Building Blocks: Landscape Installation

Building blocks are only as good as the work that is done behind them. Too much focus on the facing of a retaining wall and not enough on compaction, drainage and base preparation can easily equal disaster.

“You can get so focused on building up the veneer that you forget about the soil behind it,” said Bob Dean, president, ICD, Milwaukee, Wis. “We should not focus on the blocks or the units. The job is to increase the strength of the soil.”

Increasing the soil’s strength means landscape contractors must focus on compaction of backfill and foundation soils, which is sometimes forgotten, and developing an effective drainage system. Generally, contractors are tackling these installation challenges well, Dean said.

“We’re seeing more confidence in installation,” he said. “The good contractors are learning to follow the rules. As we follow the rules, we have success with the walls. As we have success with the walls, more contractors use them.”

IT’S A HOLD UP. Retaining walls are popular because flat property is in short supply, Dean said. “People are building more into hillsides and there is a need to create areas that are not sloped,” he said. “That’s where walls come in – to allow building on a site where you normally cannot.” Contractors also use retaining walls for decorative purposes, Dean added.

Landscape contractors may use retaining walls to prevent erosion or to create planting areas and functional walls that perhaps hold up a driveway or form a raised patio, said Jeff Nelson, marketing and creative manager, Versa-Lok, Oakdale, Minn. “(The wall) doesn’t just retain soil, it creates a useable space,” he said.

The retaining wall market is divided into three segments, according to Bob Gravier, president, Allan Block Corp., Edina, Minn.

• Garden walls. Installed by homeowners, these are mostly light, do-it-yourself projects.
• Landscape walls. These walls that go “from knee to nose” involve studying the site and analyzing soil, drainage and structures on the property.
• Retaining walls. Their most important function is structure. “The wall must absolutely, positively not fall down,” Gravier said. “Its primary function is to retain soils and hold up whatever is above the wall. These walls require engineering and thoughtful, careful review by a professional.”

A MATERIAL WORLD. A wide variety of construction materials is available for building retaining walls. Boulders and field stones are the most natural and durable, but expensive, Gravier said. While treated wood is less expensive and easier to work with, it lasts only about 15 years, he added. “Treated wood is also an environmental nightmare,” he said. “It’s essentially a dead tree soaked with chemicals. In many places, if you pull a timber wall out of the landscape because it has rotted, it has to go to a toxic landfill.”

Concrete materials range from cast-in-place systems to mortared walls that may require skilled craftsmen to build. “You have to pour footings below the frost line and place steel reinforcement inside,” Gravier explained. “Someone who is skilled in masonry has to stack the bricks. This type of wall is beautiful, but expensive.”

Segmental retaining walls can be constructed without mortar and concrete footings. Products differ by shape, color, interlocking mechanism and block type – hollow or filled. “(The segmental system) is a versatile way to build walls,” Dean said.

WALL INSTALL TIPS. No matter what the wall size is, getting the job done right the first time is essential, Gravier said. “Coming back afterwards and repairing a retaining wall is expensive because you are dealing with landscaping, paving and maybe even a house – and you can’t get equipment through,” he said.

First, establish a level, sound base for the wall, Gravier advised. “We take out soft soils, put crushed stone in and pack it down so it’s firm,” he said.

Then, contractors may begin stacking blocks, filling in and compacting along the way. “By doing proper compaction, you are dramatically increasing the strength of the soil and the integrity of the wall,” Dean said.

Proper compaction of foundation and backfill soil is critical to the long-term performance of retaining wall systems, Nelson agreed. Foundation and backfill materials should be compacted to at least 95 percent of standard Proctor density – the maximum soil density achieved in a laboratory using a standard amount of compaction effort, according to Versa-Lok’s retaining wall design and installation guide.

Loose backfill will add pressure on walls, collect water, cause settlement and will not anchor soil reinforcement materials properly. “When the soils settle, the low spot attracts water, the water flows in there and creates a swimming pool effect,” Gravier said. “Now you can have a blow out, or the wall can tip over forward or backward.”

A proper drainage system within the wall and at the site is key. Wall sites should be graded to avoid water flows, concentrations or pools behind retaining walls. If the wall is not drained, Dean said, the water will decrease the friction resistance of the soil.

Setting up screed rails will help contractors keep the retaining wall level, Nelson said. Some level the wall block by block, which makes the process more difficult, he added.

In addition, geosynthetic soil reinforcement, or geogrids, can help reinforce soil backfill, Gravier explained.

Soil type is another important consideration in retaining wall installation. “Clay is more difficult to build walls into than sandy soils,” Gravier said.

Additional training may help contractors boost their installation know-how. For example, the University of Wisconsin, Madison, and Auburn University offer soil engineering courses. Also, the National Concrete Masonry Association offers a segmental retaining wall installation guide.

BACK TO THE FUTURE. Expect more retaining walls to be installed because they work well and are cost effective when they are built properly, Dean said. “The industry is really going to continue to grow, but we have to convince the design community that we will build retaining walls to specs,” he said.

Some new wall construction materials may debut – more designer blocks and possibly an alternative to chemically treated wood, Gravier forecasted.

Despite better building blocks, Gravier said there will be no automation of installation tasks. “The small, modular pieces give you so many advantages over a large panel brought in by a crane,” he said. “You are probably still looking at getting out in the dirt every day.”

The author is Associate Editor of Lawn & Landscape magazine.

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