Pervious Paving Alternatives

Storm Water & Tree Asset Management

July 2001

by Chere Peterson

Development and Drainage
     Governmental Intervention

Permeable Paving Arrives
     Plastic Paves the Way
      Pervious Concrete

Best Management Practices

Exploring the Best of the Alternatives
     Grasspave2
      Gravelpave
      Slopetame, Draincore and Rainstore
      Beachrings

Asphalt as Permeable Paving

Gravel as Permeable Paving

Development and Drainage

Drainage. Before development it was not much of a problem, except when river banks overflowed, hurricanes brought major tidal surges, or heavy rains hit areas barren of vegetation. For the most part, the ground simply absorbed the rains that fell. In the twentieth century, urban America began to sprawl over the countryside. Even then, paving didn't really become a national issue - until the Eisenhower years and the federal highway paving program.

Suddenly, in the fifties and the sixties, concrete became king and asphalt, landed gentry. Grass lanes and gravel parking lots disappeared, having been banished as untidy and outmoded, replaced by asphalt and concrete. Huge shopping malls sprouted in the middle of former farmland, surrounded by an inky sea of asphalt. In the summertime, cool, green farm land vegetation had been replaced by a giant, black heat-sink, radiating waves of heat visible to the human eye.

When rains fell, they washed the oil, grease, and rubber of a thousand, thousand vehicles, fast food detritus and the leavings of scavenger fowls who feed on it, into our storm drains, and into our streams and rivers, untreated. Subdivisions proliferated around these suburban commercial areas of shopping malls, car dealerships, vast apartment complexes, cinemas, office "parks," and fast food franchises. The green countryside turned urban black, and businesses and residences cranked up the air conditioning to compensate, literally, for the greater heat generated by asphalt.

Flooding was inevitable. Clear-cutting trees to make room for urban sprawl, and covering the earth with impervious surfaces, left heavy rainfall nothing to do, but collect and back up into streets and yards, making driving dangerous, if not impossible, in numerous sections of the cities and their adjoining suburbs. Tunnels and low-lying streets became urban arroyos, subject to flash flooding.

Tree canopies dwindled, but some good-intentioned, early developers planted new trees in monolithic black parking lots only to see them fail to thrive, or starve and die, because their roots could not get air and water. Wherever asphalt was placed up against mature trees, the fight was on: roots broke through the asphalt constantly in search of air and water, the owner or municipality asphalt-patched over (or poured concrete) - only to have repaired surfaces torn up by a tree determined to survive. In many cases, the axe gave urbanization one more victory over the environment. That's pretty much how it was through the seventies and eighties, a fight between development and the environment.

While not a new idea, sometime in the eighties, a collective thought occurred to planners, residents, officials and developers that environmentally sensitive development not only made sense, it solved some of the problems created in the preceding decades by unconstrained development. And, it presented assurances that our children would experience the same quality of life we expect, and hopefully better, because of choices we make today. If paving is too important to stop doing, why not make it work for us, by making it environmentally friendly? The idea of pervious paving began its slow, but sure, journey to institutionalization.

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Governmental Intervention

Rules seldom appear, the old saw goes, until there is a glaring lapse of common sense, followed by a loss of control, which hurts people and/or their property. Reaction to these offenses becomes codified as a rule, regulation or a law. So it was with the growing problems of polluted bodies of water and urban flooding. The running joke (please pardon the pun) in the seventies was that the City of New Orleans would have drinking water as long as the folks up in the Twin Cities in Minnesota and all those downstream from them would keep flushing their toilets. Art is said to imitate life. Humor laughs at it. That was the first area of federal regulations to deal with water pollution, to regulate water treatment of effluent. Today that joke is neither funny, nor appropriate.

Moving from regulating effluent to regulating storm water runoff was not a quantum leap, when it was discovered that even when effluent was treated to acceptable levels, our rivers, lakes and streams were still being polluted. Non-point source pollution - storm water runoff is what municipalities are trying to cope with today.

The availability of water diminished as the need for it increased with urban growth. Savannah's Historic District was built around 26 squares, a plan with which the city's founder General Oglethorpe was credited. Each of those squares (twenty-three of which are still beautifully intact) sported an artesian well spouting water two to three feet in the air as late as the nineteenth century. The end of the twentieth century has seen artesian wells across the continent disappear, like those of Savannah's historic squares. Potable water tables have dropped, in some places of the country, as much as several hundred feet. Surface water has become the most critical source of drinking water, and we need to protect it from pollutants.

Storm water runoff became the focus of regulators. Words like detention, absorption rates, impervious, footprints, non-point source pollution, tree ordinances, and flood mitigation all related to the amount of impervious surfaces a development had and the expensive remedies to counter their effect on the environment. These regulations and laws were argued against vehemently on the basis that they could be construed as a legal "taking" of their property's usefulness in development. How many parking spaces would be lost to a detention pond? What is more important - a few 50 year old hardwoods - or the ability to develop land without constraints? The tug of war continued between the environment and development, until acceptable alternative means of permeable paving were developed.

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Permeable Paving Arrives

Plastic Paves the Way

In the early sixties, the Army Corps of Engineers developed a kind of collapsible gill net system of low impact plastic on a roll, for the purpose of providing temporary landing places for tanks on beach heads. It was called GeoWeb. Still sold today, its manufacturer Presto Company indicates in its specifications that its use design is for "infrequent passes" of passenger vehicles.  GeoWeb was the precursor to a stable, functional system of high impact plastic rings on a flexible grid, manufactured in rolls: these re-engineered rings can be used any place there is a need for a pervious surface to handle high volume, low speed traffic. While GeoWeb is still a functional, inexpensive way to temporarily deal with erosion, in a soil stabilization function, it is limited by the fact that it is low density plastic (LDPE), with an Environmental Stress Crack Resistance of only 3000 hours, 30% of the durability of a High Density plastic (HDPE).  The fact that low impact plastic degrades rapidly is undoubtedly the reason it is specified for infrequent, low speed travel - and is against the law to use in fire lanes or around fire stations in cities like San Diego, California. TerraCell, one of GeoWeb's cheaper knock-offs, is specified with only an "Environmental Stress Crack Resistance of 2000 hours," meaning early degradation is 500% more likely to occur than for a High Density (HDPE) plastic like Invisible Structure's Gravelpave2.  GeoWeb's other cousins are interlocking rigid, high density plastic mats, approximately one meter by half meter. These, too, are specified by their manufacturer for only infrequent passes of low speed passenger vehicle wear. Pop-up problems are inherent in this design with any amount of travel or frost problems, they are labor intensive to install, and they require a separate non-woven fabric installation first.  As an urban tree surround they are perfectly acceptable as long as the area is relatively traffic free.

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Pervious Concrete

Also in the early eighties, Florida, where the water table is critically high and flooding has become almost a way of life ( because of the Sunbelt growth phenomenon), began to use another, somewhat more familiar, porous paving alternative, pervious concrete. Pervious concrete is basically a sand-free, low water mix with #89 stone (1/4 to 3/8 of an inch size gravel), instead of the #56 or #67 stones (3/4" or 1/2" respectively), and, of course, Portland cement. Initially, fly ash was considered an acceptable filler or additive (pozzolith). As a filler, fly ash added little, if anything, to pervious concrete. Extensive engineering tests proved the mix to be stronger and more reliable if the fly ash was replaced by additional Portland cement. Simply stated, pervious concrete is predicated on the fact that adhesion supercedes cohesion to the point where stable air pockets are encased within, allowing water to drain through into the ground below it. It can be installed directly on a well-draining (sandy) sub-base compacted to a minimum of 92% Modified Proctor (to a maximum of 95%).  For extra detention, it is installed as a system of four to eight inches of #67 stone base (or in combination with #56 stone), with six to eight inches of pervious concrete installed over it.  Over clay sub-surfaces, an additional augmentation of 4 to 8 inches of compacted sand, beneath the stones, enhances permeability, detention, and primary filtration properties.  Non-woven fabric is often specified to be installed beneath stone layer, where slope exceeds 2 or 3%, to avoid sheeting during a heavy storm event.

In areas where flatwork installers, attempting to install Portland Cement Pervious Paving for clients as On the Job Training, using conventional untrained crews, the results have been nothing short of disastrous.  Regulators, responding to this phenomenon offer conditional permitting, contingent upon the selection of experienced Portland Cement Pervious crews, specify that every 3000 square feet of commercial installation be tested; if tests indicate failure to meet the specifications, the owner is required to remove and replace the installation.  More knowledgeable engineers and architects require their approval of the contractor/installers to be employed on their projects as a quality assurance measure.  A good installation has the look of stabilized aggregate, a poor one looks like cement coated gravel or an ugly cool deck job.  If a crew is OJT on a job, too much water added at the site renders it worthless as permeable paving.  Where space constraints permit, finishing should be done with a vibrating screed, being careful not to overwork the surface.  Expansion joints should be placed every 15 to 20 feet apart.  Because of the entrained air, isolated control joints are not necessary.  As detention, 14% volume is an optional figure.  An acceptable range for voids should fall between 12% and 18%.

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Best Management Practices

The most effective way to promote acceptable pervious paving alternatives is to adopt Best Management Practices as they relate to solving urban drainage and flood control problems:

"In addition to concrete pavers, an alternate approach is to use stabilized grass porous pavement consisting of high impact plastic rings in a flexible grid system placed on a compacted sand and gravel road base. This Best Management Practice is intended to be employed in low traffic areas to facilitate stormwater infiltration."

"A Best Management Practice for high frequency, low speed traffic is to employ pervious concrete or a stabilized gravel porous pavement consisting of high impact plastic rings in a flexible grid system placed on a compacted sand and gravel road base. This Best Management Practice provides functional alternatives to facilitate stormwater infiltration."

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Exploring the Best of the Alternatives

Grasspave2

In 1982, Invisible Structures, Inc (ISI) started producing the first of their high impact plastic ring products, Grasspave. Mats evolved into rolls, as a practical matter, for ease and stability of installation, and manufacturing from recycled plastic was a natural. The patented high impact plastic ring system , installed on a road base of sand and gravel affords a 92% void in each ring, important because this gives grass a chance to develop a good, strong root network. Aesthetically, ISI's Grasspave2 is the superior grass paving system, because it looks no different than an adjoining grass yard, unlike concrete grass pavers-the speckled look- or the checkerboarding of GeoBlock's rigid, common-walled mats. Both products have a kind of "grass on chemo" look.  Grass grown on top of Grasspave2 is hardier. The 5700 psi is impressive, too - since most concrete drives are only 3000 psi. The Orange Bowl in Miami has all its parking bay areas in Grasspave2. West Farms Mall in Farmington, Connecticut features four acres of Grasspave2 overflow parking which has more of a park appearance than that of a parking lot. Heavily used in the high shopping seasons and plowed in the winter when it snows, this durable surface is the darling of green community proponents.  Charley's Crab on Hilton Head Island, South Carolina, has 68 Grasspave2 parking spaces, for a restaurant which turns its tables four times an evening.

Gravelpave

Gravelpave, like all of ISI's products, is evolving to meet the needs of some very demanding customers, folks who have been brought up to believe concrete is king and asphalt is the answer if you can't afford concrete. For high frequency, low speed traffic, Augmented Gravelpave10's only competitor is pervious concrete in the permeable arena. Gravelpave drains faster and, when installed over a road base of sand and gravel, it sports 5700 psi, as compared to pervious concrete's 1800 to 2400 psi. A geofabric is fused to the bottom of the high impact plastic ring/ flexible grid to prevent gravel from migrating. Maintenance is occasional box blading. It is significantly cheaper (as is Grasspave2) to maintain than asphalt, approximately 40% lower in cost. Installation cost varies due to economies of scale, unlike pervious concrete, but Gravelpave can run anywhere from $5.75 a square foot installed for small jobs, down to near $4.25 per square foot for large ones. UV protectant is built right into ISI products as opposed to coatings which tend to disappear due to natural abrasion dynamics.

When taking the normal asphalt or conventional concrete parking costs of land use loss from detention ponds, and conventional underground piping to handle stormwater run-off, Grasspave and Gravelpave become highly competitive from a cost perspective.

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Slopetame, Draincore and Rainstore

For erosion control that is easy to install (ISI's rolls come in 100 to 1300 square foot quantities) and a cost effective way to handle storm water runoff, Slopetame is head and shoulders above alternative solutions. The largest installation in Georgia is holding the banks of the Chattahoochee River intact in Columbus. Again, high impact plastic rings in a flexible grid are backed with a lightweight geo-fabric; installation is easy, using 12 inch spikes with 1 - inch fender washers to anchor the surface and split duck bill anchors to "hang" it from the top. A two foot overlap is recommended over the crest of the bank. If rip rap is necessary around outflows because of "underwashing", it should only have to come out of the water onto the bank by about a foot. Sand is placed and leveled on top and sod installed.  Colorado Springs' Garden of the Gods City Park has more horse traffic on its trails than most other parks in the state, and they are holding up well and no longer are subject to erosion, thanks to Slopetame.

Draincore is a french drain system built to handle large volumes of water rapidly. Its versatility makes it a first choice for golf course greens, where it is installed horizontally, as well as tight fit drainage problem areas where it can be installed vertically.

Raincore is custom built, light weight, therefore easy-to-install , sub-surface detention.  It can be used to store run-off for irrigation, storage conducive to infiltration/exfiltration, or storage for transfer to stormwater system.  Installing Grasspave2 or Gravelpave over Rainstore is the ideal for optimal Stormwater management.  However if the developer, designer or client insists on impervious paving surfaces, Rainstore can be readily installed under any or all of the paved area.  Depending on the building footprints; post-development run-off CAN EQUAL pre-development run-off using Rainstore, very possibly eliminating the need for unsightly, land-use usurping, mosquito-breeding retention ponds and/or costly stormwater hook-up fees, along with most or all piping, catch basins, flumes and drains.  Spray N' Bake, a automotive paint business in Hinesville, Georgia used Rainstore in order to avoid having to buy more expensive road frontage property for surface detention.

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Beachrings

For handicap accessible beaches, nothing is easier to install, more environmentally friendly, or more durable than Beachrings.

Asphalt as Permeable Paving

Professor Bruce Ferguson, Associate Dean of the University of Georgia's Landscape Architecture Graduate School, is a noted scholar and author whose publications on Stormwater Management are seminal in this field.  His research into the viability of Pervious Asphalt found the product's function to diminish so rapidly in a few year's time under heavy use such that its permeability appears minimal.  His findings were that "porous asphalt is self-clogging, due to gradual asphalt draindown."

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Gravel as Permeable Paving

This used to be one of the few permeable paving alternatives, along with sand and crushed oyster shells. Drainage is not a problem, as long as the gravel is placed on a sandy substrate. (Crusher run exacerbates drainage problems). Gravel is relatively inexpensive to install, but has a high cost maintenance factor. Depending on the travel, quarterly replacement of gravel is pretty standard - if maintaining an attractive appearance is desirable. Optimal depth is three inches, but sometimes, in order to avoid frequent early gravel replacement, the tendency is to increase the depth. At four inches of gravel, pedestrian and vehicular traffic is impeded. Much deeper than that, and it becomes impossible to drive or walk on it.

The problems of vertical and horizontal gravel migration create messy issues, as gravel tends to go where it is not supposed to go; and loose, straying gravel on impervious surfaces is hazardous for both vehicles and pedestrians. For some businesses the risk exposure is too high to use it by itself. This is why, during the sixties, when concrete became king, municipalities made plain gravel illegal as a surfacing alternative. Since the early eighties and the advent of the high impact, UV treated plastic rings on a flexible grid, with a geofabric fused to the bottom side to prevent gravel migration, gravel can be employed for its superior drainage capabilities, mixed with sand to provide primary filtration.  Four inches of half inch stone under any pervious material provides conservatively 25% of its volume for storage; storage capacity for three-quarter inch stone is 30%.

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Trees, Drainage and Flood Control

Besides providing shade and beauty, a mature oak tree consumes nearly 500 gallons a week from the ground, a significant factor where high rainfall is the norm. Permeable paving preserves and protects tree assets, while permitting developers to maximize land use. Pervious concrete permits water to run through it, and offers a small amount of detention as well, when the ground reaches field capacity. With ISI's Gravelpave or Grasspave2 systems, dirty water drains through a sandy gravel base, a primary filtration process, on its way to the recharge area - and without adding to an already serious street flooding problem. A typical installation is six to eight inches thick, is less intrusive to existing tree roots, as it can be feathered down to four inches along the edges, and provides a small amount of detention during extremely heavy rains as well.

Brick and Stone Pavers

While brick pavers, Pennsylvania Blue Stone, and cobblestones have also been considered as porous paving alternatives in the past, their permeability is relatively insignificant: with only 8% water penetration, run off is still a problem. The only downside to paving under tree drip lines is that roots may cause some breaching of finished surfaces. Stone and bricks offer an elegant, decorator look, and the price for product and installation is considerably more expensive. If drainage is not a problem and money is not a concern, these can be superior paving choices.

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CU Structural Stone

Cornell University has been developing a system combining stones of a particular size and shape in very specific quantities with specific measures of sand and clay, and adding an engineered high water absorbing polymer to create a compatible base for asphalt and conventional concrete, which will permit air, nutrients and water to travel to the bacteria that carry nourishment to tree roots.  While the system allows trees to thrive where they otherwise would not, it is still in the testing stage, but holds much promise in this arena.

About Petrus UTS, Inc.

Established in 1997, Petrus is a vertically integrated Stormwater Management concern whose core business lies within education concerning, distributorship and installation of environmentally sound economically feasible paving alternatives. We are the National Manufacturer Installation Rep for Invisible Structures, Inc. (ISI) Gravelpave2, Grasspave2, and Augmented Gravelpave10. Petrus has the only certified Pervious Concrete Installation Crews (trained under ASAP, Inc., Florida’s oldest and largest pervious concrete company) outside of Florida.  A true Scottish StoneMason sets our cement and brick pavers and stones.

Whether a client requires a drainage system to handle large volumes of water rapidly, erosion control, subsurface detention, porous paving, or a Stormwater Management system combining several or all of ISI’s superior, cutting edge products, we can do it. Petrus has the capabilities and standards of excellence to satisfy the most discriminating clients, with materials supplies – to total turnkey installation.

And the Key People Behind It. . .

President Chere Peterson has over twenty years of commercial construction experience, where her exposure to storm water drainage and flood control formed the basis of her knowledge. Augmented by the subsequent ownership and operation of an eighteen hole, PGA regulation golf course, her growth continued in the field. In the spring of 1996, she was central to collecting, compiling and disseminating information on Chatham County, Georgia's Stormwater Drainage Plan as it pertained to a Special Local Option Sales Tax referendum in a public information function for the city of Savannah. She has developed a Stormwater Management and Tree Preservation Symposium format, has been a guest lecturer at the University of Georgia Graduate Landscape Architecture School, a presenter at an International Summit conference for Invisible Structures in Denver, Colorado, and a frequent keynote speaker for developers, contractors, design professionals and regulators.  Her company Petrus UTR, Inc. is the only vertically integrated, environmentally friendly paving company east of the Mississippi.  She is a board member of the Portland Cement Pervious Association, a member of the Georgia Chapter of the American Concrete Institute, a Certified Georgia Master Gardener, a freelance writer, member of the Society of Professional Journalists, and a country girl at heart who gains much satisfaction from earning a living by being a steward of the earth.

Vice President Dan Sprecher brings twenty-two years of construction management experience and before that, a stint working for the Pennsylvania Fish and Game Commission. His hand is seen in the planning and execution of every job.  Manpower, production and costs are his areas of expertise.

Sensitive Sites owner Dr. Don Gardner earned his undergraduate degree in Turfgrass Management at the University of Tennessee and his Doctorate in Plant Pathology at The Ohio State University. He served as Park & Tree Director in Savannah, Georgia for 16 years; developing a comprehensive tree management plan which contained Savannah's Land Clearing and Tree Protection Ordinance, guiding landscape development and tree protection for public and private lands.  His knowledge is key to saving trees on highly developed sites while reducing flooding and run-off and working with regulators.  Gardner and his company pair up with Petrus on projects requiring his special cadre of skills and experience.

Master Stone Mason Alex Skellon comes to us from Scotland where he started his apprenticeship at the age of eight. His most notable accomplishment was his involvement in relocating the Cape Hatteras Lighthouse this past year.

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