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Paved Driveways Can Be Invisible

By Dustin Glist, Invisible Structures, Inc.

This driveway manages the stormwater runoff into environmentally sensitive Chesapeake Bay. The system can percolate about 2/5 inch (.4) inches per hour, through the entire depth of base of 12 inches. The EPA guidelines say a porous paver needs to percolate .25 to .5 inches per hour.
Photos courtesy of Vicki Bohnhoff, Invisible Structures, Inc.

The look and feel of a paved surface is changing. Landscape contractors are at the forefront of this change, transferring their knowledge of porous and permeable pavers from the commercial sector to residential driveways.

Concerns over stormwater management, heat island mitigation and multifunctional surfaces have made the use of porous pavements, such as structural grass and gravel aggregate surfaces, popular in commercial venues. More frequently, landscape contractors are installing porous pavers for commercial and business developments. These sites are benefiting from porous paving technology in fire lanes, parking lots, access roads, drivable swales and trails.

Eighty percent of a tree's roots are concentrated in the top 12 inches of soil. The root system diameters are similar to the canopy diameters. Porous pavers prevent compaction of the roots, and allows ample amounts of water and air to reach the roots and make it possible for cars to drive and park below mature tree canopies without damaging the roots.

Now, landscape contractors have found a way to transfer these benefits to their residential customers by suggesting installations in driveways. Residential customers may have limited playground or drive space, need additional car, boat or RV parking, guest parking or in some cases, have local stormwater regulations to comply with. For such cases, landscape contractors have the knowledge and skills to help.

"I like how versatile and easy the porous pavers are, compared to the old cinderblocks," says Jean Fogg of Jean Fogg Landscape in Pleasant Hill, Calif. Fogg estimates he has installed six to eight Grasspave2 porous paving systems for residential driveways.

The porous paving for this 33-car parking lot at the Grand Canyon Trust in Arizona has tremendous load bearing strength--5,721 psi--strong enough for any street-legal truck or twice the compressive strength of conventional driveway concrete.

Manufactured by Invisible Structures, Inc. of Golden, Colo., the form is a structural ring and grid system, which reinforces turf for parking or driving with a life span of 60 years. The open structure allows rainwater to percolate through the living grass, back into the ground water supply, and at the same time keep the grass roots protected from deadly vehicle compaction.

Fogg contends grass pavers are ideal for residential driveways. "You cannot tell the stuff is there. In the places we have installed it, it looks exactly like the regular turf right next to it." In other words, it's invisible.

The plastic pavers can be easily cut with garden shears to go around trees and curbs and conform to the shape of a driveway and allow for irrigation heads. RainBird popup heads with swing joints "give a little," providing added longevity to the irrigation head. They cannot withstand tire pressure, however, any irrigation head below and within a Grasspave2 ring, will not receive direct pressure therefore should not be damaged.

An alternative to grass pavers, but one that maintains the benefits of porous paving, is a structural gravel containment system. A normal gravel surface will lose its ability to remain permeable over time since the surface gravel becomes more and more compacted by vehicular traffic. Gravel porous pavers, like Gravelpave2, also from Invisible Structures, Inc., have a life span of 25 years and transfers the load of a vehicle to the base course. That prevents the top dress gravel from migrating. Gravel pavers also prevent rutting, washboarding, and erosion associated with normal gravel surfaces. Both systems require less maintenance than asphalt and the installation costs are comparable to asphalt and less than concrete.

Gravel porous pavers may be suggested as an alternative to grass porous pavers since they do not require irrigation - which is good for communities with water restrictions. However, the gravel pavers have their unique qualities for residential driveways as well. Constructing a driveway with exposed aggregate adds color and texture while reducing glare. Gravel pavers can sustain the same frequency and volume of traffic as asphalt or concrete (grass pavers require less frequent traffic). The geo-textile filter fabric is molded directly to the ring and grid structure and functions in two capacities; to keep the top-dress gravel from mixing with the base course and to inhibit weed growth into the matrix.

Jean Fogg finds it easy to convince residential customers who have never heard the term porous paving of the products' effectiveness by having them visit an installed site. "We send them to a couple of different addresses, and they look at it, touch it, and walk around. They like it."

Porous pavers can be used in driveways with slopes up to five to seven percent and they are less susceptible to surface freezing. A porous paved driveway can thaw ice and snow faster than hard surfacing and is flexible. The heaving and shifting problems found in brick pavers under freeze/thaw conditions also don't occur.
Photo courtesy of Roger Tadlock, MKM

When you add the environmental benefits to the list of cost saving advantages, it's hard to pass up. Porous pavers clean pollutants out of stormwater, reduce offsite flooding, recharging ground water supplies, cool the air (urban heat island mitigation), reduce soil erosion, promote tree growth and capture and retain airborne dust. "Our products our made from 100 percent recycled plastic," states Chris Spelic of Invisible Structures, "That plastic goes toward helping the environment and not into a land fill."

Another cost saving benefit of porous paving is the omission of drainage pipe, some curbing (to convey water), and catch basins, associated with asphalt or concrete installations.

Grass pavers provide an attractive park-like lawn or play surface that doubles for parking or driving. Topsoil becomes too compacted over time for healthy grass roots, so the rings should be filled level to the tops with sand to provide structural integrity and drainage. A thin cut layer of sod is laid directly on top of the sand.

Installation procedures are straightforward. Examine the site and check the slope. Porous paving systems are designed to absorb most of the rain that falls on them. However, torrential rainfall amounts still should be directed away from structures by a minimum of one percent slope.

Next, determine the depth of the base course if it has not already been specified. A contractor may want to consult with a soils engineer and determine the depth based on existing soil conditions and vehicle loads. "Generally, we see base course depths in the range of six to twelve inches, " Spelic adds, "But, it is a good idea to examine every site individually."

Excavate down to the depth of the base course plus one inch - the height of the plastic structures. The existing soils are then compacted to 95 percent density.

Irrigation lines for grass paver driveways or to traverse to a different section, can be laid. Extra trenching may be necessary if the base course depth is not enough to accommodate a larger head. Place and compact a sand and gravel base course, similar to a road base. Porous pavers require that the base be pervious. Pervious base courses usually have less fines, approximately four to five percent, 200 pound sieve, by volume. Check the manufacturer's literature for sieve sizes and requirements. Compact the base material in minimum six-inch lifts. Hosing down the area with water can aid this process.

For grass pavers, spinkle on the Hydrogrow mixture - a polymer/fertilizer compound to help aid growth - with a fertilizer spreader or by hand. Hydrogrow should be applied at a rate of one pound per approximately 100 sq ft. For gravel pavers, no Hydrogrow is needed.

Connect each roll to its adjacent roll via the male and female connectors. The most technically challenging part of installation is putting a seven-sixteenth inch nut driver over the connector and pressing the part onto a one-by -four inch piece of scrap wood. Gentle curves can be achieved through the flexibility of the plastic rolls and dramatic curves can be achieved by cutting the rolls to shape. Extra pieces can be re-used in other spots.

The next step is anchoring down the gravel paver system with the provided eight-inch nails and washers (grass pavers do not need to be anchored, as the grass roots will permanently stabilize the matrix). Hammer the anchors into the base course at a rate of one anchor every sixth ring. Use the extra anchors in the perimeter of the driveway and at the interface of any concrete or asphalt areas such as the garage pad, sidewalk, or street.

For grass pavers, the next step is to fill the rings level to the top with clean sand. Sand provides the best balance for strength, drainage, and air infiltration. Then lay thin cut sod or hydroseed directly on the sand. "I use Dwarf Fescue," says Fogg, "It's drought tolerant, hearty, and can withstand traffic. In cooler climates you might consider a Bermuda." Invisible Structures recommends that each driveway site be examined individually for the proper grass type. Rainfall amounts, sun exposure, and aesthetics are all variables to be considered when choosing grass. If sod is applied, use a sod roller to ensure good contact is made between the roots and the sand/top of the rings. If hydroseeding is applied, extra watering may be necessary during the first growing season. In both cases, wait three mowing cycles to drive or park on the installation.

The last step on the gravel paver installation is to fill the rings with your top-dress gravel two-and-a-quarter to two-and-a-half inches above the top of the rings. Some compaction and settling will occur naturally and with traffic. Gravel fill should be clean, sharp, hard, and about 3/16 inch to 3/8 inch. Larger gravel and rounded gravel will too easily migrate out of the rings. The installation in now complete and the driveway is ready immediately.

For both systems--grass and gravel--minimal maintenance is required. Grasspave2 requires mowing, fertilization, and water - just like any other lawn. It does not need to be aerated, as the rings and sand combination will prevent compaction. Commercial aerators may also damage the plastic structure. Gravelpave2 maintenance may include occasionally brooming the aggregate back into low spots. An additional quarter to half inch layer of top-dress gravel may be necessary every five or 10 years.

"It is our belief that we offer the landscape contractor more options to help their clients," says Spelic. "We can provide quality products to alleviate parking constraints, aesthetic issues and cost limitations."

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December 7, 2019, 4:38 am PDT

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