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Porous asphalt does not necessarily require
additives or proprietary ingredients, although polymers and/or fibers can be
used to prevent draindown and to improve durability and shear strength.
Constructing a permeable surface does not require the contractor to have
special paving equipment or skills. With the proper information, most
asphalt plants can easily prepare the mix and most paving contractors can
install it.
This article will discuss the background and costs
of porous pavement, cite examples of successful installations, explain how
it works, and explore design considerations.
Background
First developed in the 1970s at the Franklin
Institute in Philadelphia, porous asphalt pavement consists of standard
bituminous asphalt in which the aggregate fines (particles smaller than 600
um, or the No. 30 sieve) have been screened and reduced, allowing water to
pass through the asphalt. Underneath the pavement is placed a bed of
uniformly graded, clean-washed aggregate with a void space of 40%. Storm
water drains through the asphalt, is held in the stone bed, and infiltrates
slowly into the underlying soil mantle. A layer of geotextile filter fabric
separates the stone bed from the underlying soil, preventing the movement of
fines into the bed.
Porous pavement is especially well suited for
parking lot areas. Several dozen large, successful porous pavement
installations, including some that are now 20 years old, have been developed
by Cahill Associates of West Chester, Pennsylvania, mainly in Mid-Atlantic
states. These systems continue to work quite well as both parking lots and
storm water management systems. In fact, many of these systems have
outperformed their conventionally paved counterparts in terms of both
parking lot durability and storm water management.
Cost
Porous pavement does not usually cost more than
conventional pavement. On a yard-by-yard basis, the asphalt cost is
approximately the same as the cost of conventional asphalt. The underlying
stone bed is usually more expensive than a conventional compacted subbase,
but this cost difference is generally offset by the significant reduction in
storm water pipes and inlets. Additionally, because porous pavement is
designed to fit into the topography of a site, there is generally less
earthwork and are no deep excavations.
When the cost savings provided by eliminating the
detention basin are considered, porous pavement is generally an economically
sound choice. On those jobs where unit costs have been compared, the porous
pavement has generally been the less expensive option. Current jobs are
averaging between $2,000 and $2,500 per parking space for parking, aisles,
and storm water management.
A recent installation at the University of North
Carolina in Chapel Hill included parking lots where some sections were
constructed from porous asphalt and some sections used porous concrete. The
cost differential was approximately 4:1 — that is, the porous concrete
pavement cost four times as much as the porous asphalt pavement. All other
installations cited in this article are asphalt pavements.
Installations old and new
One of the first large-scale porous
pavement/recharge bed systems that CA designed is located in a corporate
office park in the suburbs of Philadelphia (East Whiteland Township, Chester
County). This particular installation of about 600 parking spaces posed a
challenge because of both the sloping topography and the underlying
carbonate geology that was prone to sinkhole formation. The site also is
immediately adjacent to Valley Creek, designated by Pennsylvania as an
Exceptional Value stream where avoiding nonpoint source pollution is of
critical importance.
Constructed in 1983 as part of the Shared Medical
Systems (now Seimens) world headquarters, the system consists of a series of
porous pavement/recharge bed parking bays terraced down the hillside
connected by conventionally paved impervious roadways. Both the top and
bottom of the beds are level, hillside notwithstanding. After 20 years, the
system continues to function well and has not been repaved. Other early
1980s sites, such as the SmithKline Beecham (now Quest) Laboratory in
Montgomery County, Pennsylvania, and the Chester County Work Release Center
in Chester County, Pennsylvania, also used the system of terracing the
porous paved recharge beds down the hillside to overcome the issues of
slope. At the DuPont Barley Mills office complex in Delaware, the porous
pavement was constructed specifically to avoid the construction of a
detention basin, which would have destroyed the last wooded portion of the
site.
More recently (1999), the porous parking lots at the
Pennsylvania State Berks Campus were constructed to avoid destroying a
wooded campus hillside. The Pennsylvania State Berks lots, also on
carbonate bedrock, replaced an existing detention basin and have not
experienced the sinkhole problems that another campus detention basin has
suffered.
How it works
The porous asphalt mix has a lower concentration of
fines than traditional asphalt, accomplished by straightforward screening.
In all other manufacturing aspects, porous asphalt is the same as
conventional asphalt and can be mixed at a standard asphalt plant. With
fewer fines, the asphalt is porous and allows water to drain though the
material through virtually imperceptible openings. (To the untrained eye,
porous pavement properly prepared is difficult to distinguish from
conventional pavement.) There are several variations of the mix, including
gradations developed by various state transportation departments seeking a
pavement that also can be used to reduce noise and skidding.
The underlying stone recharge bed consists of a
uniformly graded (i.e., screened) 1.5- to 2.5-inch clean-washed stone mix,
such as an AASHTO No. 3. Depending on local aggregate availability, both
larger and smaller size stones have been used. The important requirement is
that the stone be uniformly graded (to maximize void space) and clean
washed. The void space between the stones provides the critical storage
volume for the storm water.
The stone bed is usually between 18- and 36-inches
deep, depending on storm water storage requirements, frost depth
considerations, and site grading. This depth provides a significant
structural base for the pavement. As a result, porous asphalt exhibits very
few of the cracking and pothole formation problems encountered in
conventional pavement.
The bottom of the recharge bed is excavated to a
level surface and is not compacted. This allows water to distribute and
infiltrate evenly over the entire bed bottom area. Compaction of the soils
will prevent infiltration, so it is important that care be taken during
excavation to prevent this. The bottom of the bed cannot be placed on fill
material unless that fill material is stone. A layer of non-woven geotextile
at the bottom of the bed allows the water to drain into the soil while
preventing the soil particles from moving into the stone bed.
Very often, the underlying stone bed can also
provide storm water management for adjacent impervious areas such as roofs
and roads. To achieve this, we convey the storm water directly into the
stone bed and then use perforated pipes in the stone bed to distribute the
water evenly.
Design considerations
In the late 1970s and early 1980s, as we designed
our first systems, we were uncertain how well the porous asphalt would hold
up over time and use. In these first systems, we designed the parking spaces
with porous pavement but constructed the aisles and connector roadways with
conventional asphalt. We extended the stone storm water storage/infiltration
bed under the entire parking area, however, including the areas with
impervious paving.
Over time, we have found that the porous asphalt
material has held up as well as, or better than, the conventional asphalt,
largely due to the solid subbase provided by the stone storage/infiltration
bed. In subsequent designs, we have paved the entire surface in the porous
asphalt. We have found that sufficient asphalt content is essential to
pavement durability (5.75 to 6.0% bituminous asphalt by weight).
We have also taken the belt-and-suspenders approach
to all of our systems. If the pavement were to be paved over, forgotten, or
clogged, storm water still must reach the stone bed below the pavement.
Often, we have used an unpaved stone edge for this purpose. We have also
used catch basins that discharge to perforated pipes in the bed.
Additionally, in case the bed bottom clogs (which
has not happened yet), we have always designed the underlying bed systems
with a positive overflow. During a storm event, as the water in the
underlying stone bed rises, it must never be allowed to saturate the
pavement. We have used a catch basin with a higher outlet than inlet to
provide positive release. In this way, the bed also serves as an underground
detention basin, eliminating the need for a separate basin.
The storm water component of the system should be
designed by an engineer proficient in hydrology and storm water design.
Essentially, the bed acts as an underground detention basin in extreme storm
events, albeit one that also reduces volume. A storm can be routed through
the bed using the same calculation methods employed to route detention
basins to confirm peak rate mitigation.
As a final design consideration, infiltration
systems also work best when the water is allowed to infiltrate over a large
area. We usually use a rule of thumb and design to a ratio of 5:1 impervious
area to infiltration area. That is, the runoff from five acres of impervious
area would require a one-acre infiltration bed. Because parking tends to
consume so much of our landscape relative to other impervious surfaces,
meeting this ratio is rarely a problem.
Deicing and Freezing Issues
One of the most common questions relates to concerns
about freezing conditions. Freezing has not been an issue, even in very cold
climates. We were quite surprised when the owners of early installations
first told us that there was less need to snowplow on the porous pavement
surfaces. The water drains through the pavement and into the bed below with
sufficient void space to prevent any heaving or damage, and the formation of
black ice is rarely observed. The porous surfaces tend to provide better
traction for both pedestrians and vehicles than conventional pavement. Not a
single system has suffered freezing problems.
Obviously, the use of sand or gravel for deicing
would be detrimental to the porous surface. Salt may be used, however, and
the surface may be plowed if needed, though the plow blade should be
elevated 1- to 2-inches so as to not scrape the porous surface. Most sites
have found that light plowing eliminates the need for salt since the
remaining snow quickly drains through the asphalt. This has the added
benefit of reducing groundwater and soil contamination from deicing salts.
Thomas Cahill is President, Michele Adams is
Principal Engineer, and Courtney Marm is a Planner, all at
Cahill
Associates.
Reprinted from Better Roads Magazine
November 2004 |
