The Cost-Effective Retaining Wall

March 1, 2010

These days, the function of a retaining wall isn’t merely to hold back soil or rock. They have come to serve a dual function of becoming aesthetically pleasing and blending into the environment while providing the necessary strength to do a proper job.

Another benefit is that engineers and project managers are finding more cost-effective ways of building retaining walls.

Four years ago at Hall Winery in St. Helena, CA, winery owners sought to cut a 2-acre aboveground detention pond in half to gain a building site for constructing a processing plant.

John Bowman Construction, a general contracting company in Napa, CA, had priced out a poured-in-place straight concrete wall of approximately 300 feet. The company began the job, cutting the 2-acre pond in half and, in an area that wasn’t cut, taking out 6 feet and recompacting it; this had been the bottom of the old pond.

After finishing that portion of the job, company owner John Bowman thought that the straight concrete wall would not be aesthetically pleasing.

“I got the owner’s representative to look at a Keystone wall,” says Bowman. Keystone manufactures a concrete segmental block retaining wall system that includes a high-strength fiberglass pin system.

Bowman says the choice gave the architect the freedom to be creative, and the design then focused on a curved wall instead of a straight, poured-in-place concrete wall with a 12-foot footing.

“Because you offset each block by an inch, you don’t get a straight, ugly wall,” Bowman says. “It’s all offset, so it looks pretty.”

John Bowman Construction drained for pressure using the same type of drainage that would have been used for a poured-in-place wall.

Bowman was pleased with the onsite clay material, as it meant he could sink the geogrid into clay instead of another type of material.

In addition to aesthetics, Bowman likes the wall because his company didn’t have to put down a 12-foot footing to hold a 12-foot concrete wall. The construction time was shorter as well, he notes.

Plus, there was a money savings. The footing would have cost about $150,000.

Bowman says he had never worked with Keystone walls previously, but after doing so has added it to his menu of retaining wall options. He’s now using it in a subdivision project.

Lakefront Property-Almost
In Minnesota, owners of an expensive home had a great view of the lake behind their property. Getting to the lake, however, was a problem-the ground sloped steeply, and the area next to the lake was marshy.

The homeowners wished to have an outdoor patio space, but water and wind erosion from the nearby lakeshore was a concern. The contractor’s job was to build a system on the marshy soil that would not fail. Even walking in the area meant one’s feet sinking in, creating an issue for compaction.

Jim Hartwell, owner of Franklin Outdoor Services in Burnsville, MN, performed a cost analysis that included installation of a quality geogrid and soil-stabilization fabric for proper reinforcement in the marshy native soils, an approach he takes with many difficult projects.

Hartwell says, however, that the project was not typical of the type of work his company has done. “We were next to a lake, and we also had some conditions with the gutters,” he says. “We had to put quite an effort into this project because of the gutters and runoff that was being caused between the two homes.”

To create a stable base, Franklin Outdoor Services chose SRW Products’ SS5 soil-stabilization fabric and SRW geogrid. The homeowners, comparing the bid to a cheaper one from a competitor, were reticent, but Hartwell got the job by spending time educating them on the benefits of proper engineering and installation.

“We had to use that material; otherwise it would have been just a spongy patio and there would be issues in the future,” says Hartwell.

He chose SRW products at the recommendation of a supplier. “More engineers are going that route, because it helps stabilize the wall and keeps it from going outward,” he says.

One of the leading causes of retaining wall failures is improper or inadequate reinforcement of backfill. The benefit of the geogrid-composed of high-molecular-weight polyester yarns protected by a PVC coating-was rooted in its easy installation and its being more cost-effective than replacing native soils. The interlocking grid resists biological degradation and allows interaction between the soils above and below.

Franklin Outdoor Services supervisor Dale Wolf transformed the marshy area into a solid patio by clearing brush, smoothing out the subgrade, and removing such sharp objects as tree branches and large rocks that could puncture the soil stabilization fabric. He then placed SRW SS5 fabric over the area to be compacted. The fabric was pulled taut and secured with fabric staples. Where more than one piece of fabric was needed, it overlaps by 18 inches to 3 feet.

Franklin Outdoor Services used up to 8 inches of Class 5 soil as base material and up to 5 inches of gravel behind the wall, with smaller particles behind that for traction and stabilization and to allow water to seep through. The dirt was compacted in 2-inch lifts until the site was back up to grade. Bedding sand was spread to support the patio blocks.

The work also involved removing some 5 feet of onsite clay. “It immediately helped with the drainage,” says Hartwell. “We also used perforated drainage tile behind it, stretched along the length of the wall. If for some reason there was a downpour and it went through all of the material behind it and there’s still water retention at the base of the wall, that drainage tile would act as a fire hydrant would-if you had a rush of water, it would be able to handle it. That way it doesn’t affect the base.”

Wolf had to create a double-tier, 6-foot retaining wall to hold back soil from the steep slope leading up to the house. Debris was removed and the soil was prepared for the geogrid base. The geogrid was then laid horizontally back into the soil, measured to fit the area, and cut to the desired length with a utility knife.

Wolf had to ensure that the geogrid was laid out smoothly without any folds. Folds are problematic when compacting a higher course of base material. While the base pushes down on the folded geogrid, causing tension and helping to hold soils in place, folds in previous courses can allow the geogrid room to move, giving the wall room to move as well. Wolf compacted the backfill soil to 95% standard proctor after each course while completing the vertical construction of the 6-foot retaining wall.

Access was a challenge. “It seems like it was a 45% increase from the top of the wall to the base,” says Hartwell. “We had to bring in a track machine, and we also had to bring in an excavator. Because of the reach, sometimes we couldn’t get down there with a wheel loader. It would get stuck.”

The finished result features retaining walls planted with native vege, which assist with additional erosion control. Copper landscape down lights were placed to highlight the wall plantings.

“We had to use the geogrid behind the wall to stabilize it because of the height,” says Hartwell. “Otherwise, you’re building more tiers for the wall. But when you use tiers, you actually minimize the space, because you have to go further back toward the wall. So we used two tiers and extended the backyard by about 10 feet.”

The tiered retaining walls marry the natural surroundings of the lake to the home’s vertical architecture and, with a small stone fire pit, provide the outdoor leisure space the homeowners sought.

The project took his company longer than usual, but that time investment was necessary in order to stabilize everything, Hartwell notes. The efforts paid off: Four years after its completion, the project is still intact and looking good, he adds.

“In my mind, after a year or so, if you don’t have any settling, I don’t think it’s going to occur in the future,” he adds.

Hybrid Walls
Keith Moser says his job is to figure out how to design a retaining wall in such a way that his contractor clients can bid a job competitively and get the work.

“To do that, I have to think outside the box,” he says.

Moser is an engineer with Geomo Enterprises, an engineering consulting company in Fairfax, VA. He works with contractors using Lock+Load Retaining Walls.

He cites an example: “If someone wants to build a retaining wall, but there’s not enough room to put in the geogrid, they might have to excavate that 10 feet to put in the geogrid-but maybe it’s only feet to the property line. That doesn’t work.

“The way it has been done in the past is that soldier piles or soil nails were used as different ways to support the soil, and then some type of facing was put on top of the soldier piles or soil nails,” he adds. Moser favors Lock+Load Retaining Walls as a way to approach such challenges.

“We produce a final product that looks much nicer than cast-in-place concrete for a cost that is equivalent or less than cast-in-place concrete,” he says. “Lock+Load Retaining Walls help us do things other modular block systems have more difficulty doing.”

Moser calls them “hybrid walls.” In the past, the approach would be to construct one wall system and connect it to another wall system with a joint, resulting in two systems that don’t look like a single wall, Moser says.

“We can use several of those structural technologies behind the same wall,” he says. “The hybrids we use are different technologies. We can use the mechanically stabilized earth [MSE] technologies, soldier piles, tiebacks, or soil nails. We put the Lock+Load face over the whole thing. It looks like one wall; nobody knows the difference.”

In one such project a few years ago-at Alcova Row in Arlington County-developer Centex Homes called upon Geomo Enterprises for help.

“Somebody else had installed a system of soldier piles and tieback anchors,” says Moser. “They needed some temporary support because they had a deep excavation for a stormwater basin. They also had to leave this in place permanently because there was a tree-save area that the county required in order for Centex to develop this lot.”

Original plans called for a three-tiered, cast-in-place retaining wall system with the bottom tier being a cantilever-reinforced concrete retaining wall in front of the piles, the middle tier a cast-in-place structure cast onto the piles, and the upper tier a cast-in-place reinforced concrete cantilever structure behind the piles.

The developer’s concerns were rooted in expense and aesthetics.

“It was going to be really expensive, and not only that, but it’s directly across the parking lot from high-end luxury town homes,” notes Moser.

“You’d be walking out the door and the first thing you’d see is this huge, ugly cast-in-place concrete wall. Not only would they have to pay for the cast-in-place concrete structures, but they would also have to pay for some sort of architectural veneer on top of it.”

Moser took a different approach. For the first tier, he used conventional MSE geogrid reinforcement, backfilling with gravel. He designed the second tier to have the Lock+Load modules structurally attached to the soldier piles using reinforced cast-in-place concrete, with the modules being used as a stay-in-place concrete form.

For the upper tier, he once again used MSE reinforcement and gravity walls systems with just Lock+Load modules and no other structural elements.

The total vertical cut was 28 feet, with about 5 feet between each of the three tiers. The gravel used for the bottom tier was imported.

“It was a very small site,” says Moser. “There was no soil available onsite for us to use. In that type of space, the gravel was going to work well for us, so we used that. When we got to the top tier, we used the onsite soil that was available from the tree-save area. We had to excavate out to build the wall and put the dirt right back.”

The bottom tier, with the gravel backfill, provided drainage.

“On the middle tier of the wall, we alternate lifts of the Lock+Load with layers of gravel so that we’re able to weave water through the face of the wall,” Moser says. “At the very upper tier, it was a very clean gravel, so there wasn’t any drainage concern just using the onsite soil.”

Moser says the approach facilitated faster construction and yielded a good-looking architectural finish on the wall “so we didn’t have to come back later and face the wall with something else to make it look nice.”

Access was quite difficult, Moser says. With the townhouses already constructed, as well as an existing parking lot, getting trucks in and out of the site was a challenge, and the equipment used to execute the job had to be small.

There were engineering challenges as well: The tieback design was done by someone else, and Moser had to examine the design to make sure he was comfortable with it.

“Other than that, the main thing was getting people to understand that we were doing something different in terms of how it looked, but it wasn’t different from the conventional structural engineering practices that were out there,” says Moser. “We were using a different process to produce the same result, using the same kind of governing building codes for concrete design.”

Since then, Moser has designed a Lock+Load project every couple of weeks, with quite a few of them similar in scope to the Alcova Row project.

Protecting Against Wave Action
Richland Chambers Lake in Fort Worth, TX, is used by the Tarrant Regional Water District as an area water supply. It is the youngest lake in Texas, but the third-largest inland lake, measuring 45,000 surface acres.

Richland Chambers Lake has an average water depth of 25 feet and a maximum depth of 82 feet. It has 330 miles of shoreline.

In 2009, PremierCrete Products-a Justin, TX, project management company that does everything from designing to installing big-block retaining walls and visual and sound mitigation walls-was called upon to provide a solution to a failing cast-in-place seawall along Richard Chambers Lake.

Working with engineer Dan Thiele, who owns Thiele Geotech in Omaha, NE, PremierCrete replaced a portion of the failed cast-in-place wall by incorporating a Stone Strong Systems precast block system into the existing seawall. In total, 6,614 square feet of precast modular block retaining wall was installed for the 10-foot gravity seawall.

PremierCrete also provided long-term bank stabilization with a wall system adequate to handle the drainage and wave action of Richland Chambers Lake.

“Stone Strong was the product of choice because the system is very quick to install,” says Chris Heiser, vice president with PremierCrete, adding that a quick installation was possible because no large onsite equipment was necessary as would be necessary for a cast-in-place installation.

“Due to the large blocks, we were able to install the wall in six weeks, taking advantage of the low lake levels and saving the client the expense of dewatering,” says Heiser. “We were able to put in about 1,000 square feet a day.”

Heiser says one of the biggest challenges on the job was addressing the wave action of the lake. “We had to design an adequate wall that could withstand the hydrostatic pressure from the wave action and from the drainage that was coming off of the bank,” says Heiser.

“With a seawall with water against it, drainage was a big issue for us,” says Thiele. “We were able to handle it with the Stone Strong because of the large voids, and using the aggregate behind the wall as an added measure to maintain drainage in the wall system.”

The backfill used was imported three-quarter-inch aggregate. The drainage system was twofold. The Stone Strong Systems wall is self-draining, “so we were able to take care of a lot of the hydrostatic pressure, where with a normal retaining wall system, you put a typical drain behind the system,” says Heiser. And although the wall is self-draining, PremierCrete gave the project additional drainage by adding a through-drain every 100 linear feet.

Heiser says the 15-year-old cast-in-place wall had failed because of drainage problems.

“It had no drainage in it, none whatsoever,” he says. “As time went on, the lake levels dropped. The water doesn’t equal out, so there is more water on one side of the wall and less on the other, and that causes the wall to fail.”

Thiele adds that the new wall not only is functional, but also looks good.

“It was a very efficient design,” he says. “It was a perfect height for this product in its application. The wall ranged from 8 feet to 10 feet tall and 900 feet long, really well suited to the productivity associated with precast concrete. Just unload blocks, stack the blocks, fill them with aggregate, and just keep going.”

Living Walls
Vegetated retaining walls-also called living walls-are a “green alternative” to conventional systems, points out Ed Severance, operations manager for Northeast Environmental Solutions, a division of Taylor Davis Landscape and Construction in Amherst, MA. Among the company’s many services is its construction of living walls.

Northeast Environmental Solutions builds a mechanically stabilized earth system reinforced with vegetation and geotextile. The locking system integrates geogrid, vegetation, and a fascia made from Filtrexx FilterSoxx filled with Filtrexx GrowingMedia.

The vegetated retaining walls rely on GrowingMedia to provide a fertile growing environment that encourages vegetation establishment and successfully helps anchor the roots to the wall and the site substrate, says Severance, adding that such walls are structurally sound.

Like all retaining-wall projects, vegetated retaining walls have challenges in construction. “Living walls, as all retaining walls, are always a challenge, because after excavation to create a sill or shelf on the slope, you are always working from the top of the wall,” says Severance.

There is usually a slope below the wall, and as the wall is constructed, it becomes too tall to reach from below, points out Severance. Onsite soils significantly influence the type and depth of the base materials used to make the shelf to begin the wall, he adds. Backfill material is commonly a structural fill, which is angular and helps to hold the wall in place. Severance’s company also favors using a stone with porous space for drainage.

“The geogrid is used to tie the wall back into the slope for stability,” he says. “Compaction in lifts is crucial to prevent settling and slumping and for long-term stability.”

Drainage approaches depend on the project’s site. “Unlike hardscape-type walls, our walls are permeable and porous, so relieving hydrostatic buildup behind the wall tends to be less crucial,” says Severance. “Access is always an issue and safety a concern as you are working with heavy equipment above a typically unstable slope.”

Native backfill is typically the only material removed from the wall excavation to begin the lifts, he says.

“The socks, which are the facia of the wall, have compost and seed blend blown into them via a blower truck with an injection system for the seed and any nutrients we may want to add,” says Severance. “At the top of the wall, we install loam or compost as a seed bed to revegetate the top of the wall to blend it back into the surrounding area.”

Severance says his company has constructed living walls that are now seven years old and holding up well.

“They tend to fade into the site and, aside from stabilizing the slope, are nearly unnoticeable,” he says. “Maintenance is minimal after the initial watering to get germination. We do ask customers to be careful when using any weed control or power equipment in the area so as not to kill the vegetation on the wall or accidentally cut into the wall.”

About the Author

Carol Brzozowski

Carol Brzozowski specializes in topics related to resource management and technology.