Concrete block pavements have been available for many years and have been used primarily as aesthetic
treatments to parking areas and low volume roadways. In the last 20 years, high-density plastic grids
have also entered the market place. There are many configurations and applications that have been
developed for each of these materials. Most of the systems are supported by a stone base that
has large pore spaces, such as AASHTO #57 Stone. This base acts both as pavement support and as a
reservoir to store water so that it can be infiltrated, if the soil conditions allow, or detained and
slowly released to the storm drain system. Supplemental storage facilities, such as underground
vaults or drainage blankets, can be used in conjunction with these systems. Each pavement type is
generally described below.
Porous Concrete: This pavement has stable air pockets encased within it that allow
water to drain uniformly through into the ground below, where it can be naturally filtered. The
material becomes stronger and more stable when it gets wet and so does not deteriorate as fast
as other paving materials. Its use should be restricted to parking lots and local roads since
it supports lighter loads than standard concrete. Since it is cement based, it will not release
harmful chemicals into the environment such as with oil-based asphalt. It has been in use
throughout Europe for about the last fifty years, and a domestic formula known as the Portland
Cement Pervious Pavement has been used successfully since the 1970s in the U.S., particularly in
Florida. The pavement is a special blend of Portland cement, sand-free coarse aggregate rock,
and water.
Grass Pavers: Plastic rings in a flexible grid system are placed on a base of blended
sand, gravel and topsoil, then filled with a topsoil such as sandy loam and planted with
vegetation. This pavement gives designers a turfgrass alternative to asphalt or concrete
for such low-traffic areas as firelanes, overflow and event parking, golf cart paths,
residential driveways, and maintenance and utility access lanes. The support base and the
rings’ walls prevent soil compaction and reduce rutting and erosion by supporting the weight
of traffic and concentrated loads, while the large void spaces in the rings allow a strong root
network to develop. The end result is a load-bearing surface covered with natural grass and
which is typically around 90% pervious, allowing for stormwater pollution filtration and
treatment. Ancillary benefits include airborne dust capture and reductions in the urban heat
island effect. Most manufacturers also produce the paver’s rings from post-consumer recycled
plastic materials.
Gravel Pavers: This pavement option is intended for high frequency, low speed traffic
areas. The same ring structure as with the grass paver is used, but the voids in the rings
are filled with gravel in order to provide greater load bearing support for unlimited traffic
volumes and/or parking durations. Manufacturers provide specifications on the sieve analysis
that should be used to generate the clean gravel fill for the rings, and a geotextile fabric
is used to prevent the gravel infill from migrating to the soil subbase. Gravel pavers can be
used for automobile and truck storage yards, high-throughput parking lots, service and access
areas, loading docks, boat ramps, and outdoor bulk storage areas.
Interlocking Concrete Paving Blocks: The unique shape of these interlocking precast
units leaves drainage openings that typically comprise approximately 10% of the paver’s surface
area. When properly filled with permeable material, the voids allow for drainage of stormwater
through the pavement surface into the layers below. The system is a highly durable, yet
permeable pavement capable of supporting heavier vehicular loads than grass or gravel pavers
and offering the most flexibility in widespread application. Interlocking concrete paving
blocks are resistant to heavy loads, easy to repair, require little maintenance, and are of
high quality. These systems also have the highest materials and construction costs.
Manufacturers should have detailed design and construction specifications available.
For an excellent web-based course on hydraulic and structural design of permeable pavers, see
North Carolina State University Cooperative Extension's Continuing Education
Course on Permeable
Pavements developed by Bill Hunt in the Department of Biological & Agricultural Engineering.
Site
Permeable pavers have the potential to be used for a wide range of applications including
park-and-ride facilities, low volume roads, parking lots, and walkways. The most successful
installation of alternative pavements has been stated to be in coastal areas with sandy soils
and flatter slopes.1 In general, the use of permeable pavers requires:
low traffic volume
sandy or loamy sand in-situ soil (Soils that contain significant levels of silt or clay
or that are highly compressible, lack cohesion, or expand or contract with moisture may not be
feasible for permeable pavers without the use of geotextiles to provide support. A detailed
analysis of the soils and feasibility should be conducted when these conditions are
encountered.)
a seasonally high water table at least 3 to 4 feet from the surface (Water tables
approaching the surface will prevent the water from exfiltrating and can cause structural
damage to the system through freeze/frost and floatation processes.)
minimal upstream disturbance (This will prevent clogging of the system, which can
significantly shorten the pavers lifetime. Permeable pavers should not be used to treat
runoff from large, sparsely vegetated upland areas or areas prone to wind erosion. Sediment
control measures should also be carefully followed when upland construction activities take
place, and for the longest system lifetime, active street sweeping programs should be employed
in the contributing area.)
Load and Gravel Base
When designing the chosen site with permeable pavers, several calculations and considerations must
be made. First, the load requirements over the site’s expected lifetime must be determined based
on the typical vehicle weight, the typical number of vehicle passes per day, and the design life.
Most permeable pavement applications will have life spans ranging from 10 to 20 years, with a
conservative design life of 10 years recommended as a general rule.2 The designer must
ensure that over this lifetime, the permeable paver system is strong enough to support the applied
traffic. Part of this ability will depend on the inherent strength of the actual paver chosen.
Generally the design strength of grass and gravel pavers is listed near 5700 psi, interlocking
concrete paving blocks are typically designed to meet a minimum of 8000 psi, and porous concrete
supports from approximately 1800 to 2400 psi. The other component contributing to system support
is the underlying soil strength. Highly permeable soils, such as sands and sandy loams, have the
best ability to carry loads. Depending on the paver chosen and the underlying soils, the depth
of the system’s final gravel base can then be calculated in order to ensure that load requirements
are met.
For example, in some locations, a gravel layer may not be needed at all for infrequent car and
pick-up truck access, while a layer of approximately 4 to 6 inches depth may be required for
infrequent fire truck access. For interlocking concrete paving blocks, certain manufacturers
recommend a minimum base of 4 inches for pedestrian applications over well-drained soils and 8 to
10 inches for residential streets. In locations with numerous freeze-thaw cycles, weak soils or an
extremely cold climate, a thicker base should be used. The stone and gravel base layer not only
provides support but also acts as a storage reservoir to achieve extra detention and serves as a
buffer from frost problems.
Infiltration and Water Release
Regional environmental factors, such as the amount and frequency of rainfall and the local soil’s
permeability, will determine the ability of the paver system to pass stormwater easily through its
top layers and then store and release the water in a timely manner into the underlying soil.
Whether or not runoff will be generated from the paver for a given storm will depend on the paver’s
ratio of open to impervious spaces, the storm’s precipitation rate, the surface’s slope, and the
storage capacity in the base layer below. The depth of this storage layer is dictated by the
structural considerations discussed above, while the void space in the layer is a function of the
stone fill. The system should be designed to infiltrate the design storm and then complete release
of the water within at least 48 hours (24 hours is recommended). If the in-situ soil does not
allow for release within 48 hours, the site is not suitable for the use of permeable pavers.
Possible modifications to the system, however, include the use of an overflow drainage pipe for
low permeability subgrades and / or for storms exceeding the design storm. Systems can also be
designed to drain water away from the pavement to more pervious layers that can accommodate the
inflow, to storage areas that allow for slow infiltration, or to a pipe for discharge as filtered
stormwater. In situations with a discharge pipe, infiltration does not occur, but the system is
used to enhance storage, reduce peak runoff rates and filter pollutants.
