From Rooftop to Pavement

Sept. 4, 2014

Within the Appalachian Mountains in the western part of North Carolina, a seasonal home was being built on the shore of Lake Toxaway, the largest private lake in the state. It was designed by Platt Architecture.

The home site is narrow and steep, sloping down from the road to the lake. The result is that the floor of the house is 20 feet below the adjacent road, and the roof of the house is more or less level with the roadway. This topography inspired architect Parker Platt to design a 3,959-square-foot green roof to better blend the home into its natural surroundings.

Living Roofs Inc. designed the green roof and selected the Xero Flor Green Roof System for the project. In addition to prevegetated mats, delivered as fully vegetated rolls or flats ready for installation over base components, the system includes a root barrier, a drain mat, a retention fleece, and additional engineered growing media. Both the root barrier and the drain mat are produced by global manufacturer Bonar.

The XF 112 Root Barrier protects a roof’s waterproofing membrane from direct contact with, or encroachment by, the roots of the green-roof plants. The product is a monolayer membrane consisting of highly flexible, 20-mil, low-density polyethylene (LDPE) manufactured with a mixture of virgin and 15% to 20% post-industrial recycled resins.

Installed atop the root barrier, the XF 108H Drain Mat allows excess water to flow to roof drains. The product is a three-dimensional, lightweight, and flexible composite material made from 50% recycled materials. It is manufactured to form a drainage core of looped polyamide filaments bonded to a specially perforated, nonwoven filter fabric.

The retention fleece, a nonwoven fabric, goes on top of the drain mat. It serves as a filtration barrier against media erosion and facilitates distribution and temporary storage of rainwater. The additional growing medium, a lightweight mix of porous mineral aggregate and composted organic material, is spread out over the retention fleece before the prevegetated mats are rolled out to complete the installation.

The relatively steep slope of the roof posed special challenges.

“Roof slope angle is a significant design factor in any green roof project. Not accounting for the slope of a roof is a leading cause of green roof maintenance problems and even failure,” explains Clayton Rugh, Ph.D., general manager and technical director, Xero Flor America. “Roofs sloped from 10 to 45 degrees require green roofs with more depth and subsequently heavier load-bearing capacity on the roof. To prevent erosion and slippage, sloped roofs also require specialized components to hold and stabilize the growing medium and vegetation.”

“Sloped roofs drain more efficiently and are consequently drier than low-slope, flat roofs,” says Emilio Ancaya, cofounder of Living Roofs Inc. “That is why we increase the rooting depth of green roof assemblies for sloped roof installations. This is achieved with a deeper layer of growing medium, which helps the plants establish greater root mass and increases the water-holding capacity of a green roof.”

For the Lake Toxaway installation, 2 inches of growing medium were used-double the depth generally required for a low-slope roof in this geographic region.

The supplemental stabilization grid integrated within the green roof assembly for the Lake Toxaway project utilizes the TerraCell 140, a geosynthetic cellular confinement system made with high-density polyethylene (HDPE) strips bonded together to form a honeycomb configuration.

Anchored to the aluminum drain-through edging at the perimeter of the roof, the grid was installed over the water retention fleece. The Living Roof installers filled the grid with 2 inches of growing medium before rolling out the mats. The cells of the grid hold the growing medium on the slope.

“Roof slope also affects irrigation requirements and irrigation system design,” notes Ancaya. “The portions of a green roof’s vegetative field at the top of a slope will dry out more quickly than lower portions.”

An automated irrigation system pulls water from the lake to water the roof vegetation from early May to early October, with the irrigation system regularly checked for proper coverage, pressure, and timing. It is also adjusted as needed due to seasonal temperature and rainfall changes.

The green roof was completed in April 2012. During the first year after installation, Living Roofs made 10 scheduled maintenance visits. Subsequently, this has been reduced to six annual visits.

Preventing more than 99,800 gallons of annual stormwater runoff into Lake Toxaway, the green roof extends the natural landscape at eye level out toward the lake and serves the desired unification of the house with its setting. According to Platt, it is something of a local landmark. Nearby residents often give directions to their homes in relation to the “green roof” house.

Two Roads, One Solution
A new regional hospital was being built in Owensboro, KY, but neither of the two access roads adjacent to the hospital was suitable for the needs of the site.

“The contractor had responsibility for improving the access roads,” says Jim Sanneman, business development manager with Tensar International. “One was on the east side of the hospital and one on the west side. Interestingly, one road was owned by the county, Davies County, and the other road was owned by the city of Owensboro. So they had to get approvals from both municipalities.

“The problem was that the roads were not designed for the kind of increase in traffic that was going to occur because of the new hospital going in. The city road was a residential street, basically, and the county road was a farm market road.”

The main problem was that there were soft soils below the conventional pavement sections.

Proof-rolling the prepared subgrade resulted in rutting between 1 and 2 inches. Sanneman describes the possible options: “The contractor could have done a chemical stabilization, which would involve mixing cement and making a subgrade that would be harder, so that he could then pave on it. Or he could have done a full-depth undercut with just stone, but he chose to redesign the pavement section to incorporate a layer of grid into the existing pavement section and address two issues. One was constructability, and the other was long-term performance of the roadway.

“He was able to redesign it with a thicker aggregate section such that the geogrid and stone could be constructed over the soft soil, and asphalt could be placed, and it still met long-term design requirements.”

For this purpose, the contractor selected Tensar Triax TX5 geogrid.

“We have two different types of Triax to use in pavement applications: TX5 and TX7,” Sanneman explains. “Within our Spectra pavement design module, these are the two products that we can choose from. For the most part, TX5 will address most of the design challenges that we have. TX7 might be utilized for a thicker pavement section, where you’re trying to get a more robust product in the ground to provide a higher level of structure.

“The geogrid allowed him to build over that soft subgrade. Had he not changed it-had he not utilized the geogrid-he probably would have had a tough time building the section.”

Whereas geogrid used in a retaining wall provides lateral earth reinforcement, in a pavement application, its primary function is enhancing the modulus of the aggregate part of the pavement, making it stiffer. The geogrid extends the life of the pavement.

“The geogrid is under the entire roadway,” Sanneman notes. “It becomes a component of the flexible pavement. It contributes to its structural performance over time. As an element in the pavement section, it has a structural contribution. It provides a life enhancement compared to that same section if it had been constructed without the grid.”

An important issue that the contractor faced was that while these two roads had to be upgraded, there were entrances and exits from both roads leading to businesses and homes. Access had to be maintained for these neighbors, so only a portion of each road was reconstructed at a time.

“The geogrid solution provides for that ability to sequence a job,” says Sanneman, “where you keep half the roadway open and close down only one lane, allowing access to these businesses and homes. In contrast, a chemical stabilization would have required shutting down the road for a week to allow the chemicals to impact the stability of the subgrade soils.”

According to Sanneman, there were other benefits as well. “As a result of the solution they selected, there was a decrease in labor and equipment required, and a reduction in the aggregate and asphalt requirement to build the roads. It was a green solution, from the standpoint of reducing the amount of footprint that was required to construct the roadways.”

Each road was a standard 24 to 25 feet wide, and it required two side-by-side layers of the Triax geogrid to cover that width of road. In total, approximately 30,000 square yards of the geogrid were used on the project.

Shoulder Surgery Through a Busy Mountain Pass
Located about an hour east of Seattle, Snoqualmie Pass carries Interstate 90 through the Cascade Mountain Range. The most traveled east-west corridor in Washington State, I-90 connects large populations and business centers of Puget Sound with the agricultural industries and recreational activities of eastern Washington. It is said to be the nation’s busiest mountain pass highway, carrying approximately 29,000 vehicles a day, approximately 25% of which is made up of vehicles in the trucking industry, making it a critical corridor for freight mobility.

To maintain I-90 as a primary statewide transportation corridor, the Washington State Department of Transportation (WSDOT) is adding lanes, adding and replacing bridges, stabilizing rock slopes, and improving bridges and culverts along a 5-mile section between Hyak and Keechelus Lake Dam. Construction started in 2009 and will be complete in 2018, according to Bob Hooker, WSDOT assistant project engineer.

“During the summer of 2012, they were widening the road in the area of Snoqualmie Pass, with all sorts of new construction going on,” explains Pat Gowan of ACF Environmental West. “They had to divert traffic over onto the shoulder of the road. They didn’t know exactly what kind of load that shoulder would support, and there are a lot of semi trucks going over this roadway. It’s a major highway.”

WSDOT was planning to dig out the shoulder and rebuild it to be used as a detour for traffic so crews could start building new lanes. However, because the detour would be temporary, WSDOT decided to use a less expensive method, choosing a type of inter-layer to put over the existing shoulder and adding a lift of asphalt over the top, with the hope it would support heavy freight traffic.

“This was a unique application of the geosynthetic paving fabric in that it wasn’t installed in a permanent long-term application,” says Hooker. “It was installed to provide additional strength to a section of roadway that only had a thin section of existing asphalt. It didn’t make much sense to dig up the entire section of roadway and rebuild the base to allow for a thicker section of asphalt for one summer.”

WSDOT looked into several different options, but ultimately determined that the most economical option would be to utilize the existing roadway with the geosynthetic fabric and a thin asphalt overlay. This combination of materials would prevent having to rebuild this section of roadway and provide the structural strength required to support truck traffic. This method not only accelerated construction but also minimized delays to drivers.

Hooker explains that the existing shoulder had been approximately 12 feet wide through that section of highway. “I’m estimating that the rebuilt shoulder was about two lanes wide, so we had to widen it out to about 24 to 26 feet, and it went on for a length of around 2,000 feet.”

WSDOT used a product called HaTelit, manufactured by Huesker, to reinforce the asphalt overlay.

“It’s a geosynthetic, high-quality polyester asphalt reinforcement grid combined with an ultralight nonwoven material,” says Gowan. “It is installed going down flat on a roadway, either on an existing road or on a pre-leveled layer of asphalt. Then new asphalt gets paved right over the top of it.

“The company that laid it down was very efficient at it,” adds Hooker. “They laid it down on top of the existing asphalt surface, tacked it well, rolled it in nice, and laid down the final layer of asphalt on top of it.”

Gowan notes that the new asphalt and the HaTelit geosynthetic were installed in a single day. While weather could have been a problem, he says, “It was a gorgeous few days” surrounding the project.

According to Hooker, once the asphalt overlay cooled, the roadway was ready for use.

He says HaTelit and similar products have been used on other WSDOT projects as well. “When we do work adjacent to and through local municipalities, sometimes we have seen fatigue cracking-stress cracks-in the asphalt on city streets and around intersections. For repairs on asphalt with stress cracks and rutting, we have used similar geosynthetics mainly as a membrane to block the reflective cracks from coming through the new asphalt mat,” says Hooker.

Reflective cracking is the result of thermally induced or traffic-induced fatigue in the asphalt. The cracks form in asphalt laid over concrete roads through horizontal movements of individual concrete slabs. The slabs expand and contract as a result of daily or seasonal temperature fluctuations. This can create high tensile strains in asphalt, which can lead to cracks forming directly above the joint in the concrete.

Traffic-related stress can also occur when a wheel load passes over a crack in the old pavement beneath the overlay. Bending and shearing forces are thus induced in the new asphalt overlay. The strength of the overlay is reduced by each loading event until reflective cracking occurs.

The HaTelit asphalt reinforcement grids are designed to delay this reflective cracking. According to the company, such cracking may be delayed by a factor of three to four, compared with unreinforced overlays.

Although the Snoqualmie Pass shoulder rebuild occurred close to a mountain lake, no special erosion or sediment control BMPs were required as a result of the use of the geosynthetic. The nearby road construction, however, used numerous BMPs that had been built into the project design. Various check dams, silt fences, and erosion control-modified mulch were put on exposed slopes to control stormwater runoff from the site.

When the shoulder detour was no longer needed, it was removed.

“After we were done with our roadway construction this section was removed because it was at a different grade than the final roadway. We were able to utilize the existing shoulder as a viable detour for I-90 traffic the summer. It allowed us to keep traffic flowing even during the heavy construction of this very busy section of highway,” says Hooker.

Trail Bridge Stabilization
The Ohio to Erie Trail, running from the Ohio River to Lake Erie, spans the state of Ohio from Cincinnati to Cleveland. When complete, it will total some 300 miles of off-road trails for bicyclists, hikers, and other nature lovers.

A portion of the trail courses through Westerville, OH, intersecting busy County Line Road. City fathers wanted to ease the way for trail users by building a bridge over this road. Supporting the approach to the bridge is a retaining wall that incorporates a cellular confinement system by Presto GeoSystems.

“The best way to describe it might be like an old-style ice cube tray,” says Victor Meredith, president of Meredith Brothers, a civil engineering company based in Columbus, OH. “It’s like an accordion in a closed state, when it’s not being used. But then you stretch it out into larger sections and fill the spaces with aggregate and topsoil. The front cell that faces out would be filled with topsoil, a good growth medium where we could put plant life. The next two cells, and on back approximately 10 to 12 feet, would have a crushed aggregate to act as good drainage media, and also to really hold that grid in place, to keep that slope stable.”

The trail will go over the area containing the aggregate, leading to the bridge. When complete, the retaining wall will total 5,600 square feet and reach about 12 feet high, allowing trail users to safely reach the bridge, rather than trying to traverse the busy city road. Below ground, the wall extends about another foot and a half. At the top of the wall will be a railing, adding another four and a half feet to its height.

Meredith notes that Presto Geoweb is being used in trails and greenways around the country. Westerville city administrators had also previously used similar walls for other projects adjacent to bridges and were very pleased with this method of stabilizing slopes and paths.

“The Presto system uses a green retaining wall,” he adds. “So this product is perfect because it is going through a residential area. There is a nice footbridge that is aesthetically pleasing, so they wanted to go with the Presto Geoweb again. When the people in their houses look out back, they don’t see concrete; they actually see some green space.”

He explains, “We always use a 6-inch Geoweb for retaining walls, because you can get better compaction in the cells than you can get in smaller or larger sections. This material is made in 8-inch-thick and 12-inch-thick sizes, but we just don’t use it for walls because we can’t get as good compaction in those as we can in the 6-inch.

“To assist drainage, we’re putting in a 6-inch pipe about every 20 to 25 feet, going from the back of the aggregate through the front of the wall and acting as an outlet for additional drainage. Very short walls don’t require any drainage, but the taller you get, the more drainage will be necessary for global stability.”

Meredith notes that because of the railing on top of the wall, crews had to add 10-inch plastic pipe sleeves to hold the posts for the railing fencing.

The only maintenance that will be required, he says, will be some watering at the beginning to obtain good establishment of plant growth.

When the project has been completed, the city will have contributed a safer and more attractive portion to the beautiful Ohio to Erie Trail.

Stabilizing a Gem
Riverside County, in California, has been one of the fastest growing regions in the US over the past 50 years. The city of Murietta, in the southwest portion of the county, has contributed significantly to that growth. In 1980, the city’s population was 2,200. It grew to 24,000 in 1990, to 44,000 in 2000, and to more than 103,000 in 2010.

Murietta bills itself as the “Gem of the Valley,” and work taking place at the interchanges at Interstate 215 and Los Alamos Road highlight the name.

“They constructed a concrete diamond on two of the steep slopes on these interchange ramps,” explains Michael Hutchinson, landscape architect with Hernandez, Kroone & Associates, based in San Bernardino, CA. “They put SlopeTame2 geosynthetic fabric down in the gem itself and all around the slope and covered it with gravel in different colors.”

SlopeTame2, manufactured by Invisible Structures, is a permanent three-dimensional reinforcement and stabilization matrix consisting of 1-inch-high rings, vertical bars, and a connecting grid. These elements are bonded to a geotextile filter fabric, which is attached to the ground with a combination of duckbill anchors and quarter-inch rebar. It is especially useful on steep slopes.

The project in Murietta used just shy of 35,000 square feet of the product across four slopes on the interchange ramps. The decorative diamonds contain a mixture of white gravel and blue glass within the structures to create the appearance of a sparkling gem.

The designers wanted a geotextile with a fabric backing that would not allow stones to penetrate underneath, and with a three-dimensional structure to hold the stones in place. The SlopeTame2 matrix met all these requirements.

Vegetation is able to grow through the geotextile, and a variety of grasses, shrubs, and trees are being planted along the slopes. Hutchinson notes that sediment control BMPs utilized on the project included a gravel berm, drain inlet protection, and fiber rolls.

The project has proceeded well, but Hutchinson describes an interesting problem that arose.

“The contractor was having a little trouble getting all the elements of the diamonds connected. As the slope curved, they weren’t really sure how to deal with it. One time they worked inward from opposite sides, to meet in the center. It didn’t quite work out–like a transcontinental railroad meeting in the middle somewhere. It put a big wrinkle in the diamond.

“They tried to lift it up and pull it up, but it still had a wrinkle. I think they had to take it apart and reset it.” 
About the Author

Steve Goldberg

Steve Goldberg writes on issues related to erosion control and the environment.