Residential Retaining Walls

Nov. 1, 2009

In days past, cast-in-place concrete was the basic option for retaining walls-not always pretty, and not always effective for the long-term, but useful for at least awhile.

These days, the options are virtually limitless, with vendors offering a plethora of products, many of which are designed for unique situations.

For engineers and contractors, the challenge is often selecting from among the wide range of products available to select the one that will best meet the needs of the environment, both physically and aesthetically. And that’s not always easy.

Tacoma, WA: Residence
When the new owners of a home in Tacoma, WA, saw their backyard, they knew it had potential. The yard, about 85 feet long, had about a 45-degree slope.

The homeowners contacted Tacoma-based Ostrander Enterprises. When owner D.J. Ostrander saw the property, he, too realized its potential. “We terraced the whole back of the property by building three retaining walls and creating three flat surfaces,” he says.

The first level was a sports court at the bottom of the property. Ostrander went in and excavated the bottom 50 feet of the property, then did a “cut and fill” for a wall. “In other words, we cut back 50 feet to create a flat playing surface for the sports court,” he explains. Ostrander built an 8-foot wall, 2 feet of which were buried. He then filled behind it.

Next, for the middle section, he created a flat surface for a lawn area where the kids could play. “We cut back another 25 feet, and then created a 25-foot level lawn and playing area,” he states.

The final 10 feet or so included a wall that connected to the homeowner’s patio, so they could walk out into their backyard onto a flat surface.

For the walls, Ostrander used GravityStone MSE (Fat Face) from WestBlock Systems. Applications for the product include commercial retaining walls, residential retaining walls, bioswales, and retention ponds. When used with geogrid, the product allows for structures up to 25 feet high.

“WestBlock System blocks are easy to work with,” says Ostrander. “We did corners and round corners, and we used some of their corner blocks. The blocks also allowed us to do a nice radius.” Ostrander also used a 4-inch cap on the top of each wall. The caps, which were glued down, stick out about an inch over the wall, which gives it a nice little shelf.

Because these were engineered walls, Ostrander put 7-foot lengths of geogrid mesh in place. “At every 2 feet, we had to cut back,” he explains. Crews also added ballast backfill (drain rock) behind the walls in 2-foot increments. “In terms of backfill, the engineer said we could use most of what was onsite,” he continues. “We used inch-and-a-quarter rock for drainage backfill behind the wall and also imported some backfill.”

With the walls complete, Ostrander built a ramp down the side of the property from the house to the bottom, so family members could walk from the house to the sports court. The ramp connects all three levels. “Since there aren’t any steps in it, it can be used for walking, wheelbarrows, lawn mowers, and easy access by elderly people,” he explains.

Challenges? Fortunately, there weren’t many. For example, even though the project was built during the winter, weather wasn’t a problem. In addition, crews were able to complete the project in about four weeks.

“The city had some height restrictions on walls, but we were able to accommodate those,” he continues.

So what were the challenges? One was water. “There was a spring that percolated through the surface at the bottom of the property,” states Ostrander. While direct winter weather-that is, potential downpours of rain during the construction process-wasn’t a challenge, the fact that it was winter when the project was being built, and that the project was in the Pacific Northwest, meant that there was more water than usual. “At first, we had no idea where the water was coming from,” he admits. “However, once we excavated, we found the crevice in the earth and the stream that came through.” Initially, Ostrander set up pumps to pump the water out.

Then, after excavating and finding the source, crews were able to contain it. “Behind the bottom wall, we installed a drain,” he says. Crews excavated a 3-foot-by-4-foot trench behind the 2-foot section of the wall that was belowground, then installed drainpipe behind the wall. This caught all the water and moved it to the greenbelt that was next to the property. (A greenbelt is a land-use designation used to retain areas of largely undeveloped land.) “Now, the whole bottom of the property is dry,” he reports.

Another potential problem would have been access. However, luck was on Ostrander’s side. “For some reason, there was an asphalt driveway between the two houses that led to nowhere but belonged to the neighbor,” he explains. “This was the only access to the backyard.” Fortunately, the homeowner next door was getting ready to do a remodel himself, so he allowed Ostrander’s equipment to enter and exit via the unused driveway. “If it wasn’t for this, I don’t know if that that job could have been accomplished,” he concludes.

Cathlamet, WA: Residence
When Lemmie Rockford and his wife decided to retire in 2001, they purchased some property and a house on the Columbia River near Cathlamet, WA. The property sloped steeply down to the river. It provided a wonderful view, but little usable yard space. In addition, the steep bank was difficult to maintain, because it was covered with brambles and weeds. Rockford elaborates: “We had 70 to 80 feet of hillside in front of our house that was absolutely useless because it was so steep, and it was also eroding. In addition, it could almost be considered a hazard.”

By 2007, the Rockfords decided that they wanted to free up some space for a yard and a flat area where they could plant a garden and some fruit trees. They also wanted to prevent the bank from continually eroding.

Building a retaining wall seemed like the best solution. “We decided to terrace it and make some useful space out of it,” he says. Rockford and his wife happened to see some Redi-Rock retaining wall blocks at a landscaping yard one day while they were out driving. In learning more about the product, they found that the blocks were more suitable for building taller gravity walls than many other retaining wall products on the market.

While Rockford was impressed with the product, he wasn’t as impressed with the design that the landscaper had proposed. Instead, he contacted Redi-Rock directly, and the company put him in touch with the Puget Sound Precast in Tacoma. “They were great to work with,” he states.

Being a former pipeline construction company owner, Rockford was no stranger to construction projects, so he elected to build his own wall with the help of a small crew. The project began in 2008 and was completed in mid-2009.

To prepare the site, Rockford poured a 3-foot-by-3-foot footing for the bottom tier of blocks that would be placed along the beach. For the footings in the other tiers above the bottom one, he designed a 2- to 3-foot-deep leveling pad of crushed rock that he purchased from a local quarry and that he was able to compact to make a suitable leveling pad. “For these tiers, I had a large compactor, and I dug down about 3 feet and compacted the crushed rock in,” he explains.

Rockford used Redi-Rock 60-inch base blocks for the first tier and 41-inch retaining blocks for the tiers above. There are actually three terraces, which separate the tiers by about 10 feet. “I filled the bottom two terraces with topsoil, and then filled the top one with crushed rock and use it as a backyard,” he states.

He backfilled behind each wall with 6 to 8 feet of crushed rock to allow for adequate drainage. “I also put drainage pipe behind each wall and 4 to 6 feet of drain rock,” he adds. He purchased the crushed drain rock backfill from Burns Gravel in Cathlamet, as well as the crushed rock for the top tier.

Rockford then used Redi-Rock 28-inch top blocks at the top of each tier. A staircase cuts through the walls and provides beach access.

So far, the project has been a success. “We went through our first winter, which was very wet, and we didn’t have any settling at all, and no water backing up,” he says.

In addition to the retaining wall, Rockford also decided to create a Redi-Rock archway, the first of its kind. He took his sketch to Puget Sound Precast, which created the blocks using Redi-Rock 28-inch column caps. Blocks were grouted together to form an arch.

Rockford finished the wall in the summer of 2009 and plans to start landscaping this fall. “In addition, I will be planting some fruit trees and ornamental shrubs next year,” he concludes.

Pickwick Dam, TN: Residential Resort Area
Pinnacle Design/Build Group of Cumming, GA, completed a retaining wall project in 2008 at The Retreat at High Point, a resort area near Pickwick Dam, TN. The resort, which is lakefront residential property, is near Pickwick Lake on the Tennessee River, just north of the Mississippi state line.

“They had a severe change in grade from the property entrance to the shoreline,” says Joe Harris, P.E., president of Pinnacle Design/Build Group. To make the best use of the land and get as many properties on it as he could, the developer needed a retaining wall. In addition, the wall would allow him to build closer to the water’s edge.

The biggest challenge was that, because of the wall height-36 feet-Pinnacle had to cut into a hillside. “We had to minimize the amount of geogrid embedded into the hillside,” Harris explains. “As a result, we did soil nailing, which is drilling in anchors to provide a stable hillside embankment.”

Pinnacle used a Tensar Mesa Retaining Wall System (a segmental block retaining wall) in conjunction with a soil-nail wall. This combination provided an aesthetic retaining wall that the developer needed in order to sell the high-end lakefront lots.

Creating the soil-nail wall involved a top-down excavation. The soil nails are installed in a grid pattern, stabilizing the soil. The stabilized soil helps retain the less-stable material behind it. Once the first cut is made, shotcrete is applied and the nails are drilled in and grouted. The process is repeated in sections down to the subgrade.

“We connected the Mesa Retaining Wall System to the soil nail wall to get the large retaining wall into the bottom of the property by the shoreline,” states Harris.

Pinnacle selected Tensar because the company had used it in the past in conjunction with soil nailing. In addition, Tensar geogrid lent itself well to the connection between the soil nail wall and the block facing.

Beyond that, according to Harris, there we no challenges. “We were able to use onsite backfill soil, and we used standard drainage,” he concludes.

Watertown, CT: Drainage Improvement Project
When Guerrera Construction Co. of Oxford, CT, began work on the Turkey Brook Drainage Improvements project in Watertown, there were several things that needed to be done. “We did several bridges,” reports Joseph A. Guerrera Jr., president. “In addition, we did about 3,500 feet of channel improvement.”

At one point along the brook, Guerrera ended up replacing an existing culvert on a road with a box culvert. There was also a small wing wall that came off the culvert. “However, there was a house in close proximity to the river, so we needed to build a retaining wall to retain the earth and protect the foundation,” he explains. In sum, a retaining wall needed to be designed to retain the bank of Turkey Brook behind existing homes, and to prevent erosion.

“We went down, poured the footing, and then built the wall,” says Guerrera. There was a short distance from the face of the wall to the existing foundation. “We were actually lower than the foundation footings, so we had to protect that,” he adds. As a result, the wall was designed by filling some of the units with concrete (pouring concrete behind the units), and then continuing to build the wall in standard fashion with stone inside the units and with some stone backfill, as well as with some pervious backfill.

There were several challenges with the project, according to Guerrera. Fortunately, soil was not one of them. “The soil was good,” he states. However, a major challenge was that crews were working in a flood-control area. “When we did the Turkey Brook Drainage project, we bypass-pumped the river around us, so we would we able to work somewhat in the dry,” he notes.

Second, there were space constraints. This meant that the company would need to select a retaining wall system that would allow crews to work in an area that was very near to the closest home, as well as some other nearby homes, with a minimal amount of disruption. A cast-in-place wall could have been used in this application. However, it would have taken significantly longer to install, and the installation would have caused some disruption to the neighborhood.

Along the same lines, the distance between the wall and the house foundation was so small that crews couldn’t install any type of geogrid. “Since there was no room for geogrid, we had to pick a wall that could be built without geogrid,” explains Guerrera. In the past, the company had dealt with V.I.P. Supply, which was its Arrow Concrete Products supplier. “V.I.P. was aware of the project we were doing, and they told us about a new type of Arrow wall that they felt would be perfect, and it worked out well,” he states. Arrow is a licensed manufacturer and distributor of the Stone Strong retaining wall system.

A third challenge was that the project was being built in the dead of winter-January and February. This was another reason a precast solution made more sense than trying to use a cast-in-place solution. Still, the weather posed problems. “When the blocks were delivered, they ended up with snow and ice on them from the night before, so we ended up having to heat them to melt the snow and ice before we put them in place,” states Guerrera.

In addition, each night, the crew covered the blocks, as well as the wall as it was being built, with insulated blankets, to keep the pervious structure backfill from freezing. The one bit of good news was that the project was able to move along so quickly that it took only a few days to build.

Drainage? This wasn’t a problem. “It was all in the design,” he explains. “The water drains through the stone at the joints.”

In terms of results, the appearance of the completed retaining wall provided an aesthetically pleasing feature in this residential and commercial area along Turkey Brook. “In fact, the town and the engineering firm loved the wall so much that we ended up building another wall at the senior center to retain a parking lot,” he concludes.

Santee, CA: Residential Community
The Sky Ranch Village in Santee, CA, is a collection of four premier communities in a mountainous area northeast of San Diego. Along with a view overlooking the city, the mountain terrain also provided the developers, Lennar Homes, with a challenge-terrain that consisted of 1.5:1 slope ratios, large boulders, and limited access points.

To create buildable land on the mountain, engineers designed a system that would use Keystone segmental retaining walls (Keystone Compac II) and Vista DSM (dry stack modular) block walls in combination to build approximately 100,000 square feet of walls throughout the development, with one continuous natural stone aesthetic throughout.

Initially, the developers wanted a random-pattern, natural-stone aesthetic for the project. However, the original plans were already drafted with cast-in-place and masonry walls. The wall contractor, Geogrid Retaining Wall Systems of Vista, CA, redesigned the project using Keystone, featuring one size of block, to provide a more cost-effective solution.

However, to combat the steep slopes, the use of Keystone walls exclusively would require extensive use of geogrid for reinforcement. This posed a challenge, because property lines ended up conflicting with the reinforced zones for some of the proposed walls, which would not allow sufficient room for geogrid installation. Because the originally desired random-pattern look was also important, Geogrid proposed integrating Vista DSM walls, which do not require geogrid, with the Keystone walls.

“We value-engineered the project,” reports Mike Stevenson, chief executive officer of Geogrid Retaining Wall Systems. “It was all designed with soil nails and masonry blocks. We met early on with the city and Lennar Homes, and value-engineered the use of Keystone and Vista DSM.”

He continues, “Because of the steep slopes above the walls, we couldn’t use geogrid reinforced retaining walls on portions of it. This lent itself to a new product called Vista DSM blocks, a system that uses a concrete footing.”

Vista DSM is a dry stack modular block system that features vertical rebar for footing construction, horizontal rebar throughout the wall, and concrete grouting for reinforcement. This combination eliminates the need for traditional retaining wall batter and geogrid.

RCP Block and Brick produced the customized Keystone Compac and Vista DSM units, using a specially created fluted mold design and block color. The fluted design allowed the use of a single-sized block and the originally requested random-pattern look.

“We were able to match the look of Keystone Geogrid walls with the Vista DSM blocks,” states Stevenson. “It was a challenge to blend the two types of walls, though.” Because the developers required a specific look, Geogrid Retaining Wall Systems designed a new block specifically for the project, which it calls the “Sky Ranch Blend” block. “It is unique to that site,” Stevenson says.

The integration of the two wall systems, in addition to the site challenges-steep slopes, boulders-required a collaborative engineering effort by Red One Engineering (Keystone Walls), ZCS Engineering (Vista DSM walls), and various other consultants involved in the overall soil testing, grading plan, and other efforts.

The first major engineering effort was to establish those areas in which it would be appropriate to build a Keystone wall and those areas in which it would be appropriate to build a Vista DSM wall. The presence of property lines usually dictated a Vista DSM wall.

When Geogrid began building the first walls on the project (which were the Vista DSM walls for the model home lots), crews had to deal with large boulders, 10-15 feet in diameter. Geogrid called in ZCS Engineering for consultation. ZCS determined that it would be too expensive to excavate the boulders and then import the compacted engineering fill for the gaps produced by the excavation. (The boulders extended 2 to 4 feet deeper than required for the footings of the Vista DSM wall.) In addition, the soil around the boulders was sandstone. It was determined that the footings for the Vista DSM wall could be poured in and around the boulders. Geogrid used epoxy to set the wall into the boulders and also used epoxy around the rebar that would run through the footings. In sum, the boulders and wall were turned into a single monolithic structure.

The Vista DSM walls vary from 2 to 21 feet and feature three major designs: level backfill and no surcharge, building surcharge, and parking surcharge with a 2:1 upslope.

The Keystone walls reach heights of between 28 and 30 feet, with exposed heights of approximately 20 feet. These walls are designed to withstand a fairly aggressive combination of terrace walls with slopes above them. Some of the Keystone walls required geogrid, reaching 35 feet in length.

There are a total of 75 walls, almost 11,000 total linear feet. Of these, 43 are Keystone walls (almost 8,000 linear feet), and 32 are Vista DSM walls (just over 3,000 linear feet.)

“As a result of everything we did, we were able to overcome both engineering and aesthetic hurdles,” states Stevenson.

The results are impressive. In fact, the collaborative efforts resulted in the integration of two different wall systems so that it is almost impossible, to someone other than an engineer, to discern one wall system from the other.

Minnetonka, MN: Residential Development
While some engineers and contractors face challenges in designing and building retaining walls due to the physical environment, the main challenge in a 2007 project at Ridgewood Heights, a high-end residential development, was due to the regulatory environment.

The challenge related to gaining approval for a new type of retaining wall technology from Millenia Wall Solutions. “This was one of the first Millenia walls we did,” recalls Michael Johnson, P.E., president of Civil Solutions, an engineering firm based in Bloomington, MN. “Our goal was to introduce the wall to the “˜retaining wall society,’ since this area is considered the “˜retaining wall capital of the world.'”

So, while there were a few minor engineering and construction challenges, such as maneuvering around all of the existing trees on the site, the main challenge wasn’t in the construction of the walls themselves, but in getting them approved in the first place. The sequence of walls was designed to hold back soil to create space for three or four houses.

“It took awhile to get people used to the product, which is made with poly-resin,” Johnson explains. “It wasn’t easy to get approved, especially because city engineers tend to be conservative and often aren’t comfortable with something that is new.” In addition, since part of one of the walls was very close to a foundation on one of the homes, there was concern about the potential for wall failure, which could jeopardize the foundation.

“Based on my experience with the company and its product, I was able to assure them that it would work,” he continues. “However, it still took about six months to get it approved.”

Once approved, it took a while for the contractor to get comfortable with the technology, too. One difference was the weight of the blocks. Typical reinforced concrete blocks are 70 to 120 pounds each, according to Johnson. “Millenia blocks are seven to eight pounds,” he explains.

However, everyone ended up being pleasantly surprised and satisfied with the durability of the walls. One interesting event that helped allay concerns was that, when the contractor began to run his machinery behind the wall, it moved a bit, but then snapped back in place. The blocks have a Smart Lock feature on the sides, which locks the corners of the blocks together. In addition, the units are locked together from top to bottom. “When they were rolling the area with their compaction equipment and drove up on top of the wall, the wall seemed to start to move outward a bit, but it then snapped right back in place,” states Johnson. “This turned out to be another selling point.”

About the Author

William Atkinson

William Atkinson specializes in topics related to utilities and infrastructure.