Retaining Walls That Disappear Into The Landscape
RETAINING WALLS ARE PRACTICAL THINGS. They serve a vital purpose in construction and erosion control, bearing loads and holding slopes back from sliding down, but sometimes they’re not the nicest things to look at.
However, there are retaining walls that are sturdy and functional, yet unobtrusive— green, living or vegetated retaining walls. Like the others, they still fulfill their primary load-bearing or erosion-prevention purpose, but with an extra, environmentally-beneficial flourish.
After some time, this type of wall can even do a disappearing act. Once its outer coating of vegetation grows in, all that’s seen is green; the wall itself vanishes. Yet, it continues its strong and silent vigil behind the scenes.
A green light for green walls
The demand is increasing for livingwalls. “Green walls are gaining in popularity with our residential clients,” said Mike Deboer, account manager at Superior Groundcover in Grand Rapids, Michigan. “We’ve done some commercial ones, too. But residential seems to have more traction for us; the market is better.”
“We’ve had clients that have had us put in vegetated walls in lieu of timber or concrete walls,” says Deboer. “We’ve even done vegetated shoreline stabilization with the same technology, instead of sheet pile or pylon systems for water breaks.”
“There’s definitely been increased interest in and awareness of these solutions,” said Carey Witt, P.E., president and principal engineer at Austin, Texas-based GeoSolutions, Inc. “I don’t think it’s restricted to any particular demographic. I think all people appreciate things that are green and natural. And these days, there’s more of a desire for a back-tonature, organic approach.”
When asked if the growth he’s seen is mainly with his commercial or residential clients, Witt says, “It’s really across all sectors of the economy, up to even larger kinds of infrastructure type of clients, such as municipalities.”
The severe drought in Texas is a main driver of green walls in that state, according to Witt. “The drought has created an awareness of and a sensitivity to our ecosystem as it relates to rainfall and water usage.”
Along with that sensitivity comes governmental mandates. Pino Cignarella, southeast region manager for Maccaferri USA, Inc., attributes much of living walls’ growing popularity to this. “All of the regulatory agencies in the U.S. are getting vegetation-sensitive. Europe was a few years ahead of us, but we’re catching up.”
“Over the past two years, there’s been a real upswing in LID (lowimpact development) solutions,” adds Witt. “Many of the cities and agencies throughout Texas are beginning to implement and codify requirements for green infrastructure.”
Green walls have many benefits. They reduce the urban “heat island” effect; they increase habitat for wildlife and insects. They’re just plain beautiful and calming to look at. And they’re a relatively easy way for contractors or developers hungry for LEED points to get them.
Methods of construction
There are a number of materials with which to build living walls. Among them are compost-filled filter socks interlocked with polyethylene plates between them; mechanically stabilized earth (MSE); gabion baskets; gabions wrapped with turf reinforcement mats (TRMs); poly geocells; and concrete cellular confinement systems.
All of these systems have their advantages and disadvantages. In Florida, “the two things we sell the most of down here are the polyethylene cellular confinement systems and trapezoidal gabion baskets made of PVC-coated woven steel wire,” says Larry Larson, director of operations, sales and product applications at R.H. Moore & Associates in Tampa, Florida, a distributor of soil erosion products.
“Both of those definitely work well, but do have some limitations on velocities. If you’re putting a green retaining wall within a channel lining, for instance, then you have to be careful about the height and speed of the water, so you don’t wash the plant material out of the face.”
Superior Groundcover prefers to build its green walls out of compostfilled filter socks. “We mix seed right into the growing media, which is typically compost. We then fill the socks with this seed/compost combination. The vegetation then grows from the inside out.”
This method works well because no other special equipment is needed to fill the bags. The firm uses the same blower trucks they also use for their core business, bulk delivery of mulch materials and hydroseeding.
Landscape architect Kirsten Coffen, owner of Garden Architecture, LLC in Fork, Maryland, used a concrete cellular confinement system to build several low retaining walls in a client’s backyard, on a large estate he owns in Pikesville, Maryland. “They’re on a 2:1 slope, almost the steepest you can get,” she said. “Before this, we’d been stabilizing it with a meadow seed mix.”
In 2009, the client decided he wanted more usable garden space. “He’d seen pictures of terraced hillside rice farms, the kind that are typical in Vietnam and China,” said Coffen. “He thought that kind of image would be really cool, and give a softer feel to that slope.”
“I had found this cellular concrete at a local garden trade show, and thought it would give us the effect we were going for. We laid the walls out in a serpentine design, picking up on the topography,” she said.
“There is some sort of architectural element to these walls, but eventually, the vegetation will take over completely,” says Coffen. Drought-resistant perennials that can withstand full afternoon sun were selected for the pockets, including creeping phlox, ice plant, silver mounds and a variety of sedums.
Seattle, Washington-based Herrera Environmental Consultants is involved in an environmentally-sensitive green-wall project for the city of Lacey. Compost socks were chosen for this application. “It’s for a wetland restoration, so we want to establish it with indigenous native vegetation,” said senior landscape architect Jason King.
“The walls we’re creating are actually in the water, in the wetland itself.
We’re creating an overlook with a trail that goes out into the wetland, to provide people with a means to get out there and see it.” Dogwood and willow trees are being staked in, along with plugs of quick-growing groundcover to provide a “green edge.”
“This method was recommended by our geotechnical engineer,” adds King, “because of the natural setting, and some pretty sandy soils that we’re also dealing with. We originally went with an MSE wall, with layers of fabric tied back into the slope and wrapped around the soil.”
“Those can be planted as well, and you can get a little bit steeper with them. But in this case, we only needed about a five- or six-foot wall that sort of tapers back into the banks. The sandy soil, though, always gives us a hard time maintaining steeper slopes.”
For this wall, the disappearing act is a must. “We don’t want people who are looking out at the wetland to see this big retaining wall.”
“We do a lot of streambank restoration,” says Witt, “where there’s a hydraulic element involved, and it’s potentially a very erosive environment because of that. So that leads us to a stronger element, what the industry calls a ‘wrapped-face geosynthetic.’”
“We take an 18-inch tall, L-shaped wire basket form. It’s not really a gabion; there’s no back and top, just a face and a bottom panel. We put poly geogrid directly inside of the wire basket form, and a permanent TRM behind that. Then, we put a growing medium inside that facial wrap.” Afterwards, the whole thing can be hydroseeded.
Costing them out
It’s impossible to say in general whether a green wall costs more or less than a non-green wall. Facing a wall with plants may cost less than a fascia using an expensive natural stone; but generally speaking, there are too many variables involved to do a real cost comparison.
Of concrete cellular confinement systems, “I would say the cost is about the same to possibly less,” than a conventional, non-green concrete wall, says Coffen. “There’s not as much concrete material, but then you have to consider that you have to purchase the plants. It’s probably about the same.”
Then there are the costs that come in after the wall is built. “You can just walk away from a plain concrete block wall,” says Witt. “But with a green wall, there’s going to be some maintenance, some pruning and replacing of dead plants and obviously, irrigation—at least through the establishment phase. Depending on your location, and what you’re trying to grow, you may have to have a permanent, ongoing irrigation system.”
“In our business, we talk a lot about ‘unfinished face-feet,’’ says Pat Estenson, national sales representative at Oakdale, Minnesota-based VERSA-LOK, a manufacturer of concrete cellular confinement systems.
“You have to ask, ‘How many square feet is this retaining wall; how much area will it cover, and how much concrete will be showing?’ I’d say if you’re just doing a block-vs-block comparison, the price of green retaining wall block is very close in price to any other retaining wall block.”
“Planted walls use fewer blocks per square foot, as there’s green space incorporated into the wall. So from a purely how-much-concrete-must-youbuy standpoint, it’s going to be a little less.”
Some makers of compost sock products claim that they can stand up to high water velocities as well as hard armor does, once the vegetation is fully rooted in.
Cignarella doesn’t think so. He’s worked on a number of vegetated retaining wall projects, many of them in channels. “When they talk about ‘high’ velocities, what kind of velocities are we talking about?” he asks. “Five feet per second, six? When I say ‘high velocity,’ I’m talking about 12 to 16 feet per second.”
“The most difficult aspect of living walls is being able to sustain gestation beyond the initial germination, to get maintainable vegetation,” says Witt.
Another obstacle that Witt has run into is that “there can be some hesitancy on the part of permitting entities to accept green solutions as permanent structural components for critical features.”
This is because “a green wall is looked at as a hybrid of a structural solution and a landscaping solution. Part of the sustainability of it as a structure is the ability to maintain vegetation, because you’re relying partly upon the root system to give strength to the overall structure.”
“There’s a fear that the vegetation could die, or the bags could deteriorate, or soil could be washed out of the bags. It doesn’t seem as permanent to them as good old concrete. That’s something for the industry to overcome.”
On the other hand, green solutions that are kinder to wildlife may have an easier time getting approved in certain situations. “A compost sock system is good for sensitive applications,” says Richard Kim, sales director for Singapore-based Deltalok, makers of a compost bag system that uses interlocking poly plates between the bag layers.
“If you armor your riverbanks with hard armor such as concrete, riprap and so forth, they get baked by the sun. Even at night, the heated surfaces will heat up the water, killing fish.” When hard-armored systems are fully vegetated, however, this effect is mitigated.
Some conventional wisdom says that you shouldn’t build green walls in hard northern climates such as Michigan’s, as they won’t look good much of the year. “I would disagree with that,” says Deboer. “It doesn’t look that different from any other vegetation during the winter months, when lots of plants go dormant. It all looks brown, or doesn’t look vibrant. But there’s still plant material there, covering the face, so you still can’t see the wall underneath it. It just blends into the landscape.”
Drainage can’t be ignored, either. A buildup of hydrostatic pressure behind a retaining wall can cause failure. With a green wall, you’re adding water. “It’s a critical design feature to make sure there’s a balance between available moisture and excess moisture,” says Witt. “With most green walls, the wall material is usually porous, so if there is hydrostatic pressure, it’ll just flow through the face.”
Drip systems are the usual go-to for living walls. Witt also uses sprays on some of his walls. Deboer says, “If it’s a spring or fall installation, there’s usually enough rainfall, but we typically suggest a drip system just for insurance. You just never know when you’re going to have a dry spell.”
Moisture is essential during the establishment phase, which usually takes about six months. “By then, the vegetation is at maturity and pretty well established,” says Deboer.
Irrigation became a big problem on the terraced-wall project Coffen built. “The block system we used is designed to accept drip tubing. However, the client was on a well, and having trouble with the yield. He really didn’t have enough water supply to irrigate. If you’re going to build this type of wall, the irrigation system is really critical.”
Hand-watering is an option, but Coffen says that won’t get the best performance out of the plant material. You really need an irrigation system to get it firmly established.
Living walls, along with green roofs, permeable pavers and other green infrastructure elements are going to become increasingly important erosion control options for the foreseeable future. Building a retaining wall that ‘disappears’ allows a contractor to protect the environment quietly, from behind the scenes. It’s not only a beautiful solution, it’s downright magic.