HomeMy Public PortalAboutPermeable Paving for Commercial ProjectsMASSACHUSETTS LOW IMPACT DEVELOPMENT TOOLKIT
FACT SHEET #6
PERMEABLE PAVING
Overview
Since impervious pavement is the primary source of stormwater runoff,
Low Impact Development strategies recommend permeable paving for
parking areas and other hard surfaces. Permeable paving allows rainwater
to percolate through the paving and into the ground before it runs off.
This approach reduces stormwater runoff volumes and minimizes the
pollutants introduced into stormwater runoff from parking areas.
All permeable paving systems consist of a durable, load bearing, pervious surface overlying
a crushed stone base that stores rainwater before it infiltrates into the underlying soil.
Permeable paving techniques include porous asphalt, pervious concrete, paving stones, and
manufactured "grass pavers" made of concrete or plastic. Permeable paving may be used for
walkways, patios, plazas, driveways, parking stalls, and overflow parking areas.
` Applications and Design Principles
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a Permeable paving is appropriate for pedestrian -only areas and for very low -volume, low -speed areas
such as overflow parking areas, residential driveways, alleys, and parking stalls. It can be constructed
where the underlying soils have a permeability of at least 0.3" per
hour. Permeable paving is an
excellent technique for dense urban areas because it does not
require any additional land. With proper design, cold climates are not a
major limitation; porous pavement has been used successfully in Norway,
Management Objectives
incorporating design features to reduce frost heave.
13 Reduce stormwater runoff volume
Permeable paving is not ideal for high traffic/high speed areas because it has
from paved surfaces
lower load -bearing capacity than conventional pavement. Nor should it be
"hotspots"
° Reduce peak discharge rates.
used on stormwater with high pollutant loads because stormwater
cannot be pretreated prior to infiltration. Heavy winter sanding may clog
o Increase recharge through
joints and void spaces.
infiltration.
o Reduce pollutant transport through
direct infiltration.
Metropolitan Area
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o Improve site landscaping benefits
(grass only.)
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} Three Major Types of Permeable Paving
•. = , a Porous asphalt and pervious concrete appear to be the same as traditional asphalt or
r=i , concrete pavement. However, they are mixed with a very low content of fine sand, so
ate!? that they have 10%-25% void space and a runoff coefficient that is almost zero.
o Paving stones (aka unit pavers) are impermeable blocks made of brick, stone, or
concrete, set on a prepared sand base. The joints between the blocks are filled with
sand or stone dust to allow water to percolate downward. Runoff coefficients range
from 0.1— 0.7, depending on rainfall intensity, joint width, and materials. Some
concrete paving stones have an open cell design to increase permeability.
Cover: A driveway in Connecticut
built with manufactured paving
stones. Lower photo shows paving
stone detail. Photo: University of
Connecticut, Jordan Cove Urban
Monitoring Project
Above: A parking lot with concrete
grass paver parking stalls. Lower
photo shows grass paver detail.
Photos: Lower Columbia River
Estuary Partnership
Right: A schematic cross section of
permeable paving. In some
applications, the crushed stone
reservoir below the paving is
designed to store and infiltrate
rooftop runoff as well. Image: Cahill
Associates, Inc. 2004
a Grass pavers (aka turf blocks) are a type of open -cell unit paver in which the cells
are filled with soil and planted with turf. The pavers, made of concrete or synthetic,
distribute the weight of traffic and prevent compression of the underlying soil.
Runoff coefficients are similar to grass, 0.15 to 0.6.
Each of these techniques is constructed over a base course that doubles as a reservoir
for the stormwater before it infiltrates into the subsoil. The reservoir should consist
of uniformly -sized crushed stone, with a depth sufficient to store all of the rainfall
from the design storm. The bottom of the stone reservoir should be completely flat
so that infiltrated runoff will be able to infiltrate through the entire surface. Some
designs incorporate an "overflow edge," which is a trench surrounding the edge of
the pavement. The trench connects to the stone reservoir below the surface of the
pavement and acts as a backup in case the surface clogs.
Benefits and Effectiveness
Porous pavement provides groundwater recharge and reduces stormwater runoff
volume. Depending on design, paving material, soil type, and rainfall, permeable
paving can infiltrate as much as 70% to 80% of annual rainfall.
Porous pavement can reduce peak discharge rates significantly by diverting
stormwater into the ground and away from the pipe -and -pond stormwater
management system.
Design Guidelines for
Porous Asphalt with
Subsurface Infiltration
UNCOMPACTED
SUBGRADEIS
CRITICAL FOR PROPER
INFILTRATION
FILTER FABRIC
LINES THE
1(0 nwunswaares SUBSURFACE BEDim
Above: A parking lot with
o Grass pavers can improve site appearance by providing vegetation where there
conventional asphalt aisles and
would otherwise be only pavement.
paving stone parking stalls. Paving
stones are most appropriate For
o porous paving increases effective developable area on a site because portions of
P g P
low -speed, low traffic areas. Photo:
the stormwater management system are located underneath the paved areas, and
Lower Columbia River Estuary
the infiltration provided by permeable paving can significantly reduce the need
Partnership
for large stormwater management structures on a site.
Limitations
o Permeable paving can be prone to clogging from sand and fine sediments that fill
void spaces and the joints between pavers. As a result, it should be used carefully
where frequent winter sanding is necessary because the sand may clog the surface
of the material. Periodic maintenance is critical, and surfaces should be cleaned
with a vacuum sweeper at least three times per year.
o In cold climates, the potential for frost heave may be a concern for the use of
permeable paving. Some design manuals recommend excavating the base course
to below the frost line, but this may not be necessary in rapidly permeable soils.
In addition, the dead air and void spaces in the base course provide insulation so
that the frost line is closer to the surface.
o Permeable paving should not receive stormwater from other drainage areas,
especially any areas that are not fully stabilized.
o Permeable paving can only be used on gentle slopes (<5%); it cannot be used in
high -traffic areas or where it will be subject to heavy axle loads.
o Snow plows can catch the edge of grass pavers and some paving stones. Rollers
should be attached to the bottom edge of a snowplow to prevent this problem.
Above: A handicap -accessible park pathway made of
permeable paving stones.
Photo: GeoSyntec Consultants, Inc.
Maintenance
n Post signs identifying porous pavement areas.
o Minimize use of salt or sand during winter months
n Keep landscaped areas well -maintained and prevent soil
from being transported onto the pavement.
n Clean the surface using vacuum sweeping machines.
For paving stones, periodically add joint material (sand)
to replace material that has been transported.
a Monitor regularly to ensure that the paving surface
drains properly after storms.
n Do not reseal or repave with impermeable materials.
n Inspect the surface annually for deterioration.
o Grass pavers may require periodic reseeding to fill in
bare spots.
Design Details
o For all permeable paving, base course is a reservoir
layer of 1"-2" crushed stone; depth to be determined by
storage required and frost penetration.
o Permeable paving require a single -size grading of base
material in order to provide voids for rainwater storage;
choice of materials is a compromise between stiffness,
permeability, and storage capacity. Use angular crushed
rock material with a high surface friction to prevent
traffic compaction and rutting.
o The design may also include a 2" thick filter course of
0.5" crushed stone, applied over the base course. A
geotextile fabric may be laid at the top of the filter layer
to trap sediment and pollutants.
o For grass pavers, use deep-rooted grass species whose
roots can penetrate the reservoir base course. Irrigation
may be required but should be infrequent soakings so
that the turf develops deep root systems. Grass pavers
are not suitable for every day, all day parking because
the grass will get insufficient sunlight. Better for use as
occasional overflow parking.
o The introduction of dirt or sand onto the paving surface,
whether transported by runoff from elsewhere or carried by
vehicles, will contribute to premature clogging and failure
of the paving. Consequently, permeable paving should
be constructed as one of the last items to be built on a
development site, after most heavy construction vehicles are
finished and after the majority of the landscaping work is
completed.
Cost
On most sites, permeable paving costs more than
conventional asphalt or cement paving techniques. In the
case of porous asphalt and pervious concrete, construction
costs may be 50% more than conventional asphalt and
concrete. Construction costs of paving stones and
grass pavers varies considerably and will depend on the
application. As with any site improvement or stormwater
management structure, property owners should provide a
budget for maintenance of permeable paving, at an annual
rate of 1%-2% of construction costs.
Permeable paving reduces the need for stormwater
conveyances and treatment structures, resulting in cost
savings elsewhere. Permeable paving also reduces the
amount of land needed for stormwater management and
may satisfy requirements for greenspace, allowing more
development on a site.
Local Case Study
West Farms Mall — West Hartford, CT
Grass pavers were installed at the West Farms Mall off of I-84
at exit 40, to handle peak -season overflow parking associated
with a mall expansion. Over four acres of reinforced turf was
designed to accommodate 700 spaces of overflow parking for
the peak shopping seasons. There are a few drains installed
in the reinforced turf but are only used during very heavy
storms. Because the reinforced turf works so well the existing
storm drainage system did not have to be enlarged for the
additional parking. The overflow parking area needs to be
mowed on a regular basis and treated like a regular lawn. The
area also needs to be plowed as any parking would be. Rollers
were fit to the bottom of the snow plow so the reinforced
turf would not be damaged. The manager of the Westfarms
facility is satisfied with the turf.
Websites
www.unh.edu/erg/cstev/index.htm
www.invisiblestructures.com/GP2/whole—lotof turf.htm
www.uni-groupusa.org/case.htm
www.nemo.uconn.edu/
www.lowimpactdevelopment.org/epa03/pavespec.htm
www.epa.gov/ednnrmrl/repository/abstrac2/abstra2.htm
www.forester.net/sw-0503—advances.html
This publication is one component of the Massachusetts Low Impact Development Toolkit, a production of the Metropolitan Area
Planning Council, in coordination with the I-495 MetroWest Corridor Partnership, with financial support from US EPA.
The Massachusetts Low Impact Development Interagency Working Group also provided valuable input and feedback on the LID Toolkit.
FOR MORE INFORMATION, VISIT: WWW.MAPC.ORG/LID AND WWW.ARC-OF-INNOVATION.ORG.