Going Wall-To-Wall: Retaining Walls

A popular movie of a few years back carried the signature line, “If you build it, they will come.”

To most people, turning a cornfield into a baseball field would be an insurmountable chore, much the same way constructing a retaining wall on a residential or commercial landscape would be.

But landscape contractors have put their own twist on that line from “Field of Dreams,” and they have come to build it – the retaining wall – and more and more of them are offering this increasingly popular service.

FUNCTION FIRST. Seeing that building a retaining wall is no easy or cheap chore, contractors are understandably hesitant to construct an unnecessary wall.

“I would say 75 percent of the walls we install are because of a grade change that is too steep to be made up in a slope,” noted Alan Strohbehn, landscape designer, Prescription Landscape, St. Paul, Minn., adding that a 3 percent grade change is the maximum for safe mowing. “Any greater slopes are tough to deal with using grass and landscape mulches because the wind and water movement moves the materials as well.”

Paul Barton, president and chief landscape architect, Landshapes, Bloom-ington, Minn., went even further than Strohbehn’s 75 percent estimate. “We only install retaining walls when there’s a functional need – period,” he stated.

“In my opinion, a wall in the landscape is often a sign of failure to site a building properly or take care of grading naturally,” related Bill Weiss, landscape architect, Greenland Landscape Co., Paramus, N.J. “There are not too many walls in nature, so from a landscape point of view, I don’t think walls should be considered for decorative purposes.”

Retaining walls will also enter into play for an installation where the neighboring property is at a different height from the client’s property. “Or, sometimes we’ll build a wall to preserve an existing tree if the job requires changing the ground level and we have to keep the tree at the old ground level,” commented Rick Kier, president, Pro Scapes Inc., Syracuse, N.Y.

Although many people will initially think of walls as separating or dividing devices, retaining walls can actually create more useable space of value in a landscape.

“We’ll often suggest a wall that is 3 feet high or 42 inches high for areas that can’t be maintained easily,” added Barton. “That way we can create a planting area or garden that’s actually at waist height for the client and we can put a taller wall above that one to correct the water flow or hold the water back from the structure.”

“A lot of people like walls of 18 or 24 inches high for backyard borders or they like using walls to raise planting beds,” related Strohbehn. “That way we can put good soil on top of bad soil and improve the planting that way.”

MATERIALS MATTER. When it comes to constructing a wall, contractors have options as to what materials to use.

“The most popular style of wall for us has been the pre-cast concrete type,” noted Kier. “Pressure-treated timber walls are favored by some people, but a lot of times the timbers are purchased from a railroad and they’re being sold because they’ve expended most of their useful life.

“The concrete walls go up pretty quickly once you have the base course put in,” Kier continued. “Along with setting the base, the real challenge is making sure the wall has the proper batter or pitch backwards for structural and drainage purposes.”

“We always encourage customers to use what looks best, and while the commercial areas almost always go with modular block systems, residential customers also like using natural stone walls,” commented Barton. “The modular block walls are also the most time consuming to install and are more costly because they have to be engineered.”

But Barton has seen a real increase in the popularity of natural stone or boulder walls over the last two years, despite the challenges usually associated with acquiring sufficient quantities of quality stones to produce an installation.

“We have some suppliers hauling materials from a couple hundred miles away, and there are times in the summer where we can’t get a 25-ton load for three to five days,” Barton noted. “So sometimes a supplier who has a quality supply of stone can name the price, which means $40 to $50 a ton isn’t unreasonable.”

Because the company has had projects delayed due to a lack of available stone, it has become resourceful in unearthing new supply options. “We’ll often go up to construction jobs and find a crew digging up piles of 2-foot to 3-foot diameter stones it doesn’t want, so we’ll buy them and haul them away,” Barton continued. “This hauling is an expense a lot of companies don’t want to deal with because the stone is pretty hard on the equipment.”

Weiss also prefers the natural stone walls with the intent of minimizing their impact on the landscape’s aesthetics, using them as borders for an area to be filled with geofilter mats. “Then we plant a ground cover that will grow into the fibers to create a wall of green,” he explained. “After two seasons, the ground cover should have grown in enough to be effective and provide all of the technical properties of a wall.”

And the price of the different materials used to create the wall will most likely factor into the decision.

“Timber is by far the least expensive material, but it won’t last as long after it’s installed,” according to Barton, who said his company pays $10 to $12 per square foot of timber. “Boulders or natural rock usually cost us $13 to $15 per square foot, and the modular concrete blocks go from $18 to $23 per square foot.”

“In most cases, walls are expensive to install,” agreed Kier, “but they can be profitable if they are bid properly. The most important thing is having enough experience to know what to look for in terms of soil problems or structural challenges with the landscape and estimating accordingly.”

WORRISOME WATER. Although it would be easy to assume otherwise based on the number of contractors new to wall construction, installing such a feature is no easy task. In fact, walls greater than 4 feet tall generally require the design expertise of an engineer, and any flawed design or installation will only lead to maintenance chores, particularly if the wall is subjected to much contact with water.

“The biggest issue in our area with any retaining wall is having proper drainage,” Kier remarked. “If we don’t have proper drainage and water gets behind the wall, it will freeze and heave the wall out.”

To avoid this problem, Kier installs drain tile and fills with No. 2 stone behind the wall during the installation so any water behind the wall can drain out of the area.

“If you backfill the area behind the wall with a clay soil, water that gets back there won’t have anywhere to go and the wall will tip over in four or five years,” Kier pointed out.

“Sandy loam or black dirt soils tend to settle in better than heavy clay soils, so we’ll bring in backfill soil about 90 percent of the time,” Strohbehn added. “We also backfill with a lot of ¾-inch stone that doesn’t allow for a lot of pockets for water to get into.”

“The most important thing that is overlooked by contractors is what type of soil the wall is holding back,” pointed out Blaine French, sales manager, Keystone Retaining Wall Systems, Minneapolis, Minn. “Granular soils are generally more favorable to work with because they don’t hold moisture and they allow for better drainage than heavy clay soils that hold moisture and have changing properties.”

While contractors regularly term modular-style retaining walls “maintenance free,” the same is not true of natural stone walls.

“People really like the look of natural stone walls, so we stack them up with a fabric behind them and hope for the best with shifting and settling over time,” admitted Strohbehn.

Contractors do have options available to them for projects that they know will be exposed to heavy water conditions.

The author is Editor of Lawn & Landscape magazine.