Retaining walls add a layer of visual interest to any landscape, whether they are used to surround an outdoor living area or to terrace uneven land, creating more usable space. They are also used to retain earth and to help prevent erosion. Whether used for aesthetics or function, the rule of thumb for retaining walls remains the same. They shouldn’t slide, and they shouldn’t tip.
“Retaining walls that are engineered or built incorrectly will either bulge or rotate,” says Joe Kowalski of Kowalski Engineering Inc. in Silverton, Ohio.
“At some point in time, it will fail, if it happens slowly or if it happens quickly.”
A retaining wall failure can have catastrophic consequences, which is why most states and municipalities require walls over 4 feet tall to be engineered and permitted. This includes the portion that is buried beneath the ground.
“Once you start getting above 4 feet, it gets more into the need for engineering because your pressure behind the wall increases the taller you get,” says Joe
Tekulve, owner of Paramount Lawn and Landscape in Loveland, Ohio. “Most walls fail from hydrostatic pressure, or water behind the wall, so you have to make sure you backfill it with the right amount of gravel, have the right amount of water drainage and tie it in with the right amount of geogrid.”
Geogrid, a geosynthetic material that is used to reinforce soil behind a retaining wall, is a vital component of any retaining wall construction. An engineer will determine how deep the geogrid should recess into the hillside and provide specifications on exactly how the wall should be built. For walls under 4 feet in height, using geogrid to tie a retaining wall into the earth and backfilling the wall with gravel translates to a fairly affordable, eye-catching feature. For walls over 4 feet, the cost increase significantly.
“That’s where you sometimes run into problems with homeowners,” Tekulve says. “They don’t realize how expensive and involved it is when you have a wall over 4 feet. It can run anywhere from $2,000 to $6,000 just to have the wall engineered and permitted, and they get sticker shock.”
Because of the added cost of engineering some retaining walls, Tekulve is always upfront with residential customers who are envisioning terracing to increase the size of their yard.
“They don’t realize that what they’re thinking of is going to be $200,000 or more, and that’s when you run into issues on the residential side,” Tekulve says. “You just have to prep and educate them. When they look at the cost and what’s involved, they’re in disbelief.”
Tekulve says commercial clients who are developing land typically understand that the costs associated with retaining walls, some higher than 50 feet, can run in the millions.
“The taller they are, the more functional they are, and the riskier it is because you are retaining more weight and pressure,” Tekulve says. “You also have to add things like handrails, because you wouldn’t want a 20-foot wall without a handrail.”
While the failure of a wall under 4 feet might not have catastrophic consequences, proper planning and building can save thousands in repairs down the line.
“If a wall starts to move and there are items within that wall that aren’t flexible or forgiving, like utilities, storm water lines, electric lines or irrigation lines, they’re going to snap and break,” Kowalski says. “There are walls that are 2 feet tall that slowly tip over and it doesn’t seem like a big deal, until you realize it is someone’s backyard with a fire pit or patio above it, and all the sudden they’re into $10,000 worth of work to a fix a 2-foot wall.”
No matter the height or purpose of a retaining wall, Kowalski says that proper planning and preparation is key. Partnering with an engineer or landscape architect who has the experience and know-how to plan a retaining wall is good practice when taking on the heavy labor of retaining wall construction.
“It is important to have a good drawing from a landscape architect or civil engineer that can use a laser or survey equipment to determine how high this wall is going to be, where it will be located and what the final surface grades will be,” Kowalski says. “You want to know what the surface grades are, where the water is flowing and if the wall is going to be stable, because once you’ve changed the grade of a slope, you’ve also changed the direction of the water, and suddenly water is being directed at your foundation wall.”
In geographic areas filled with hills, retaining walls are very common to help prevent erosion and to transform unusable hillsides into useable space. Tekulve says that very steep and hilly yards can be transformed by retaining walls that can add features like an outdoor living area, a fire pit with lighting or a paver patio. But knowing the boundaries upfront is paramount.
“Make sure you get the property surveyed so you’re not building a wall on the neighbor’s property,” Tekulve says. “If you put a mulch bed on the neighbor’s property, it might not be a big deal, but if you build a retaining wall, you might have a $20,000 or $100,000 wall that needs to be moved.”
Knowing the location of irrigation lines and seemingly unrelated things like downspouts and drains is also important to maintaining the integrity of the wall.
“Things break, and we know that, so we try hard to make sure if that happens, we aren’t causing a lot of damage,” says Dan Barron of Timberline Landscaping in Colorado Springs, Colorado. “Drainage is a huge thing for a retaining wall, so if you have an area that has the potential for water, like downspouts off of homes, you have to make sure that your drainpipes don’t exit behind or around your wall, because that water can cause washouts or decrease the wall’s life expectancy as designed.”
Just as irrigation lines shouldn’t be installed close to a foundation, Barron says main lines should be positioned at least 6 feet away from retaining walls when possible. In cases where obstacles prevent that, Barron recommends that any irrigation lines be sleeved, so they have outer protection in the event of a break.
“If something were to break it would break into the sleeve and daylight where it was directed, allowing less chance for the water to cause damage to the retaining wall,” he says.
Know your options
Kowalski says an engineer can quickly determine what type of wall can be safely and correctly engineered for any given space. With that knowledge, contractors can then consider a variety of products to meet their client’s budget and vision.
“Walls are either a cut wall, which is where there is an excavation into the side of the hill and the wall is built up to hold that land back, or a fill wall, where you fill up above the existing grade so your land will be higher than the existing land,” Kowalski says. “Based on that, you can determine the best type of retaining wall. That could be a segmental wall, a natural stone wall mortared together, a precast modular block wall or a soldier pile wall with steel beams or concrete or wooden planks in between.”
While there are many options for materials to build retaining walls, the options for backfilling walls aren’t as flexible. Kowalski cautions that backfilling should always be completed with angular gravel.
“There is never a situation in which someone should backfill behind these walls with clay,” Kowalski says. “Gravel has a high strength. It has the ability to let water leak through so it won’t build up pressure, it is easy to compact, easy to place and easy to handle. You don’t want to backfill with clay. It will tip over, and that is guaranteed.”
Many manufacturers provide courses and certifications related to their products, Tekulve says, and it is especially important to make sure that any materials are being installed according to manufacturer specifications.
One common mistake is that contractors often neglect to fill the hollow cores in the center of some block materials, a key step in adding mass to the wall.
“That core needs to be filled with stone, but for some reason people will leave it hollow or they will try to stack up multiple layers of wall blocks and then fill the cores,” Kowalski says. “When you fill the top wall block the stone doesn’t flow all the way to the bottom. You have to fill each course of wall blocks separately and independently. That stone is an important part of the overall wall structure.”
Often, contractors believe that wall blocks, once filled with gravel, are strong enough to support fence posts. Kowalski cautions against this practice. “Fence posts need to be behind wall block,” he says. “People attach fence posts to small wall blocks, and they break and tip over. We recommend against it, and it isn’t standard in the industry.”
Barron says he often installs retaining walls made of giant, irregularly shaped boulders. With the help of excavators and heavy chains, he painstakingly places the boulders, then rotates them to reveal the boulder’s “best face.” With the size and mass of the materials used to build retaining walls, Barron says that in addition to the safety concerns of wall failures, contractors should be aware of safety risks to the human body.
“With the size of the materials we use, we make sure to work as a team and we strive in safety,” he says.
While retaining walls are a time-consuming and involved process, Tekulve says the finished product is worth the effort.
“It is heavy, hard work, but it is very rewarding when you see the finished product,” he says. “It is a labor of love.”