Evergreen precast retaining walls support vegetation to deliver strength and beauty.
By Mimi Rainero Coles
What structural product securely holds back tons of earth for infrastructure projects, boasts long service life and low maintenance, deters graffiti, absorbs sound and heat, supports plant growth, is sustainable and beautifies the environment? Hint: It is used all over the world! Any ideas?
It’s the Evergreen Wall!
The Evergreen Wall is comprised of precast concrete elements stacked in horizontal columns that quickly and easily fit together. The wall includes voids that accommodate soil and support vegetation, from flowers and vines to groundcovers like January jasmine and other low-spreading evergreens. The wall’s design promotes deep root growth that can withstand freezing temperatures and hot, dry weather conditions.
Evergreen elements can be custom built to follow almost any horizontal contour or vertical slope and manufactured for variable heights based upon project specifications and site loading conditions. Retaining and sound walls of up to 65 ft (20 m) in height and hundreds of feet long have been built around the world. Custom-designed seismic walls are in place in California to specifically help protect slopes during earthquake tremors.
Evergreen Walls are designed and certified by a professional engineer. They are manufactured in a controlled plant environment to ensure a premium quality product that guarantees durability in the challenging site conditions where retaining walls and soundwalls are typically installed. In most cases, the same equipment used for site excavation and earthwork can be used to set the wall sections and place the backfill material.
Cumberland Gap Case Study
The steep road cuts required for the highway and tunnel project along the historic Cumberland Gap region between Kentucky and Tennessee used custom-colored Evergreen Walls to meet the DOT’s and FHWA’s structural soil-retention specifications as well as the aesthetic concerns of the U.S. National Park Service. The natural stone-colored walls offered the park service the strength of precast concrete; the cost advantages of standardized elements; sound and heat reduction; and aesthetic harmony with the natural environment. The need to purchase valuable land was minimized with engineered soil retention walls. Up to 1,600 ft2 (150 m2) of wall was placed daily, allowing the project to be completed ahead of schedule.
At Cumberland Gap, the design of the Evergreen Wall units allowed sufficient ingress of rainwater and sunlight to support lush vegetation. As the plants became established and cascaded over the precast unit, noise reduction increased. “The wall system offered us an excellent medium in which to grow large plants to blend into the environment,” said Robert Leary, the U.S. Federal Highway Administrationengineer on the project.
SIDEBAR: Evergreen Advantages
Environmentalists and urban planners who seek to add natural landscaping to construction projects and other civil works will find that the Evergreen Wall offers many advantages in addition to being sustainable:
• Unmatched structural stability for soil retention, erosion control, waterfront stabilization and groundwater drainage
• Sound abatement near roadways and around busy neighborhoods, airports, parks and schools
• Minimal use of expensive right-of-way land for civil and private projects
• Stackable, 40-ft2 (3.7-m2) self-aligning erection units act independently for ease of construction and in positioning to create localized access behind walls
• Ease and efficiency of erection with readily available construction equipment
• Design and function flexibility that includes gravity-type retaining walls, rock revetments and freestanding noise barriers
• Maximized landscaping options for a variety of plant types include the creation of extended or protruding areas for planting
• Minimalization of safety issues with unstable slopes and falling-rock zones (precast benches retain boulders and rock slides)
• Elimination of unsightly graffiti near roads and walkways in urban areas
• Cost effectiveness through precast plant manufacturing efficiencies and element element uniformity and ease of installation
• Unparalleled precast concrete plant quality and long service life
• Possible LEED points based on local materials and labor.
Mimi Rainero Coles has a BA in business and economics from Hollins University and has been with Permatile Concrete Products Co. of Bristol, Va., since 1987. Coles is a current member of NPCA’s Executive Committee and Board of Directors.