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Retaining walls have come a long way from the plain gray façade.
By Roberta Baxter
Retaining walls have been essential in the civil engineer’s toolbox for many years. With the most convenient land already developed, marginal areas sometimes need retaining walls to become usable. In the past, the walls were often constructed of plain, gray concrete. Now, blocks in various colors, textures, and sizes, as well as new construction designs, provide a multitude of choices when a retaining wall is needed.
Decisions in designing and building a retaining wall depend on the soil, property lines, loading on the wall, and the desired look. Products are available for gravity walls, interlocking blocks with or without geogrid reinforcements. Blocks can be colored to blend in with the surroundings or to bring a touch of texture and color to the project.
Design Tips
Theis Engineering LLC, founded in 1997, designs all types of retaining walls. The company has focused its projects in the Pacific Northwest and most recently in Texas. From his experience with retaining walls, owner Tim Theis, P.E., offers these design tips:
- Water is generally the number one reason for retaining wall failures. Make sure you have provided adequate drainage behind the wall by using a generous amount of free-draining, well-graded gravel, and install a drainage pipe at the base of the wall. If the onsite material does not meet these criteria, imported gravel must be used as backfill. Theis stresses that good drainage is essential even in areas that don’t receive high amounts of rainfall, because it doesn’t take much water to destroy or weaken a retaining wall.
- Make sure you know what type of soil is on the site. Having a thorough geotechnical report is crucial. Forces generated by tall retaining walls may require high bearing pressures, and overexcavation below the walls may be needed to obtain proper soil bearing conditions.
- In the last few years, many areas in the country have added seismic loading as a requirement to the design of retaining walls. The seismic loading requirements should be included in the geotechnical report.
- The design must also take into consideration outside loading on the wall, including existing or planned nearby buildings or roadways. Civil engineering plans should indicate all the surrounding influences and provide a grading plan, which should also indicate the topography of the site.
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Photo: Theis Engineering |
| Construction of a 45-foot-tall by 850-foot-long wall that cost more than $1.5 million |
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Photo: Theis Engineering |
| Working to stain and carve the wall in order to enhance the overall aesthetic look |
One large project that Theis Engineering designed involved a retail site in Pierce County, WA. The main anchor of the site was a Safeway store. The site had to be cut into an existing hillside, which required a 45-foot-tall by more-than-850-foot-long wall to create the planned building pad. An added challenge was an existing 8-inch high-pressure gas line, buried 4 feet below the surface and 20 feet back from the planned wall location. Based on the height requirements for the wall and the location of the existing gas line, a soil nailed wall was chosen for the site. There was concern that electrolysis would occur between the existing gas line and the soil nails, so tests were conducted on “sacrificial” nails to ensure that this was not a problem.
This type of wall is built from the top down, which is contrary to usual retaining wall construction. After crews excavated down approximately 6 feet, a row of soil nails was installed, geocomposite vertical drain strips were attached to the face of the cut, and reinforcement was added to tie the soil nails together and provide strength to the shotcrete face. Shotcrete was added to provide a temporary wall (additional reinforcing and shotcrete were added after the temporary wall was installed to create the permanent wall). The process was repeated in stages.
The project design called for an 8-inch permanent shotcrete wall that was carved and stained to enhance the aesthetic look. The completed wall cost more than $1.5 million.
Other walls on the site were constructed using the GravityStone MSE and GravityStone Modular components from WestBlock Systems of Tacoma, WA. Theis says the GravityStone was used for walls under 12 feet in height. On this site, the GravityStone MSE system was installed with geogrid tiebacks where there was adequate room to excavate for the geogrids. Where excavation space was limited, the Modular system was used. The face of a Modular system looks the same when completed, but it uses precast interlocking blocks behind the face blocks instead of geogrid to serve as tiebacks. The spaces between the precast blocks are filled with gravel. The system is a good choice for sites that require both fill- and cut-type retaining walls due to the flexibility of the MSE and Modular systems.
Oregon Reinforcement
Blocks, rebar, and geogrid are not the only stabilizing ingredients for a retaining wall. In most cases, vegetation that strengthens the structure is desirable. That is the specialty of Agrecol Corporation of Madison, WI.
The Gerber Knife Company in Troutdale, OR, needed help with a nearby stream. An increase in impervious surface area from upstream development and runoff from I-5 had caused the stream to become flashier. A company parking lot and a 40-foot corporate sign were in jeopardy from the stormwater undercutting their footings.
Over the years, several attempts had been made to fix the problems. Fill materials of several types were found on the site, and failing geotextile wraps were also discovered. The geotextiles had been installed with a poor understanding of soil mechanics; the soil at the site was mostly hard clay.
Mark Doudlah, president of Agrecol Corporation, recalls that the company wanted to ensure only clean water was flowing from its site into a nearby creek. The site would be closely monitored to see if oil from the parking lot was reaching the creek.
Because of the urgency, the company was able to get emergency permits in about three months from the Army Corps of Engineers. A wall constructed of Envirolok—sand and soil bags—was installed in April 2007.
The bags were woven into place, providing immediate stability. The woven structure resembles a basket. Three levels of bags were placed below the level of the stream channel, rising up against the hill of the parking lot. Horizontal cinchers were added as the wall went up, and spikes were installed to prevent shear.
Plants were put in place along with the bags, and the whole structure was hydroseeded. As the plant roots grow through the bags and into the soil, they provide strength to the overall structure. Doudlah says that a wall constructed in this way is at its weakest at installation and gains strength with the emerging vegetation. The plants used were native species whose roots will reach down as far as 20 feet, and they require no added nutrients to survive. Watering after installation is required, but can be decreased once the plants are established.
Envirolok bags can be filled with onsite materials, imported fill, or other materials. For the Gerber Knife Company project, the top two courses of bags were filled with fired clay, similar in texture to cat litter. The fill material will absorb oil and other pollutants from the parking lot as runoff passes through, leaving clean water to enter the creek. Soil microbes in the other bags will help detoxify the stormwater runoff.
Eventually, the bag materials will photodegrade with exposure to sunlight. By that time, the vegetation will be established enough to stabilize the wall. Doudlah notes, “We don’t want to see the bags forever.”
Florida Beach Protection
Envirolok was also the product of choice for a project with severe beach erosion. A nature preserve near Apollo Beach, FL, had lost approximately $300,000 worth of land due to shoreline erosion since January 2007. Riprap had been installed on a neighboring strip of beach, and the erosion had increased at the unprotected area.
Forty feet of Envirolok bags were placed along the 8- to 10-foot vertical drop to the beach. This wall was tied into the riprap, and the bags were planted with native species and salt-tolerant grasses.
The unfilled bags can be shipped and then filled onsite or nearby, reducing much of the shipping cost of other materials. The total system, including bags, plants, and hydroseeding, runs about $20 to $30 per square foot of wall face. According to Doudlah, “The bags have done the thing that native vegetation can’t. They containerize the soil.” The combination makes a secure and beautiful wall.
Minnesota Condos
When ReCon Wall Systems Inc. was in the process of opening a production plant for its blocks in Cannon Falls, MN, it encountered a local client. At a planning meeting, company officials met Erich Pearson, president of Twin Rivers Development LLC, who had a condominium project in the works in the area. The site required a retaining wall but had many challenges, which included:
- The structure would be a 20-foot wall only 10 feet inside property lines
- Soil at the site was loose sand
- It proved impossible to receive easements for geogrid installation or even for passage of construction equipment from the site’s next-door neighbors
- The condominium would stand only 15 feet from the finished wall, and the basement level would be 12 feet below the face of the retaining wall
- The wall needed to be installed before condo construction, but the design would have to ensure the wall remained stable when the basement was excavated
Along with consulting engineer Mike Johnson of Civil Solutions Group LLC, ReCon president Stan Hamilton was able to design a two-tiered wall with a section of vegetation for aesthetics. The leveling pad was 2 feet of concrete to provide strength for the wall during construction of the condominiums. H-piles were installed just inside the property line. The lower-tier wall was built with geogrids extending back 10.5 feet to the H-piles. The blocks used were interlocking precast concrete manufactured by ReCon Wall Systems. This first wall was 12 feet tall. The second wall was set back 6 feet from the property line and was constructed with ReCon Wall Systems 45-inch-deep units, making an unreinforced gravity wall. The double wall is 600 feet long.
The appearance of the wall was important for this project because condominium residents view the wall from their patios. Developer Pearson was very pleased with the end result. “As design and construction unfolded, I gained a new appreciation for the complexity associated with this retaining wall,” he says. “The design presented by the ReCon team met all of the challenges. I could not be happier with the result.”
Detention Pond in Missouri
A commercial office park near the intersection of I-70 and the Little Blue Parkway in Independence, MO, was the site of retaining walls designed and installed by Thiele Geotech Inc. of Omaha, NE. Dan Thiele, president of the company, says the project’s goal was to place a needed detention pond in a small space. To save space and make the site more attractive, retaining walls fit the bill. The site had a small amount of slope, so a two-tier wall with a serpentine line was designed.
A crushed limestone-leveling pad, using onsite rock, was constructed. Then precast modular blocks from Stone Strong Systems of Lincoln, NE, were placed at the first course along the 3:1 slope. Onsite limestone was also used for backfilling.
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Photo: Theis Engineering |
| Construction of a retaining wall in Pierce County, WA |
The 6-foot-high upper tier had to support the roadway embankment and a bike trail. It would have required 60 horizontal feet to make the grade change without the wall. With the wall, the developer preserved a 75-foot by 100-foot area that can be sold for development.
The precast blocks from Stone Strong weigh 6,000 pounds and are put into place using construction equipment. Interior spaces in the block can be filled with aggregate to provide drainage, and inlet and outlet pipes can also be installed. The face of each block is 24 square feet, the largest block on the market. The blocks can be stained to match surroundings. The system can be used to construct retaining walls up to about 15 feet high without geogrids or tiebacks.
California Communities
Lennar Homes had development plans for four communities to be built on Rattlesnake Mountain in Santee, CA, on the northeastern side of San Diego. The development, called Sky Ranch, was on a challenging site because of the mountain setting with slopes of 1.5:1 and large boulders across the area.
Designers knew that a series of retaining walls would be necessary to provide usable land for the development. Geogrid Inc. was chosen as the wall contractor. The developers wanted a natural-looking stone, and the original design called for cast-in-place and masonry walls. Geogrid engineers redesigned the plan using Keystone segmental walls and Vista DSM dry stack modular blocks from Contech Earth Stabilization Solutions Inc. The whole design was more cost-effective than the previous plan and still had the natural look that the developers desired.
A challenge of the project was that in some places, extensive use of geogrids would not be possible because of property lines. For this reason, the Vista DSM blocks were an ideal choice. This system includes vertical rebar reinforcement for the footings and horizontal rebar throughout the wall. The dry stack construction eliminates the need for geogrid, so it can be used where space is at a minimum. In this project, 32 Vista DSM walls were constructed—a total of 23,049 square feet. The walls vary from 2 to 20 feet in height.
On other parts of the site, the Keystone Segmental Wall system was used, with extensive geogrids for reinforcement. The Keystone block chosen was the Compac II. Forty-three walls were constructed with these units—a total of 81,592 square feet. Keystone walls in Sky Ranch rise up to 30 feet, with 20 feet of exposed wall.
The two systems were compatible because the blocks were about the same size and color. The integration of the two systems provided flexibility and gave a coordinated look to the walls that would have been impossible with the mix of cast-in-place and masonry walls called for in the original plan.
A stumbling block for the project showed up in the form of huge boulders, 10 to 15 feet in diameter, that could not be moved because of the expense of excavation and fill dirt. Another solution had to be found. The boulders reached 2 feet lower than the footings of the Vista DSM wall, and the surrounding soil was sandstone.
Engineers solved the boulder problem by pouring the footings for the Vista DSM wall around the boulders. They used epoxy to cement the wall and rebar into the boulders, making the boulders and wall one unit. They also drilled rock dowels into the boulders and put the rebar on the dowels. With epoxy binding the entire structure together, the boulders have been included in the wall.
Montana Lake
Protecting shoreline at Flathead Lake, MT, near Glacier National Park, was a recent project for contractor Diversified Materials & Construction of Missoula. The area is under control of the Confederated Salish and Kootenai tribes. Tribal regulations specify that concrete retaining walls should be used only as a last resort; the preferred application is wood or riprap.
Homeowner Blaine Wright wished to replace a rotting wooden retaining wall in front of his residence. As part of the wall, he wanted stepping places so a person could climb down the wall to the lake. He was looking for an alternative to concrete and to wood, which has a tendency to deteriorate quickly. Tribal rules also require that walls along the lake reduce wave speed as waves break against the wall; concrete walls usually accelerate waves, so gussets are typically installed with the wall.
After seeing a brochure from Redi-Rock Inc. of Charlevoix, MI, Wright investigated the company’s products and chose to work with Diversified on a design for his wall. The final design called for placing planter blocks every 16 to 20 feet. The blocks are usually filled with soil and planted, but for Wright’s wall they were filled with concrete and provide steps down the wall. The protuberance of the planters’ blocks also met the requirement of slowing wave speed after it hit the wall. The weight of the Redi-Rock blocks allowed the 9-foot-high wall’s construction without a need for geogrids or tiebacks. Wright is pleased with the outcome: “This job worked out even better than I had hoped.”
Retaining walls will continue to be an important part of many construction projects. The ability to make a steep hillside into usable land or to protect a shoreline or slope makes retaining walls’ design a crucial ingredient. The plethora of products means that the proper fix can be found for most any project.
Frequent contributor Roberta Baxter specializes in science and technology topics.
EC - Products and Services Directory 2009
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