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Tasked with vegetating 2.2 acres of a vertical mechanically stabilized earthen (MSE) wall stretching up to 46 ft at its highest point, contractor Carolina Outdoor Services searched for a “one-time” vegetative solution. The imposing earthen walls in Raleigh, N.C., were established on a former golf course that will become a mixed-use shopping center with a pedestrian greenway area.
Already under tight time constraints, Carolina Outdoor Services brought in Susan Keen, P.E., of Trajan Inc. to vegetate the earthen basket walls. Keen contacted Brian Free, business development manager with Pennington Seed, to determine the best seed and amendment solution that would adhere to the vertical wall—and work right the first time without costly reapplications.
Living in Raleigh and having completed many other projects in the Southeast, Free was familiar with the soil conditions and challenges the heavy clay and lack of topsoil created. He suggested the use of a specially formulated seed mixture combined with agronomic soil solutions to increase organic matter and bring the soil pH up into the range of 6.3 to 7, ideal levels for ultimate nutrient uptake.
Seed-to-Soil Contact
Site challenges included soft ground at the base of the slopes, which limited vehicle access, and finding a favorable method of applying the seed mix to ensure it adhered to the geotextile element on the wall’s face.
“I suggested a two-step hydroseeding process,” Free said. “First, we would apply seed, fertilizer and soil amendments with a trace amount of Profile’s Flexterra Flexible Growth Medium (FGM), followed by a second capping application of the remaining Flexterra.”
Free had consulted with Sandy House, technical associate for Profile Products, on the use of amendments. It was decided that when mixed with the best seeds for the region, they would rejuvenate the soil for ideal seed-to-growth establishment.
Free specified a custom-blended SlopeMaster professional erosion control seed mix that contained Myco-Advantage, a beneficial mycorrhizal fungi to enhance root mass, root growth and overall nutrient uptake. The mix also was treated with Germ Max to enhance germination and plant establishment. Seeds included mohawk bermudagrass, greystone tall fescue, predator hard fescue, weeping lovegrass, vetch, ryegrass and durana white clover, an extremely persistent legume addition to any stand of grass.
House had identified Profile’s ProPlus agronomic formulations as good additives to the seed mix for this region. Specified at 2.5 gal per acre, JumpStart improves soil moisture infiltration and retention and provides hormonal plant response for root development. BioPrime was applied at 40 lb per acre to enhance long-term vegetation sustainability. With four active ingredients, including slow-release nitrogen, this biostimulant helps ensure plant health and optimizes delivery of macronutrients and micronutrients to plants. Finally, limestone derivative NeutraLime helps correct soil with pH values less than 6.3. Specified at 80 lb per acre, it mixes easily in the tank and boosts soil pH within seven to 10 days following application.
Flexterra provided a chemical and mechanical bond to lock the engineered medium in place. Thermally refined wood fibers, crimped man-made fibers and performance-enhancing additives form a lofty, interlocking matrix that creates air space and water-absorbing cavities to accelerate germination, reduce the impact of raindrop energy and minimize soil loss.
“You need to have a mechanical bond, which is why the FGM would perform so well,” House said. “You cannot have a vertical wall without a mechanical in the solution. It just won’t work.”
After receiving the order, all materials were delivered to the job site in two days. Operators used a Turbo Turf mechanical hydroseeder and an 80-ft Genie lift to apply the seed and amendment mixture, as well as 3,500 lb per acre of Flexterra. The workers applied the first layer on a 30- to 40-ft section, reloaded the 550-gal hydroseeder and returned to cap the section with the second layer of product.
Seeding began on Sept. 9, 2009, and workers completed the application seven days later.
Sustainable Growth
Following severe storms and heavy flooding that hit the region a few weeks after application, the walls were holding strong.
“All parties were looking to successfully vegetate the wall the first time,” Free said. “We found a permanent and sustainable vegetative solution by picking the right mix to establish a viable stand of grass and eliminate costly reapplications.”
Free added that the contractor insisted any reseedings would be completed at the cost of Pennington and Trajan, and he confirmed that the companies have not had to touch up the project.
“The solution was very innovative,” said Marc S. Theisen, vice president of erosion control solutions for Profile Products. “Pennington Seed did a great job of providing the best possible conditions conducive to establishing vegetation on difficult vertical slopes. We only had one opportunity to provide the correct solution out of the gate, and we were successful.”
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