Retaining Walls... Green or Traditional?
In the soil erosion business, the name of the game is controlling and conserving the earth. We build silt fences, or place plastics or fabrics to keep the topsoil on properties so it doesn’t wash away, blow away or get tracked away by human or animal activity.
The iconic symbol of the erosion control business could be the retaining wall. Its name describes what it does. Retaining walls are built, very simply, to hold the earth in place. The rest is mainly a matter of aesthetics. Traditional retaining walls can be made out of corrugated steel sheet-pile, steel gabion baskets filled with rock, articulated cement blocks, polyethylene geocells, cut stone, brick, or even geofoam.
Some of these materials are selected not merely for their structural function, but for their good looks as well. The manufacturers of articulated concrete block, for example, offer many different kinds of colors and textures of their products. What’s chosen depends upon the application and a client’s budget.
There’s also another type of retaining wall, a relative newcomer to the scene. It’s only been around for about the last 20 years or so, and it’s making its presence felt more and more. This is the “green” or vegetated retaining wall.
First, let’s consider the traditional options.
When deciding which kind of wall to do, “the first thing we always look at is footprint, whether or not you can lay it back to a slope, or if you have to go to a retaining wall,” says Larry Larson, at R.H. Moore and Associates, Inc., Tampa, Florida, a distributor of erosion control products.
“Slopes are much cheaper than walls are. Here in Florida, anything steeper than a 2:1 is usually not stable, because we have ‘sugar sand’ (a very fine soil). Our friction angle is like 28 to 30 degrees. Up north, you can cut slopes 1:1. Down here, it’ll get saturated and just fall right in. So that’s when we have to start looking at stacked wall systems.”
“If you come to me and say, ‘I need a retaining wall and I want it to be somewhat aesthetically pleasing,’ we’ll always present several options,” said Larson. “We’ll say, ‘Here’s a geocell wall; here’s an articulated concrete block wall; here’s a gabion wall; and here are the estimated installation costs.’ We take them from the lowest cost to the highest. Then they get to choose, based on their budget for the job, and the look of the products.”
As for the cost differential, Woolbright has this to say: “An articulated concrete block manufacturer’s most basic offering, essentially a grey, one-square-foot block, is going to be the cheapest buy, and will have little to no maintenance going forward. But all the block companies also offer upgraded versions, for enhanced aesthetics; split-faced, tumbled, multi-piece and color-blended systems. Those can get way more expensive than a living wall would be.”
There are certain situations in which you can’t, or shouldn’t, attempt to build anything other than a traditional retaining wall.
“Green walls don’t look very good in northern winter climates where plants aren’t green much of the year,” says Moritz. “Irrigation introduces water into a retaining wall system. That can cause discoloration and can saturate the retained soils, if and when the system leaks.”
“A vegetated wall may not be ideal when the project is located in an arid area where it’s going to require irrigation to keep the wall growing and active,” says William Handlos, P.E., a director at Presto Geosystems in Appleton, Wisconsin, a manufacturer of polyethylene cellular containment systems. “If a site has a preponderance of noxious weeds in the project area that are likely to take over, then the wall will be vegetated, but may not be particularly attractive.” A vegetated wall under a bridge abutment would not be successful; it’s too shady. “Plants don’t grow where the sun doesn’t show.”
A fast-moving river or stream edge is also no place for a green wall, according to Handlos. He uses the example of a geocell structure. “You can put concrete infill in the front cells to secure the face from the damaging effects of fast flow.
“However, if one expects to have flooding, where water will come in and exceed the level where the wall is vegetated, and the flow along the vertical face is seven feet per second or greater, well, I wouldn’t specify vegetation in that instance. Because if you’re vegetating, you’re doing it in topsoil, and those cells will scour out.”
“It’s usually always about velocities and shear stresses,” said Larson.
“If you have a situation where you have a flow of two feet per second, low shear, but have a lot of volume, you can start looking at vegetated-type systems. But if you’re at ten feet per second, that’s a whole different game, because it won’t really stay established, it’ll all get ripped out. Then you have to look at gabions or some type of hard armor.”
Many miles of retaining walls are built along highways and roadways. These are usually designed by engineers and built by contractors.
“In Texas, there have been numerous highway retaining wall failures because the soil used for the base and backfill didn’t match what was specified on the drawings,” said Joseph Kowalski, P.E., president of Kowalski Engineering, Inc., in Cincinnati, Ohio. His firm specializes in MSE retaining wall construction. “Instead, some random fill was put behind the walls, and it overloaded them.”
“You need to make sure to follow the soil parameters on the engineering drawings, whether these are MSE or panel walls, vegetated or non-vegetated,” urged Kowalski. “The biggest problem is when a contractor uses whatever soil is available on a site, rather than understanding that he should be using exactly what’s been specified, not something close to the specs just because they can get it cheaper.”
As Kowalski explains, “these walls are geotechnical structures. The engineers who design these retaining walls treat soil as a building material—something with strength—just like wood, or steel, or concrete. A certain amount of load is going to be placed on that wall. The load comes from the soil that’s behind it. If you use a different kind of soil, the load will be different.”
Blake Cervenka, co-owner of Austin-based Texas Highway Walls, LLC, agrees with this analysis. “We specialize in MSE walls, which are mostly what’s used in highway work. You’ve got precast panels that make up the facia and reinforcing metal straps that attach to the panels and go a certain distance back.
Those are held in place with select backfill that makes up the reinforced volume. If you use the wrong type of backfill, the wall can collapse.”
One of the most common reasons you see wall failure, according to Cervenka, is that the backfill has some fines (fine soil particles) in it. If water gets introduced into that backfill, from rainfall or any other source, it washes the fines right out. Then, you’ve got a void that can loosen the grip of the straps, which can then pull out of the ground.
Of course, the wrong backfill can bring any wall down. But when a collapsed wall involves a highway, it doesn’t just cause a temporary interruption to the flow of traffic; lives can be endangered. “Most of the time when you have a highway retaining wall project, you’ve got a road sitting right on top of it,” says Cervenka.
The green option
When fully covered by plant material, a green wall can be quite attractive. But this option isn’t chosen merely for aesthetic reasons, as environmental consciousness continues to grow.
A lot of factors enter into the decision to go with a traditional or a vegetated retaining wall. In some applications, a traditional wall is clearly called for. Other times, a green wall fits the bill much better. And sometimes, you—or more accurately, the client—can go either way.
There are numerous situations where a client’s need for green can be utilized for a contractor’s benefit. For instance, there are times, particularly in the case of big commercial developments, where “buffer zones” must be maintained between build sites and the public, kind of like DMZs for construction, if you will.
Developers can’t encroach with anything permanent in these areas, with one notable exception. “I’ve heard that, sometimes, regulatory agencies will allow vegetated slopes or walls to be built into buffer zones, because they’re considered environmentally friendly,” said Judd Murphy, vice president of sales and marketing for Pinnacle Design/Build Group, Inc., headquartered in Cumming, Georgia.
Many different kinds of materials are used to build vegetative walls. One of the most commonly used is geocell. It’s made out of high-strength polyethylene or other plastic, and can be green in and of itself.
Some geocell is made from recycled plastic and is recyclable again at the end of its useful life.
A geocell is a cellular confinement system for soil or growth media. Each open cell is also permeated with slots, to allow quick threading of root tendrils. Sections of geocell are shipped flat. When you pull open a panel and reveal its 3-D structure, it looks something like a honeycomb.
“We did a project for the city of Dunedin, Florida, using geocell,” said Jeff Truxton, general superintendent at Keystone Excavators, Inc., in Oldsmar, Florida. “These were very interesting walls in a drainage canal, ten feet wide on the bottom, going up ten feet high on the sides, a one-half-to-one slope, terraced back on both sides.”
“We installed geocell panels that were about eight feet long and four feet wide,” said Truxton. “Once we installed them, we filled their cells with dirt, and then set another panel on top of that, continuing that process over and over until we reached ten feet. For every six-inch-high row that went up, we would set back one row.” This gave the wall’s facing a terraced effect, with row upon row of cell “flowerpots.”
When the walls were done, they were hydroseeded with native ferns, vines and wildflowers, “a little bit of everything,” said Truxton. Construction of the walls “was a pretty easy process, and turned out really well. It was the greenest wall I’ve ever done, and it looked great.”
Another type of green wall medium that many soil erosion contractors are already familiar with is the good old-fashioned gabion basket, especially popular in channel applications. The company Truxton works for does a lot of green gabion walls and channel linings. “Gabions are being used more than ever in applications where concrete ditch paving and solid sheet pile had been the norm,” he said.
“Gabions became very popular because they can be stacked vertically, and are not nearly as ugly as concrete or steel sheeting,” continued Truxton. “On the top, you can plant jasmine or any other type of vining plant. The vines will eventually drape over the wall and weave in and out of these crabtraps (gabions). The mesh (openings) are about the same size as those found in chain link fencing, perfect for ‘weaving’ plants.”
The other advantage of gabion baskets is their porosity. If there’s one thing that’s the enemy of retaining walls, it’s water. Gabions filled with broken or crushed stone have built-in drainage.
Concrete can be vegetated, too. Several manufacturers make cast concrete blocks with pockets that can be filled with growth media. Once a wall is fully vegetated, it “disappears.”
Does green mean more greenbacks?
One of the main considerations in deciding which type of wall to build is, of course, cost. Since there’s something extra that must be added, you would think that planted walls would be the more expensive option. Not so, according to Murphy.
“Vegetated walls are a little bit less expensive (than traditional walls) if you have the room to properly put them in.” He’s referring to the common practice of setting back a vegetated wall every few feet or so, essentially turning a wall into a slope. This does require a bit more space than a straight, vertical wall.
Craig Moritz, vice president of engineering for Minneapolis, Minnesota-based Keystone Retaining Wall Systems, LLC, a manufacturer of articulated concrete block, disagrees. “Vegetated walls require the proper climate and attention to irrigation, among other details, and require year-round maintenance. A traditional wall tends to be the less expensive option, as it requires less maintenance, and thus, is preferable in most situations.”
Native plants are often used with green walls and slopes. “The idea behind using native species is that they’re very drought-tolerant,” said Truxton. “Then they don’t have to put irrigation systems in on top of (the walls).” Municipalities like them because they are virtually maintenance-free.
Mark Woolbright, founder and managing member of The Living Wall Company, LLC, St. Louis, Missouri, has been in the green wall business for 22 years. He says that the real answer lies somewhere in the middle of those two points of view. He said there are things that must be done to get plants established properly, native or otherwise.
“Natives are awesome. Once established, they take very little care. But they are a little finicky and you’ve got to get them to establishment. You can only plant them at certain times of the year, and you’ve got to get enough water to them so that they establish themselves and root in well.” His company often does maintenance on its living walls for a year or so, to make sure plants get a foothold and replace any that haven’t taken.
A greener future
Green retaining walls are never going to completely replace traditional ones. As we’ve seen, different kinds of applications require different kinds of walls. However, there are definite signs that the future is going to keep getting greener.
“I can tell you what’s going to be a game changer,” said Woolbright. “There are a lot of new codes and regulations for the disposition of runoff water, and that’s driving a different type of demand.”
“You’re looking at people who are building or improving a property and can’t let runoff water leave the site.
Now they’re looking at all these different BMPs for holding it on the site, like green roofs, rain gardens, permeable pavement and bioretention swales. If you can take a living wall and funnel your runoff water into it to treat it and detain it, then it becomes a stormwater management tool.”
Woolbright’s company recently completed a project on the East Coast that had a “zero runoff” mandate. Stormflows are captured off the building’s green roof and stored in cisterns, which must be emptied before the next rain event. The stored stormwater is pumped into the green walls’ drip irrigation system.
Success in the retaining wall business today means knowing about all the various materials and techniques that are out there, and the types of applications they’re used in. It also means having the ability to communicate to your clients what the best option will be for any given project, so that everyone’s happy with the final outcome.