications will go dormant during the dry season. While brown
vegetation is only a matter of aesthetics, it can influence public
support for this type of permeable paving.
Traditional permeable concrete paving bricks tend to lose their color in
relatively short time which can be costly to replace or clean and is mainly
due to the problem of efflorescence. = Efflorescence=
Efflorescence is a hardened crystalline deposit of salts, which migrate from the
center of concrete or masonry pavers to the surface to form insoluble calcium
carbonates that harden on the surface. Given time, these deposits form much
like how a stalactite takes shape in a cave, except in this case on a flat
surface. Efflorescence usually appears white, gray or black depending on the
region. Over time efflorescence begins to
negatively affect the overall appearance of masonry/concrete and may cause the
surfaces to become slippery when exposed to moisture. If left unchecked, this
efflorescence will harden whereby the calcium/lime deposits begin to affect
the integrity of the cementatious surface by slowly eroding away the
cement paste and aggregate. In some cases it will also discolor stained or
coated surfaces. Efflorescence forms more quickly in
areas that are exposed to excessive amounts of moisture such as near pool
decks, spas, and fountains or where irrigation runoff is present. As a
result, these affected regions become very slick when wet thereby causing a
significant loss of "friction coefficient". This can be of serious
concern especially as a public safety issue to individuals, principals and
property owners by exposing them to possible injury and increased general
liability claims. Efflorescence remover chemicals can be
used to remove calcium/lime build-up without damaging the integrity of the
paving surface. Types
Installation of porous pavements is no more difficult than that of dense
pavements, but has different specifications and procedures which must
be strictly adhered to. Nine different families of porous paving materials
present distinctive advantages and disadvantages for specific applications.
Here are examples: = Pervious concrete=
Pervious concrete is widely available, can bear frequent traffic, and is
universally accessible. Pervious concrete quality depends on the
installer's knowledge and experience. = Plastic Grids=
Plastic grids allow for a 100% porous system using structural grid systems for
containing and stabilizing either gravel or turf. These grids come in a variety
of shapes and sizes depending on use; from pathways to commercial parking
lots. These systems have been used readily in Europe for over a decade, but
are gaining popularity in North America due to requirements by government for
many projects to meet LEED environmental building standards. Plastic grid system
are also popular with homeowners due to their lower cost to install, ease of
installation, and versatility. The ideal design for this type of grid system is a
closed cell system, which prevents gravelturf from migrating laterally. It
is also known as Grass pavers / Turf Pavers in India
= Porous asphalt= Porous asphalt is produced and placed
using the same methods as conventional asphalt concrete; it differs in that
fine aggregates are omitted from the asphalt mixture. The remaining large,
single-sized aggregate particles leave open voids that give the material its
porosity and permeability. To ensure pavement strength, fiber may be added to
the mix or a polymer-modified asphalt binder may be used. Generally, porous
asphalt pavements are designed with a subsurface reservoir that holds water
that passes through the pavement, allowing it to evaporate and/or
percolate slowly into the surround soils.
Open-graded friction courses are a porous asphalt surface course used on
highways to improve driving safety by removing water from the surface. Unlike
a full-depth porous asphalt pavement, OGFCs do not drain water to the base of
a pavement. Instead, they allow water to infiltrate the top 3/4 to 1.5 inch of
the pavement and then drain out to the side of the roadway. This can improve
the friction characteristics of the road and reducing road spray.
= Single-sized aggregate= Single-sized aggregate without any
binder, e.g. loose gravel, stone-chippings, is another alternative.
Although it can only be safely used in very low-speed, low-traffic settings,
e.g. car-parks and drives, its potential cumulative area is great.
= Porous turf= Porous turf, if properly constructed,
can be used for occasional parking like that at churches and stadia. Plastic
turf reinforcing grids can be used to support the increased load. Living turf
transpires water, actively counteracting the "heat island" with what appears to
be a green open lawn. = Permeable interlocking concrete
pavements= Permeable interlocking concrete
pavements are concrete units with open, permeable spaces between the units. They
give an architectural appearance, and can bear both light and heavy traffic,
particularly interlocking concrete pavers, excepting high-volume or
high-speed roads. Some products are polymer-coated and have an entirely
porous face. = Permeable clay brick pavements=
Permeable clay brick pavements are fired clay brick units with open, permeable
spaces between the units. Clay pavers provide a durable surface that allows
stormwater runoff to permeate through the joints.
= Resin bound paving= Resin bound paving is a mixture of resin
binder and aggregate. Clear resin is used to fully coat each aggregate
particle before laying. Enough resin is used to allow each aggregate particle to
adhere to one another and to the base yet leave voids for water to permeate
through. Resin bound paving provides a strong and durable surface that is
suitable for pedestrian and vehicular traffic in applications such as
pathways, driveways, car parks and access roads.
= Bound recycled glass porous pavement= Elastomerically bound recycled glass
porous pavement consisting of bonding processed post consumer glass with a
mixture of resins, pigments, granite and binding agents. Approximately 75 percent
of glass in the U.S. is disposed in landfills.
See also Stormwater management practices related
to roadways: Bioretention
Bioswale Notes
References External links
Avoiding Stormwater Running and the Rain Tax Nitterhouse Masonry
PaveShare - Permeable Paving Resources Technical Note 14D - Permeable Clay
Brick Pavements - Brick Industry Association
"Permable Paving & SUDS" - Precast Concrete Paving & Kerb Assn.
Sustainable Drainage: A Review of Published Material on the Performance of
Various SUDS Components - Construction Industry Research & Information Assn.
Pervious Concrete Blog - Matt Offenberg "Pervious Concrete Pavement: An
Overview" - National Ready Mixed Concrete Assn.
Graphic showing water interacting with permeable paving
"Toolbase: Permable Pavement" - National Assn. of Homebuilders
Aquastone - Porous paving technical data