TEXTILES.ORG
By Chris Kelsey
Historic Cheviot Hills Golf Course opened in Raleigh, N.C., in 1930. But during the early 2000s, declining economic conditions and an abundance of golf courses in the region forced some course owners to make challenging decisions about whether to continue operations.
Cheviot Hills’ ownership chose to shutter the course and put up for sale the site’s 178 acres for redevelopment. And while golf courses are known for their beautiful transformations and green landscapes, the real greening of Cheviot Hills has been made possible through redevelopment.
Today, an impressive green wall of more than 100,000sf epitomizes the site while it hems in a 35-acre parking lot for a successful auto dealership.
Benefits beyond ‘green’
Green engineering is currently entering an interesting period of design and acceptance, because it has moved well beyond the attractive concept of just being “green.”
Performance, cost, and safety are central to realizing any useful design, and today’s contractors and clients are discovering that building green can offer these benefits and many others.
The former Cheviot Hills Golf Course site exemplifies this. The construction of a green wall rather than a conventional block-face wall has proven aesthetically pleasing while also advantageous for controlling project cost, speeding construction, and using local materials.
The result? A greener method of building a green wall!
The wall itself is a mechanically stabilized earth (MSE) system constructed of 10-ft-long, L-shaped welded wire baskets (18in. high, 18in. deep); on-site fill material; and a mix of geogrids for reinforcement, soil separation, soil containment, and drainage control.
MSE designs incorporating this type of system have been embraced in green wall systems, and rightfully so. They readily support vegetation establishment and offer designers and contractors significant flexibility without sacrificing performance, such as using local materials rather than trucking in special drainage aggregate and select backfill.
Donald Obst, the engineer on this project, noted that he has enjoyed working recently with green, wire-basket walls.
“They withstand differential settlements and the factor of safety is higher because there isn’t an issue with pH and concrete blocks,” he says.
Also, the risk of block/geosynthetic abrasion risk is removed, said Obst.
Layer by layer
Shaped like an irregular horseshoe, the Cheviot Hills wall is as high as 40ft at its back end and 20ft at its front, with the 50% slope along the highway-facing side (see photos in the box above).
Because part of the wall is located below a 100-year floodplain, the welded wire construction uses two types of wire to create the MSE layer baskets: black wire for the standard, upper layers; and galvanized wire (with free-draining stone) for the below-floodplain layer and any other portions where temporarily accumulating water might be of concern. This was particularly important for the project due to the wall’s proximity to an adjacent lake, which is part of a passing greenway.
Numerous geosynthetic materials were required to stabilize the construction. The primary reinforcement on this project is provided by two types of geosynthetics. The base is reinforced with a high-strength geotextile for mobilizing high tensile forces at low strains. It is used typically for MSE walls, embankments on soft soils, breakwater stabilization, and other reinforced earth structures.
A woven, polypropylene geotextile was also installed beneath and between the lower basket layers.
A total of more than 50,000sy of geotextiles were used in the Cheviot Hills green wall.
Higher up on the wall, where the weight acting on the reinforcement is a bit less, but the need for creep resistance is also great, geogrids were installed. The Cheviot Hills grids are made with a newer generation of polymers (aramide and polyvinyl alcohol PVA). Geogrids with these polymers are more versatile, making them good choices for numerous applications, such as noise barriers, embankments, landfill slope reinforcement, soil stabilization, and levees.
The welded-wire baskets themselves were combined with secondary reinforcement geogrids. The biaxial grid is designed for containment and reinforcement of finer materials, and in this project that characteristic grid aperture provided wrapping at the face near the bottom of the wall where stone backfill was used. A high-tenacity polyester geogrid wrapped the upper two-thirds of the wall where soil was used for backfill out to the face of the wall to support the establishment of vegetation.
The L-shaped wire baskets (18in. high x 18in. deep x 10ft wide) are stabilized by struts placed every 2ft.
The end result is a system that can stand tall, support significant loads, allow for vegetation on its face, and is reinforced by geosynthetic materials designed for long-term use. Also, in the short term the geogrids and geotextiles are manufactured to protect against installation damage—a design concern that is important in material selection. The better a project is at its beginning, the better it will perform in the long run.
Environmental benefits
The use of local materials provided savings for the project and added another environmental benefit to the construction process.
A conventional block wall would have required significant trucking in of specific stone for wall stability and drainage management. But the welded wire wall system meant that the basic wall components could be transported to the site in fewer than five full truckloads. A conventional wall could have required upward of 50 truckloads.
Savings were realized in transportation costs and the carbon footprint of the project was significantly reduced by requiring less than 10% of that conventional hauling traffic. Truck traffic wear-and-tear on the roads into the site and on the site was also reduced.
Also significant, the use of local materials enabled the wall construction quickly but safely. Obst, the engineer, noted that during the earthworks operations, on-site fill could not be harvested fast enough to keep pace with the basket system’s preparation and assembly.
Of course, the real draw of green walls to the casual observer is not all the engineering behind and below it, but its surface aesthetics. The face of this large wall—well-visible from the passing highway—has rightfully drawn second looks. The mix of deciduous and evergreen flora is appreciated by the site owner and community.
Initial vegetation was established through the application of a flexible growth medium (FGM). The FGM was applied through a hydroseeding method to produce quick, sustainable vegetation. Specific plantings were planned to bloom and flourish at different times of year.
The commercial viability of a community golf course may have faltered, but another business has taken its place and has reinstituted and furthered this greenway-abutting site’s greenness while simultaneously giving the area’s architects and engineers a new project from which to draw inspiration.