Engineering aspects of retaining wall projects

By Tara Beecham

The assortment of colors and textures that can be used in designing retaining walls today adds choices and allure for clients and engineers alike. Features such as gaps for plantings that can be incorporated into the completed wall can be pleasant diversions from the complicated engineering aspects involved in stabilizing slopes and preventing site erosion that carry on unseen.

As engineers use retaining walls to resolve the environmental challenges of their sites, they have adapted solutions as creative and complex as the walls' components themselves.

Highway Challenges, Railway Blues
Besides taxes, there are few things as unstoppable in life as the workday commute. As a retaining wall is installed beside a roadway, traffic often must be allowed to continue, posing yet another challenge for project designers.

"When the adjacent traffic is on the front side—the low side—of the retaining wall, an important aspect of both design and construction is to have a positive barrier between the adjacent traffic lanes and the work zone," notes Gregory Allen, an assistant bridge engineer with the Oklahoma Department of Transportation. "This protects the traveling public from potential falling debris and construction equipment interaction. The construction personnel are protected from the dangers of errant vehicular traffic.

"When the adjacent traffic is on the back side—the high side—of the retaining wall, an important aspect of the design is the stability of the slope of the excavated soil. Proper design and the maintenance of this slope during construction prevents slope failures that could place the safety of the traveling public in jeopardy."

PHOTO: NEEL CO.

When water threatens a roadway, retaining walls are often installed to suppress the swells. For example, last year, English Construction in Lynchburg, VA, was involved with a project to widen Interstate 95 at the Atlee-Elmont interchange. Approximately 5,800 square feet of the Neel Co.'s T-Wall panels were used at the location just north of the city of Richmond.

"The purpose of the wall is to protect the bridge where the river runs under the bridge and then turns," explains John Jordan, senior vice president of English Construction. "This wall was to protect the bridge approach from the possible ravage of the river."

Scour protection was required to keep the ChickahominyRiver at bay and became part of the Springfield, VA–based Neel Co.'s design package with English Construction. This protection is placed at the base of a retaining wall to prevent water from eroding the foundation at the area that needed it the most. Designing the wall's support required specific drainage considerations.

"This had extensive backfill with crushed aggregate," says Jordan. "It was also placed on a grid of fresh aggregate."

Economics played a primary role in the design of the retaining wall. "The original design was for a piled-panel wall. It was precast, steel H tiles with precast concrete panels," says Jordan. "We offered the State of Virginia the option to choose T-Wall with scour protection in lieu of it. We could save ourselves and VDOT [Virginia Department of Transportation] some money. We made a valued engineering proposal to VDOT to replace their contract-designed wall with T-Wall. They accepted the proposal and approved the value engineering proposal."

Although I-95 traffic had to be maintained throughout the project, it merely affected how the retaining wall would be installed, not the wall's design.

There are engineering challenges specific to erosion control along railways, as well. "Perhaps the most recent and significant use of Lock+Load is for the one-stage construction of railway embankments over soft foundations," says David Ash, president of Lock+Load Retaining Walls based in Vancouver, BC. "Previously, these situations were dealt with by using more expensive and time-consuming rigid concrete designs or two-stage reinforced soil and concrete panel construction methods. The large train loads and permanent nature of these structures were well serviced by the quality of the Lock+Load product combined with the flexibility of the system."           

Settlement can occur when railway fill embankments are built on compressible foundation soils, he explains, noting that concrete piles and pile caps used with columns supporting beams solved this problem in the past, albeit with high costs for the customer.          

"Alternatively, a costly and slow two-stage construction operation is possible, involving the construction of a reinforced soil embankment, giving it time to consolidate the foundation soils and then covering the surface with durable precast concrete facing panels," says Ash. "Due to Lock+Load's strong concrete components and independent installation the construction of permanent, flexible MSE [mechanically stabilized earth] retaining embankments can be completed in one stage for use over soft foundations."     

This was the case when Lock+Load was used to build a reinforced soil embankment for the Tacoma Light Rail Transit System. "The project involved an embankment approximately 700 feet long and 50 feet wide to support two rail lines. Engineers excepted settlement of about 1 foot in the deepest section," explains Ash. "The project progressed as quickly as the foundation soils could be loaded with the embankment fill. Caution was used placing fill to limit any excessive hydraulic pressures to occur within the foundation soils, during its consolidation." This process allowed for a quicker and less rigid instillation than if it had been completed using a two-staged system.  "The light weight and strength of the concrete components make the product price competitive with modular block," continues Ash. "It also has the structural capacities of panel wall systems, which make it a permanent solution."

Adjusting to Poor Soil
Protecting soil that doesn't exactly work with you can pose problems when installing a retaining wall, as was the case with a project at theTres Amigos Medical Center in Puyallup, WA,which needed retaining walls to support the location's parking lot.

PHOTO: WESTBLOCK SYSTEMS

PHOTO: WESTBLOCK SYSTEMS

"When they started digging it, it was just a mess. They had to go down really deep," says Ken Headley, who provides engineering assistance for WestBlockSystems in Tacoma, discussing the sloppy, wet soil conditions. "I think that's one of the reasons that they went with GravityStone modular. That system could stay narrow and work in that particular soil condition."

Because it was adjacent to an existing facility, explains Jim Hammer, president of WestBlockSystems, the project required two types of retaining walls, including a traditional MSE and a narrow modular system to conserve space.

 "These are segmental retaining walls," he says. "The geogrid is actually providing the structural support.GravityStone is different compared to the others because we can build in two different manners. We can use geogrid, and Tres Amigos has some of those MSE walls. But we can also build with a modular methodology, which doesn't require geogrid."

The system allows builders to create tall walls and also to create cells behind the base of the block—an advantage, Hammer explains, because when "it's narrow you don't have to move as much dirt. MSE structures are wide; modular structures are narrow. If you already have a hill there, you want to use modular so you don't have to remove the hill to rebuild."

The project's back wall, designed to hold the back bank with the parking lot, ranged from 12 to 16 feet high, while the front wall, designed to contain water, had a maximum height of approximately 5 feet. Both walls were designed to fit the architecture of the area because of their visibility.

"You have a street and sidewalk on one side and an embankment on the other side," notes Hammer, adding that this meant this area's system also had to be narrow. "The infiltration pond collected the runoff and that wall was built in the MSE style because there was more width. There's a culvert that goes through the block that allows water to run into the biosoil."

 Drainage issues were also a concern with the Tres Amigos project. "The pipe can run through either system. Where you have water against the wall, what you want to concern yourself with is the type of material you fill in with. If you've got the wrong material, that can create a problem in the reinforced areas," says Hammer. "They used aggregate in the reinforced zone. As the pond fills up, the water would soak through the base of the block into the reinforcement zone. If that's aggregate, that allows the water to drain properly and not create a structural problem."

Building Walls Near Waterways
In 2001, the City of Whitby, ON, Canada, contacted project engineers at RisiStone Systems, based in Thornhill, ON, to address a slope failure in a segment of Lynde Creek, which feeds into Lake Ontario. The area was adjacent to a residential development.

PHOTO: THE REINFORCED EARTH CO.

PHOTO: THE REINFORCED EARTH CO.

"The steep slope had eroded away and began sloughing, leading to slope failure of the natural embankment. The mandate of the project engineers was to find a solution to stabilize the embankment and prevent further erosion, while maintaining a high level of environmental sensitivity toward the creek and the natural wildlife," according to Tyler Matys, manager of engineering design for Risi.

After the site soils were studied, the final design integrated two types of wall systems into three wall structures for a total wall height of 9 meters, or 30 feet.

"Due to environmental restrictions, the lowest wall was designed as a live crib wall—essentially a crib structure built of cedar logs and backfilled with compacted soil. The concept was to provide a structurally stable—yet plantable—wall for erosion protection along the creek edge," notes Matys. "From the top of the creek bed, the total grade difference that required stabilization was approximately 9 meters. The live crib wall provided the lowest 1.5 to 2 meters of elevation, while the Dura-Hold segmental retaining wall system was used for the remaining 7-plus meters. For stability and overall aesthetics, a terraced arrangement was chosen for the upper Dura-Hold walls."

The walls, manufactured by Georgetown, ON–based Unilock, were chosen for both their versatility and stability, but also for the speed with which they could be installed, explains Matys, because the natural environment had to be restored before the area's fish began to spawn. The root systems of several mature trees also left little front-to-back depth available to excavators, and this is one of the reasons the crib system was selected.

"The Dura-Hold crib system allows for a reduced front-to-back depth when compared to typical geogrid-reinforced walls and can be backfilled entirely with free-draining, angular, three-quarter-inch clear stone, which requires minimal compaction. As the 100-year-flood level for the creek was determined to be above the top of this wall, the use of free-draining backfill was essential for good performance on tiers," reports Matys. "The Dura-Hold crib had proven itself to be an excellent performer in channel, harbor, and other water applications in the past, making it a logical choice. The crib wall was founded at the elevation specified in the global stability report approximately 3 meters behind the lower Dura-Hold wall and continued up to an exposed height of 4 meters.

 "The lower wall was designed as a geogrid-reinforced structure, utilizing a high-strength polyester reinforcement," he explains. "The greater depth of wall provided by the geogrid reinforcement was necessary to accommodate the significant loading of the upper wall behind it. A mild slope was set between the two walls, with a steeper slope above the upper geogrid-reinforced wall."

Sometimes it is the waterway, and not the residences that lie adjacent to it, that a company is hired to protect. The Lewis and Clark Landing Marina project on the Missouri River in Omaha, NE, required an unusual design format. "Eighty percent of our work is for the highway industry, but the most challenging projects are usually along waterways. There was a complex subdrainage system involved," says John Sankey, vice president of engineering at The Reinforced Earth Co. based in Vienna, VA. He explained that an inlet structure had to be included in the reinforced earth mass in the back of the walls. "Our most complex need is to incorporate our [design] around systems."

A scour was placed below the base of the 20- to 30-foot wall, which was in a cruciform shape, fitting together across the front and tied to steel strips in the back. Joints were covered with geotextile that could retain the backfill.

"If you get a flood condition and the waters recede immediately in front of the wall, but you still have a high water level in the backfill, the difference in the water level has to be analyzed," says Sankey. "Typically, our backfill is granular material. We don't allow any more than 15% fine. That's typical of any of our walls."

The wall is designed to handle friction between this granular backfill and the steel strips. "Strips lock in the granular soil," says Sankey. "Strips are an anchor to the panels at the front."

The area accommodated for private owners, explains Rich Allen, the regional engineer for the company's north-central United States division, who designed this project, and therefore appearance was important. He seems pleased with the result. "It's going to be a community-type gathering place," he says.

Creating Beauty That Works
When appearance tops the client's priority list, it is the company installing a retaining wall that must ensure the wall doesn't just look pretty but also performs its duty.

When a retaining wall is used for a residence, it often supports a small parcel of land around the home because of the property's slope. Because the owner will spend years looking at the work produced, appearance is of the utmost importance.

Picture then the planning involved when the land that must be supported surrounding your client's home is a mountainside. Developers in Gatlinburg, TN, had begun work on a project designed to be used as a mountain resort when they lost funding. The property was then sold to an individual who actually had to have a freeway-size bridge built to reach the large home planned for the property.

"They went in and they cut [into the mountain]. They had to build half the wall; it's holding up the entire side of the hill there and the house that will be on top of it," says Tommy Halcomb of Versa-Lok Retaining Wall Systems, based in Murfreesboro, TN. "The house has not been built yet."

Versa-Lok's weathered Mosaic segmented retaining wall system in a blended color was selected to support the mountainside, extending 35 feet in some places. Aggregate was placed behind the wall. "Water drained through the front of the wall in several different places," explains Halcomb. The majority of the drainage was designed to be diverted around the wall, with the rainwater redirected to the southeast corner.

"The big issue was the slope of the land behind it and in front of it," says Halcomb. "You just about can't walk on it, it's so steep. He [the owner] had to get the wall up so he could finish the bridge across to it."

PHOTO: RIDGEROCK

Appearance was a primary consideration for the owner, who selected the material that appealed to him first, and then allowed the engineering and design to progress. "He wanted something that matched the surroundings. It's visible from downtown Gatlinburg," Halcomb says of the wall, noting that the Smoky Mountains are a popular national park and that Gatlinburg is a huge tourist town. "Soil type determined the mount and the geogrid, but that's all."

The style selected quickly increased in popularity. "It was successful enough as a blend that it has become a standard color for Marshall Concrete, where the product was made," says Halcomb. "They use our molds, our techniques, and our colors."

In 2004, Hart Wall and Paver Systems, based in Concord, NC, completed an unusually challenging project that included a tall, very visible retaining wall. The company installed 90,000 square feet of retaining wall in part of a housing community for students attending Appalachian State University in Boone, NC. Adam Cox, an estimator at Hart, says the company worked with Ridgerock Retaining Walls in Charlotte, NC, to make the design a reality.

The sheer size of the retaining wall may have seemed daunting, but the engineering requirements also called for at least one wall that was 52 feet high, and for the extensive use of geogrid.

"More than anything, the major challenge was settlement. Our walls are designed to settle. They figure a sixteenth of an inch per foot of settlement," says Cox. "When you have a settlement issue, they use some really good material, good soil, or select fill [that] allows for very minimal amount of settling."

PHOTO: EP HENRY

PHOTO: EP HENRY

Many buildings were located within 40 feet of the wall, and this was also a concern for Hart. The granular-based material used as backfill was compacted behind the wall, which, aside from its highest point of 52 feet, was 20 feet high in most places.

"There is column drain right behind our block, 1 foot, which makes a big drain to our drainage pipe at the bottom," says Cox. Drainage solutions on a larger scale are often a serious challenge for commercial projects.

Appearance was also an important consideration in the construction of a tall retaining wall protecting the Brookworth Plaza Shopping Center in East Bradford Township, PA. Nestled in a community located just west of Philadelphia's suburban Main Line, the 2003 project was necessary to support the land surrounding a retail center with a detention basin.

"There was an additional requirement to have a green solution for the upper half of the wall to provide a unique aesthetic appearance. The total wall height is 46 feet, which includes 30 feet of SierraScape and 16 feet of Mesa block," says Kevin Earley of EP Henry Corp. in Parker Ford, PA. "The client was looking for a portion of the wall to be plantable for aesthetic reasons."

While EP Henry provided the Atlanta-basedTensar Earth Technologies' Mesa block retaining wall system, which features a concrete-block face, and Tensar's SierraScape retaining wall system that utilizes a gabion-like wire basket structure, the wall itself was designed by professional engineer Bart Shippee and installed by Pickering Valley Landscape based in Elverson, PA.

"Free-draining imported stone was used in the reinforced zone, up to the emergency spillway elevation of the basin," explains Earley. "Site soils were used for most of the project, except near the detention basin, where free-draining stone was used."

PHOTO: ERWS INC.

PHOTO: ERWS INC.

PHOTO: ERWS INC.

Teamwork was an important component in the project's success in balancing the project's engineering needs and appearance. "The combination of a great product with an experienced installer ensures that the wall will look good and perform as designed," says Earley.

When appearance is a primary concern in creating a retaining wall, making the tough work involved look easy is just another part of the project. ERWS, or Erosion and Retaining Wall Structures, is a construction company based in Lewisville, TX, that was contracted in 2003 to install a retaining wall on a Dallas homeowner's property. The challenge was creating a wall that could perform around some majestic, very old oak trees as well as several other varieties, including one that extended over the creek on the property.

"It was a three-tiered wall," describes James Fee, president of ERWS. "We had to go with a 3-foot wall along the creek, an 11-foot wall behind that one, then another that varied 2 feet to 12 feet behind the second." He says the total elevation was 9 feet with each tier being just 3 feet tall. "We used the free-draining wall, the gabions, instead of congregated earth or block.The soil itself was a clay-type soil. We used gabions because they could take the expansion and contraction of the soil without coming apart. The fill was compacted."

ERWS workers placed turf mats behind the winding wall. "He [the owner] has planted vines on it to hide the retaining walls themselves," says Fee.

Another aesthetically important project ERWS has worked on is the Buffalo Bayou hike and bike trail for the City of Houston. Located in downtown Houston, the company opted to use gravity retaining walls, where 60% of the product is underwater, in a move to save money and prevent flooding for the ongoing project.

"The bayou itself is the main drainage for the city of Houston," explains Fee, noting the maximum height of the wall is 9 feet. "They had to excavate to a good, solid clay, because there is a lot of silt. It had to be a free-draining wall. Now we're doing drainage channels into that existing structure."

Large numbers of people hike and bike the trail where the wall is visible.

"The top basket had to be architecturally hand-placed. The top basket is a 3-by-3 square, the second basket is a 6-by-3 sackgabion, and the bottom basket is a 9-foot by 3-foot gabion," says Fee. "It was more appearance driven than structurally driven to the architect."

Satisfying the client's desire for pleasing aesthetics is key today, but, as in the past, a retaining wall is only as good as it performs.

Writer Tara Beecham is based in Morgantown, PA.

EC - January/February 2006